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C++ Made Easy: Learning to Program in C++

Learn C++ with this tutorial, designed for beginners and containing lots of examples, tips and simple explanations.

Getting Set Up - C++ Compilers

The very first thing you need to do, before starting out in C++, is to make sure that you have a compiler. What is a compiler, you ask? A compiler turns the program that you write into an executable that your computer can actually understand and run. If you're taking a course, you probably have one provided through your school. If you're starting out on your own, your best bet is to use Code::Blocks with MinGW. If you're on Linux, you can use g++, and if you're on Mac OS X, you can use XCode. (If you are stuck using an older compiler, such as Turbo C++, you'll need to read this page on compatibility issues.) If you haven't yet done so, go ahead and get a compiler set up--you'll need it for the rest of the tutorial.

Intro to the C++ Language

A C++ program is a collection of commands, which tell the computer to do "something". This collection of commands is usually called C++ source code, source code or just code. Commands are either "functions" or "keywords". Keywords are a basic building block of the language, while functions are, in fact, usually written in terms of simpler functions--you'll see this in our very first program, below. (Confused? Think of it a bit like an outline for a book; the outline might show every chapter in the book; each chapter might have its own outline, composed of sections. Each section might have its own outline, or it might have all of the details written up.) Thankfully, C++ provides a great many common functions and keywords that you can use.

But how does a program actually start? Every program in C++ has one function, always named main, that is always called when your program first executes. From main, you can also call other functions whether they are written by us or, as mentioned earlier, provided by the compiler.

So how do you get access to those prewritten functions? To access those standard functions that comes with the compiler, you include a header with the #include directive. What this does is effectively take everything in the header and paste it into your program. Let's look at a working program:

#include <iostream>

using namespace std;

int main()

{

cout<<"HEY, you, I'm alive! Oh, and Hello World!\n";

cin.get();

}

Let's look at the elements of the program. The #include is a "preprocessor" directive that tells the compiler to put code from the header called iostream into our program before actually creating the executable. By including header files, you gain access to many different functions. For example, the cout function requires iostream. Following the include is the statement, "using namespace std;". This line tells the compiler to use a group of functions that are part of the standard library (std). By including this line at the top of a file, you allow the program to use functions such as cout. The semicolon is part of the syntax of C++. It tells the compiler that you're at the end of a command. You will see later that the semicolon is used to end most commands in C++.

The next important line is int main(). This line tells the compiler that there is a function named main, and that the function returns an integer, hence int. The "curly braces" ({ and }) signal the beginning and end of functions and other code blocks. You can think of them as meaning BEGIN and END.

The next line of the program may seem strange. If you have programmed in another language, you might expect that print would be the function used to display text. In C++, however, the cout object is used to display text (pronounced "C out"). It uses the << symbols, known as "insertion operators", to indicate what to output. cout<< results in a function call with the ensuing text as an argument to the function. The quotes tell the compiler that you want to output the literal string as-is. The '\n' sequence is actually treated as a single character that stands for a newline (we'll talk about this later in more detail). It moves the cursor on your screen to the next line. Again, notice the semicolon: it is added onto the end of most lines, such as function calls, in C++.

The next command is cin.get(). This is another function call: it reads in input and expects the user to hit the return key. Many compiler environments will open a new console window, run the program, and then close the window. This command keeps that window from closing because the program is not done yet because it waits for you to hit enter. Including that line gives you time to see the program run.

Upon reaching the end of main, the closing brace, our program will return the value of 0 (and integer, hence why we told main to return an int) to the operating system. This return value is important as it can be used to tell the OS whether our program succeeded or not. A return value of 0 means success and is returned automatically (but only for main, other functions require you to manually return a value), but if we wanted to return something else, such as 1, we would have to do it with a return statement:

#include <iostream>

using namespace std;

int main()

{

cout<<"HEY, you, I'm alive! Oh, and Hello World!\n";

cin.get();

return 1;

}

The final brace closes off the function. You should try compiling this program and running it. You can cut and paste the code into a file, save it as a .cpp file. Our Code::Blocks tutorial actually takes you through creating a simple program, so check it out if you're confused.

If you are not using Code::Blocks, you should read the compiler instructions for information on how to compile.
Once you've got your first program running, why don't you try playing around with the cout function to get used to writing C++?

An Aside on Commenting Your Programs

As you are learning to program, you should also start to learn how to explain your programs (for yourself, if no one else). You do this by adding comments to code; I'll use them frequently to help explain code examples.

When you tell the compiler a section of text is a comment, it will ignore it when running the code, allowing you to use any text you want to describe the real code. To create a comment use either //, which tells the compiler that the rest of the line is a comment, or /* and then */ to block off everything between as a comment. Certain compiler environments will change the color of a commented area, but some will not. Be certain not to accidentally comment out code (that is, to tell the compiler part of your code is a comment) you need for the program. When you are learning to program, it is useful to be able to comment out sections of code in order to see how the output is affected.

User interaction and Saving Information with Variables

So far you've learned how to write a simple program to display information typed in by you, the programmer, and how to describe your program with comments. That's great, but what about interacting with your user? Fortunately, it is also possible for your program to accept input. The function you use is known as cin, and is followed by the insertion operator >>.

Of course, before you try to receive input, you must have a place to store that input. In programming, input and data are stored in variables. There are several different types of variables which store different kinds of information (e.g. numbers versus letters); when you tell the compiler you are declaring a variable, you must include the data type along with the name of the variable. Several basic types include char, int, and float.

A variable of type char stores a single character, variables of type int store integers (numbers without decimal places), and variables of type float store numbers with decimal places. Each of these variable types - char, int, and float - is each a keyword that you use when you declare a variable.

What's with all these variable types?

Sometimes it can be confusing to have multiple variable types when it seems like some variable types are redundant (why have integer numbers when you have floats?). Using the right variable type can be important for making your code readable and for efficiency--some variables require more memory than others. Moreover, because of the way the numbers are actually stored in memory, a float is "inexact", and should not be used when you need to store an "exact" integer value.

Declaring Variables in C++

To declare a variable you use the syntax "type <name>;". Here are some variable declaration examples:

int x;

char letter;

float the_float;

It is permissible to declare multiple variables of the same type on the same line; each one should be separated by a comma.

int a, b, c, d;

If you were watching closely, you might have seen that declaration of a variable is always followed by a semicolon (note that this is the same procedure used when you call a function).

Common Errors when Declaring Variables in C++

If you attempt to use a variable that you have not declared, your program will not be compiled or run, and you will receive an error message informing you that you have made a mistake. Usually, this is called an undeclared variable.

Case Sensitivity

Now is a good time to talk about an important concept that can easily throw you off: case sensitivity. Basically, in C++, whether you use uppercase or lowercase letters matters. The words Cat and cat mean different things to the compiler. In C++, all language keywords, all functions and all variables are case sensitive. A difference in case between your variable declaration and the use of the variable is one reason you might get an undeclared variable error.

Using Variables

Ok, so you now know how to tell the compiler about variables, but what about using them?

Here is a sample program demonstrating the use of a variable:

#include <iostream>

using namespace std;

int main()

{

int thisisanumber;

cout<<"Please enter a number: ";

cin>> thisisanumber;

cin.ignore();

cout<<"You entered: "<< thisisanumber <<"\n";

cin.get();

}

Let's break apart this program and examine it line by line. The keyword int declares thisisanumber to be an integer. The function cin>> reads a value into thisisanumber; the user must press enter before the number is read by the program. cin.ignore() is another function that reads and discards a character. Remember that when you type input into a program, it takes the enter key too. We don't need this, so we throw it away. Keep in mind that the variable was declared an integer; if the user attempts to type in a decimal number, it will be truncated (that is, the decimal component of the number will be ignored). Try typing in a sequence of characters or a decimal number when you run the example program; the response will vary from input to input, but in no case is it particularly pretty. Notice that when printing out a variable quotation marks are not used. Were there quotation marks, the output would be "You Entered: thisisanumber." The lack of quotation marks informs the compiler that there is a variable, and therefore that the program should check the value of the variable in order to replace the variable name with the variable when executing the output function. Do not be confused by the inclusion of two separate insertion operators on one line. Including multiple insertion operators on one line is perfectly acceptable and all of the output will go to the same place. In fact, you must separate string literals (strings enclosed in quotation marks) and variables by giving each its own insertion operators (<<). Trying to put two variables together with only one << will give you an error message, do not try it. Do not forget to end functions and declarations with a semicolon. If you forget the semicolon, the compiler will give you an error message when you attempt to compile the program.

Changing and Comparing Variables

Of course, no matter what type you use, variables are uninteresting without the ability to modify them. Several operators used with variables include the following: *, -, +, /, =, ==, >, <. The * multiplies, the - subtracts, and the + adds. It is of course important to realize that to modify the value of a variable inside the program it is rather important to use the equal sign. In some languages, the equal sign compares the value of the left and right values, but in C++ == is used for that task. The equal sign is still extremely useful. It sets the left input to the equal sign, which must be one, and only one, variable equal to the value on the right side of the equal sign. The operators that perform mathematical functions should be used on the right side of an equal sign in order to assign the result to a variable on the left side.

Here are a few examples:

a = 4 * 6; // (Note use of comments and of semicolon) a is 24

a = a + 5; // a equals the original value of a with five added to it

a == 5 // Does NOT assign five to a. Rather, it checks to see if a equals 5.

The other form of equal, ==, is not a way to assign a value to a variable. Rather, it checks to see if the variables are equal. It is useful in other areas of C++; for example, you will often use == in such constructions as conditional statements and loops. You can probably guess how < and > function. They are greater than and less than operators.

For example:

a < 5 // Checks to see if a is less than five

a > 5 // Checks to see if a is greater than five

a == 5 // Checks to see if a equals five, for good measure

Comparing variables isn't really useful until you have some way of using the results--that's what lesson 2, on if statements is all about.

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You might also be interested in these beginner C++ training videos. We've found these training videos to be an excellent way to master the fundamentals of C++ Programming. Taught by a Professor Mike McMillian, these training videos come with practical working files that allow you to learn at your own pace. Try a free demo today!

Lesson (2)

If Statement

The ability to control the flow of your program, letting it make decisions on what code to execute, is valuable to the programmer. The if statement allows you to control if a program enters a section of code or not based on whether a given condition is true or false. One of the important functions of the if statement is that it allows the program to select an action based upon the user's input. For example, by using an if statement to check a user entered password, your program can decide whether a user is allowed access to the program.

Without a conditional statement such as the if statement, programs would run almost the exact same way every time. If statements allow the flow of the program to be changed, and so they allow algorithms and more interesting code.

Before discussing the actual structure of the if statement, let us examine the meaning of TRUE and FALSE in computer terminology. A true statement is one that evaluates to a nonzero number. A false statement evaluates to zero. When you perform comparison with the relational operators, the operator will return 1 if the comparison is true, or 0 if the comparison is false. For example, the check 0 == 2 evaluates to 0. The check 2 == 2 evaluates to a 1. If this confuses you, try to use a cout statement to output the result of those various comparisons (for example cout<< ( 2 == 1 );)

When programming, the aim of the program will often require the checking of one value stored by a variable against another value to determine whether one is larger, smaller, or equal to the other.

There are a number of operators that allow these checks.

Here are the relational operators, as they are known, along with examples:

> greater than 5 > 4 is TRUE

< less than 4 < 5 is TRUE

>= greater than or equal 4 >= 4 is TRUE

<= less than or equal 3 <= 4 is TRUE

== equal to 5 == 5 is TRUE

!= not equal to 5 != 4 is TRUE

It is highly probable that you have seen these before, probably with slightly different symbols. They should not present any hindrance to understanding. Now that you understand TRUE and FALSE in computer terminology as well as the comparison operators, let us look at the actual structure of if statements.

Basic If Statement Syntax

The structure of an if statement is as follows:

if ( TRUE )

Execute the next statement

Here is a simple example that shows the syntax:

if ( 5 < 10 )

cout<<"Five is now less than ten, that's a big surprise";

Here, we're just evaluating the statement, "is five less than ten", to see if it is true or not; with any luck, it's not! If you want, you can write your own full program including iostream and put this in the main function and run it to test.

To have more than one statement execute after an if statement that evaluates to true, use braces, like we did with the body of a function. Anything inside braces is called a compound statement, or a block.

For example:

if ( TRUE ) {

Execute all statements inside the braces

}

I recommend always putting braces following if statements. If you do this, you never have to remember to put them in when you want more than one statement to be executed, and you make the body of the if statement more visually clear.

Else

Sometimes when the condition in an if statement evaluates to false, it would be nice to execute some code instead of the code executed when the statement evaluates to true. The "else" statement effectively says that whatever code after it (whether a single line or code between brackets) is executed if the if statement is FALSE.

It can look like this:

if ( TRUE ) {

// Execute these statements if TRUE

}

else {

// Execute these statements if FALSE

}

Else If

Another use of else is when there are multiple conditional statements that may all evaluate to true, yet you want only one if statement's body to execute. You can use an "else if" statement following an if statement and its body; that way, if the first statement is true, the "else if" will be ignored, but if the if statement is false, it will then check the condition for the else if statement. If the if statement was true the else statement will not be checked. It is possible to use numerous else if statements to ensure that only one block of code is executed.

if ( <condition> ) {

// Execute these statements if <condition> is TRUE

}

else if ( <another condition> ) {

// Execute these statements if <another condition> is TRUE and

// <condition> is FALSE

}

Let's look at a simple program for you to try out on your own.

#include <iostream>

using namespace std;

int main() // Most important part of the program!

{

int age; // Need a variable...

cout<<"Please input your age: "; // Asks for age

cin>> age; // The input is put in age

cin.ignore(); // Throw away enter

if ( age < 100 ) { // If the age is less than 100

cout<<"You are pretty young!\n"; // Just to show you it works...

}

else if ( age == 100 ) { // I use else just to show an example

cout<<"You are old\n"; // Just to show you it works...

}

else {

cout<<"You are really old\n"; // Executed if no other statement is

}

cin.get();

}

More interesting conditions using boolean operators

Boolean operators allow you to create more complex conditional statements. For example, if you wish to check if a variable is both greater than five and less than ten, you could use the boolean AND to ensure both var > 5 and var < 10 are true. In the following discussion of boolean operators, I will capitalize the boolean operators in order to distinguish them from normal English. The actual C++ operators of equivalent function will be described further into the tutorial - the C++ symbols are not: OR, AND, NOT, although they are of equivalent function.

When using if statements, you will often wish to check multiple different conditions. You must understand the Boolean operators OR, NOT, and AND. The boolean operators function in a similar way to the comparison operators: each returns 0 if evaluates to FALSE or 1 if it evaluates to TRUE.

NOT: The NOT operator accepts one input. If that input is TRUE, it returns FALSE, and if that input is FALSE, it returns TRUE. For example, NOT (1) evaluates to 0, and NOT (0) evaluates to 1. NOT (any number but zero) evaluates to 0. In C and C++ NOT is written as !. NOT is evaluated prior to both AND and OR.

AND: This is another important command. AND returns TRUE if both inputs are TRUE (if 'this' AND 'that' are true). (1) AND (0) would evaluate to zero because one of the inputs is false (both must be TRUE for it to evaluate to TRUE). (1) AND (1) evaluates to 1. (any number but 0) AND (0) evaluates to 0. The AND operator is written && in C++. Do not be confused by thinking it checks equality between numbers: it does not. Keep in mind that the AND operator is evaluated before the OR operator.

OR: Very useful is the OR statement! If either (or both) of the two values it checks are TRUE then it returns TRUE. For example, (1) OR (0) evaluates to 1. (0) OR (0) evaluates to 0. The OR is written as || in C++. Those are the pipe characters. On your keyboard, they may look like a stretched colon. On my computer the pipe shares its key with \. Keep in mind that OR will be evaluated after AND.

It is possible to combine several boolean operators in a single statement; often you will find doing so to be of great value when creating complex expressions for if statements. What is !(1 && 0)? Of course, it would be TRUE. It is true is because 1 && 0 evaluates to 0 and !0 evaluates to TRUE (ie, 1).

Try some of these - they're not too hard. If you have questions about them, feel free to stop by our forums.

A. !( 1 || 0 ) ANSWER: 0

B. !( 1 || 1 && 0 ) ANSWER: 0 (AND is evaluated before OR)

C. !( ( 1 || 0 ) && 0 ) ANSWER: 1 (Parenthesis are useful)

If you find you enjoyed this section, then you might want to look more at Boolean Algebra.

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Lesson 3: Loops

(Printable Version)

Loops are used to repeat a block of code. Being able to have your program repeatedly execute a block of code is one of the most basic but useful tasks in programming -- many programs or websites that produce extremely complex output (such as a message board) are really only executing a single task many times. (They may be executing a small number of tasks, but in principle, to produce a list of messages only requires repeating the operation of reading in some data and displaying it.) Now, think about what this means: a loop lets you write a very simple statement to produce a significantly greater result simply by repetition.
One Caveat: before going further, you should understand the concept of C++'s true and false, because it will be necessary when working with loops (the conditions are the same as with if statements). There are three types of loops: for, while, and do..while. Each of them has their specific uses. They are all outlined below.

FOR - for loops are the most useful type. The syntax for a for loop is

for ( variable initialization; condition; variable update ) {

  Code to execute while the condition is true

The variable initialization allows you to either declare a variable and give it a value or give a value to an already existing variable. Second, the condition tells the program that while the conditional expression is true the loop should continue to repeat itself. The variable update section is the easiest way for a for loop to handle changing of the variable. It is possible to do things like x++, x = x + 10, or even x = random ( 5 ), and if you really wanted to, you could call other functions that do nothing to the variable but still have a useful effect on the code. Notice that a semicolon separates each of these sections, that is important. Also note that every single one of the sections may be empty, though the semicolons still have to be there. If the condition is empty, it is evaluated as true and the loop will repeat until something else stops it.

Example:

#include <iostream>

using namespace std; // So the program can see cout and endl

int main()

  // The loop goes while x < 10, and x increases by one every loop

  for ( int x = 0; x < 10; x++ ) {

    // Keep in mind that the loop condition checks

    //  the conditional statement before it loops again.

    //  consequently, when x equals 10 the loop breaks.

    // x is updated before the condition is checked.

    cout<< x <<endl;

  cin.get();

This program is a very simple example of a for loop. x is set to zero, while x is less than 10 it calls cout<< x <<endl; and it adds 1 to x until the condition is met. Keep in mind also that the variable is incremented after the code in the loop is run for the first time.

WHILE - WHILE loops are very simple. The basic structure is

while ( condition ) { Code to execute while the condition is true } The true represents a boolean expression which could be x == 1 or while ( x != 7 ) (x does not equal 7). It can be any combination of boolean statements that are legal. Even, (while x ==5 || v == 7) which says execute the code while x equals five or while v equals 7. Notice that a while loop is the same as a for loop without the initialization and update sections. However, an empty condition is not legal for a while loop as it is with a for loop.

Example:

#include <iostream>

using namespace std; // So we can see cout and endl

int main()

  int x = 0;  // Don't forget to declare variables

  while ( x < 10 ) { // While x is less than 10

    cout<< x <<endl;

    x++;             // Update x so the condition can be met eventually

  cin.get();

This was another simple example, but it is longer than the above FOR loop. The easiest way to think of the loop is that when it reaches the brace at the end it jumps back up to the beginning of the loop, which checks the condition again and decides whether to repeat the block another time, or stop and move to the next statement after the block.

DO..WHILE - DO..WHILE loops are useful for things that want to loop at least once. The structure is

do {

} while ( condition );

Notice that the condition is tested at the end of the block instead of the beginning, so the block will be executed at least once. If the condition is true, we jump back to the beginning of the block and execute it again. A do..while loop is basically a reversed while loop. A while loop says "Loop while the condition is true, and execute this block of code", a do..while loop says "Execute this block of code, and loop while the condition is true".

Example:

#include <iostream>

using namespace std;

int main()

  int x;

  x = 0;

  do {

    // "Hello, world!" is printed at least one time

    //  even though the condition is false

    cout<<"Hello, world!\n";

  } while ( x != 0 );

  cin.get();

Keep in mind that you must include a trailing semi-colon after the while in the above example. A common error is to forget that a do..while loop must be terminated with a semicolon (the other loops should not be terminated with a semicolon, adding to the confusion). Notice that this loop will execute once, because it automatically executes before checking the condition.

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Lesson 4: Functions

(Printable Version)

Now that you should have learned about variables, loops, and conditional statements it is time to learn about functions. You should have an idea of their uses as we have already used them and defined one in the guise of main. cin.get() is an example of a function. In general, functions are blocks of code that perform a number of pre-defined commands to accomplish something productive.

Functions that a programmer writes will generally require a prototype. Just like a blueprint, the prototype tells the compiler what the function will return, what the function will be called, as well as what arguments the function can be passed. When I say that the function returns a value, I mean that the function can be used in the same manner as a variable would be. For example, a variable can be set equal to a function that returns a value between zero and four.

For example:

#include <cstdlib>   // Include rand()

using namespace std; // Make rand() visible

int a = rand(); // rand is a standard function that all compilers have

Do not think that 'a' will change at random, it will be set to the value returned when the function is called, but it will not change again.

The general format for a prototype is simple:

return-type function_name ( arg_type arg1, ..., arg_type argN );

arg_type just means the type for each argument -- for instance, an int, a float, or a char. It's exactly the same thing as what you would put if you were declaring a variable.

There can be more than one argument passed to a function or none at all (where the parentheses are empty), and it does not have to return a value. Functions that do not return values have a return type of void. Let's look at a function prototype:

int mult ( int x, int y );

This prototype specifies that the function mult will accept two arguments, both integers, and that it will return an integer. Do not forget the trailing semi-colon. Without it, the compiler will probably think that you are trying to write the actual definition of the function.

When the programmer actually defines the function, it will begin with the prototype, minus the semi-colon. Then there should always be a block with the code that the function is to execute, just as you would write it for the main function. Any of the arguments passed to the function can be used as if they were declared in the block. Finally, end it all with a cherry and a closing brace. Okay, maybe not a cherry.

Let's look at an example program:

#include <iostream>

using namespace std;

int mult ( int x, int y );

int main()

  int x;

  int y;

  cout<<"Please input two numbers to be multiplied: ";

  cin>> x >> y;

  cin.ignore();

  cout<<"The product of your two numbers is "<< mult ( x, y ) <<"\n";

  cin.get();

int mult ( int x, int y )

  return x * y;

This program begins with the only necessary include file and a directive to make the std namespace visible. Everything in the standard headers is inside of the std namespace and not visible to our programs unless we make them so. Next is the prototype of the function. Notice that it has the final semi-colon! The main function returns an integer, which you should always have to conform to the standard. You should not have trouble understanding the input and output functions. It is fine to use cin to input to variables as the program does. But when typing in the numbers, be sure to separate them by a space so that cin can tell them apart and put them in the right variables.

Notice how cout actually outputs what appears to be the mult function. What is really happening is cout is printing the value returned by mult, not mult itself. The result would be the same as if we had use this print instead

cout<<"The product of your two numbers is "<< x * y <<"\n";

The mult function is actually defined below main. Due to its prototype being above main, the compiler still recognizes it as being defined, and so the compiler will not give an error about mult being undefined. As long as the prototype is present, a function can be used even if there is no definition. However, the code cannot be run without a definition even though it will compile. The prototype and definition can be combined into one also. If mult were defined before it is used, we could do away with the prototype because the definition can act as a prototype as well.

Return is the keyword used to force the function to return a value. Note that it is possible to have a function that returns no value. If a function returns void, the return statement is valid, but only if it does not have an expression. In other words, for a function that returns void, the statement "return;" is legal, but redundant.

The most important functional (Pun semi-intended) question is why do we need a function? Functions have many uses. For example, a programmer may have a block of code that he has repeated forty times throughout the program. A function to execute that code would save a great deal of space, and it would also make the program more readable. Also, having only one copy of the code makes it easier to make changes. Would you rather make forty little changes scattered all throughout a potentially large program, or one change to the function body? So would I.

Another reason for functions is to break down a complex program into logical parts. For example, take a menu program that runs complex code when a menu choice is selected. The program would probably best be served by making functions for each of the actual menu choices, and then breaking down the complex tasks into smaller, more manageable tasks, which could be in their own functions. In this way, a program can be designed that makes sense when read. And has a structure that is easier to understand quickly. The worst programs usually only have the required function, main, and fill it with pages of jumbled code.

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Lesson 5: switch case in C and C++

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Switch case statements are a substitute for long if statements that compare a variable to several "integral" values ("integral" values are simply values that can be expressed as an integer, such as the value of a char). The basic format for using switch case is outlined below. The value of the variable given into switch is compared to the value following each of the cases, and when one value matches the value of the variable, the computer continues executing the program from that point.

switch ( <variable> ) {

case this-value:

  Code to execute if <variable> == this-value

  break;

case that-value:

  Code to execute if <variable> == that-value

  break;

...

default:

  Code to execute if <variable> does not equal the value following any of the cases

  break;

The condition of a switch statement is a value. The case says that if it has the value of whatever is after that case then do whatever follows the colon. The break is used to break out of the case statements. Break is a keyword that breaks out of the code block, usually surrounded by braces, which it is in. In this case, break prevents the program from falling through and executing the code in all the other case statements. An important thing to note about the switch statement is that the case values may only be constant integral expressions. Sadly, it isn't legal to use case like this:

int a = 10;

int b = 10;

int c = 20;

switch ( a ) {

case b:

  // Code

  break;

case c:

  // Code

  break;

default:

  // Code

  break;

The default case is optional, but it is wise to include it as it handles any unexpected cases. Switch statements serves as a simple way to write long if statements when the requirements are met. Often it can be used to process input from a user.

Below is a sample program, in which not all of the proper functions are actually declared, but which shows how one would use switch in a program.

#include <iostream>

using namespace std;

void playgame()

    cout << "Play game called";

void loadgame()

    cout << "Load game called";

void playmultiplayer()

    cout << "Play multiplayer game called";

int main()

  int input;

  cout<<"1. Play game\n";

  cout<<"2. Load game\n";

  cout<<"3. Play multiplayer\n";

  cout<<"4. Exit\n";

  cout<<"Selection: ";

  cin>> input;

  switch ( input ) {

  case 1:            // Note the colon, not a semicolon

    playgame();

    break;

  case 2:            // Note the colon, not a semicolon

    loadgame();

    break;

  case 3:            // Note the colon, not a semicolon

    playmultiplayer();

    break;

  case 4:            // Note the colon, not a semicolon

    cout<<"Thank you for playing!\n";

    break;

  default:            // Note the colon, not a semicolon

    cout<<"Error, bad input, quitting\n";

    break;

  cin.get();

This program will compile, but cannot be run until the undefined functions are given bodies, but it serves as a model (albeit simple) for processing input. If you do not understand this then try mentally putting in if statements for the case statements. Default simply skips out of the switch case construction and allows the program to terminate naturally. If you do not like that, then you can make a loop around the whole thing to have it wait for valid input. You could easily make a few small functions if you wish to test the code.

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Lesson 6: Pointers in C++

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Pointers are an extremely powerful programming tool. They can make some things much easier, help improve your program's efficiency, and even allow you to handle unlimited amounts of data. For example, using pointers is one way to have a function modify a variable passed to it. It is also possible to use pointers to dynamically allocate memory, which means that you can write programs that can handle nearly unlimited amounts of data on the fly--you don't need to know, when you write the program, how much memory you need. Wow, that's kind of cool. Actually, it's very cool, as we'll see in some of the next tutorials. For now, let's just get a basic handle on what pointers are and how you use them.

What are pointers? Why should you care?

Pointers are aptly named: they "point" to locations in memory. Think of a row of safety deposit boxes of various sizes at a local bank. Each safety deposit box will have a number associated with it so that the teller can quickly look it up. These numbers are like the memory addresses of variables. A pointer in the world of safety deposit boxes would simply be anything that stored the number of another safety deposit box. Perhaps you have a rich uncle who stored valuables in his safety deposit box, but decided to put the real location in another, smaller, safety deposit box that only stored a card with the number of the large box with the real jewelry. The safety deposit box with the card would be storing the location of another box; it would be equivalent to a pointer. In the computer, pointers are just variables that store memory addresses, usually the addresses of other variables.

The cool thing is that once you can talk about the address of a variable, you'll then be able to go to that address and retrieve the data stored in it. If you happen to have a huge piece of data that you want to pass into a function, it's a lot easier to pass its location to the function than to copy every element of the data! Moreover, if you need more memory for your program, you can request more memory from the system--how do you get "back" that memory? The system tells you where it is located in memory; that is to say, you get a memory address back. And you need pointers to store the memory address.

A note about terms: the word pointer can refer either to a memory address itself, or to a variable that stores a memory address. Usually, the distinction isn't really that important: if you pass a pointer variable into a function, you're passing the value stored in the pointer--the memory address. When I want to talk about a memory address, I'll refer to it as a memory address; when I want a variable that stores a memory address, I'll call it a pointer. When a variable stores the address of another variable, I'll say that it is "pointing to" that variable.

C++ Pointer Syntax

Pointers require a bit of new syntax because when you have a pointer, you need the ability to request both the memory location it stores and the value stored at that memory location. Moreover, since pointers are somewhat special, you need to tell the compiler when you declare your pointer variable that the variable is a pointer, and tell the compiler what type of memory it points to.

The pointer declaration looks like this:

<variable_type> *<name>;

For example, you could declare a pointer that stores the address of an integer with the following syntax:

int *points_to_integer;

Notice the use of the *. This is the key to declaring a pointer; if you add it directly before the variable name, it will declare the variable to be a pointer. Minor gotcha: if you declare multiple pointers on the same line, you must precede each of them with an asterisk:

// one pointer, one regular int

int *pointer1, nonpointer1;

// two pointers

int *pointer1, *pointer2;

As I mentioned, there are two ways to use the pointer to access information: it is possible to have it give the actual address to another variable. To do so, simply use the name of the pointer without the *. However, to access the actual memory location, use the *. The technical name for this doing this is dereferencing the pointer; in essence, you're taking the reference to some memory address and following it, to retrieve the actual value. It can be tricky to keep track of when you should add the asterisk. Remember that the pointer's natural use is to store a memory address; so when you use the pointer:

call_to_function_expecting_memory_address(pointer);

then it evaluates to the address. You have to add something extra, the asterisk, in order to retrieve the value stored at the address. You'll probably do that an awful lot. Nevertheless, the pointer itself is supposed to store an address, so when you use the bare pointer, you get that address back.

Pointing to Something: Retrieving an Address

In order to have a pointer actually point to another variable it is necessary to have the memory address of that variable also. To get the memory address of a variable (its location in memory), put the & sign in front of the variable name. This makes it give its address. This is called the address-of operator, because it returns the memory address. Conveniently, both ampersand and address-of start with a; that's a useful way to remember that you use & to get the address of a variable.

For example:

#include <iostream>

using namespace std;

int main()

  int x;            // A normal integer

  int *p;           // A pointer to an integer

  p = &x;           // Read it, "assign the address of x to p"

  cin>> x;          // Put a value in x, we could also use *p here

  cin.ignore();

  cout<< *p <<"\n"; // Note the use of the * to get the value

  cin.get();

The cout outputs the value stored in x. Why is that? Well, let's look at the code. The integer is called x. A pointer to an integer is then defined as p. Then it stores the memory location of x in pointer by using the address-of operator (&) to get the address of the variable. Using the ampersand is a bit like looking at the label on the safety deposit box to see its number rather than looking inside the box, to get what it stores. The user then inputs a number that is stored in the variable x; remember, this is the same location that is pointed to by p.

The next line then passes *p into cout. *p performs the "dereferencing" operation on p; it looks at the address stored in p, and goes to that address and returns the value. This is akin to looking inside a safety deposit box only to find the number of (and, presumably, the key to ) another box, which you then open.

Notice that in the above example, pointer is initialized to point to a specific memory address before it is used. If this was not the case, it could be pointing to anything. This can lead to extremely unpleasant consequences to the program. For instance, the operating system will probably prevent you from accessing memory that it knows your program doesn't own: this will cause your program to crash. To avoid crashing your program, you should always initialize pointers before you use them.

It is also possible to initialize pointers using free memory. This allows dynamic allocation of array memory. It is most useful for setting up structures called linked lists. This difficult topic is too complex for this text. An understanding of the keywords new and delete will, however, be tremendously helpful in the future.

The keyword new is used to initialize pointers with memory from free store (a section of memory available to all programs). The syntax looks like the example:

int *ptr = new int;

It initializes ptr to point to a memory address of size int (because variables have different sizes, number of bytes, this is necessary). The memory that is pointed to becomes unavailable to other programs. This means that the careful coder should free this memory at the end of its usage.

The delete operator frees up the memory allocated through new. To do so, the syntax is as in the example.

delete ptr;

After deleting a pointer, it is a good idea to reset it to point to 0. When 0 is assigned to a pointer, the pointer becomes a null pointer, in other words, it points to nothing. By doing this, when you do something foolish with the pointer (it happens a lot, even with experienced programmers), you find out immediately instead of later, when you have done considerable damage.

In fact, the concept of the null pointer is frequently used as a way of indicating a problem--for instance, some functions left over from C return 0 if they cannot correctly allocate memory (notably, the malloc function). You want to be sure to handle this correctly if you ever use malloc or other C functions that return a "NULL pointer" on failure.

In C++, if a call to new fails because the system is out of memory, then it will "throw an exception". For the time being, you need not worry too much about this case, but you can read more about what happens when new fails.

Taking Stock of Pointers

Pointers may feel like a very confusing topic at first but I think anyone can come to appreciate and understand them. If you didn't feel like you absorbed everything about them, just take a few deep breaths and re-read the lesson. You shouldn't feel like you've fully grasped every nuance of when and why you need to use pointers, though you should have some idea of some of their basic uses.

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Lesson 7: Structures in C++

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Before discussing classes, this lesson will be an introduction to data structures similar to classes. Structures are a way of storing many different values in variables of potentially different types under the same name. This makes it a more modular program, which is easier to modify because its design makes things more compact. Structs are generally useful whenever a lot of data needs to be grouped together--for instance, they can be used to hold records from a database or to store information about contacts in an address book. In the contacts example, a struct could be used that would hold all of the information about a single contact--name, address, phone number, and so forth.

The format for defining a structure is

struct Tag {

  Members

};

Where Tag is the name of the entire type of structure and Members are the variables within the struct. To actually create a single structure the syntax is

struct Tag name_of_single_structure;

To access a variable of the structure it goes

name_of_single_structure.name_of_variable;

For example:

struct example {

  int x;

};

struct example an_example; //Treating it like a normal variable type

an_example.x = 33;  //How to access it's members

Here is an example program:

struct database {

  int id_number;

  int age;

  float salary;

};

int main()

  database employee;  //There is now an employee variable that has modifiable

                      // variables inside it.

  employee.age = 22;

  employee.id_number = 1;

  employee.salary = 12000.21;

The struct database declares that database has three variables in it, age, id_number, and salary. You can use database like a variable type like int. You can create an employee with the database type as I did above. Then, to modify it you call everything with the 'employee.' in front of it. You can also return structures from functions by defining their return type as a structure type. For instance:

database fn();

I will talk only a little bit about unions as well. Unions are like structures except that all the variables share the same memory. When a union is declared the compiler allocates enough memory for the largest data-type in the union. It's like a giant storage chest where you can store one large item, or a small item, but never the both at the same time.

The '.' operator is used to access different variables inside a union also.

As a final note, if you wish to have a pointer to a structure, to actually access the information stored inside the structure that is pointed to, you use the -> operator in place of the . operator. All points about pointers still apply.

A quick example:

#include <iostream>

using namespace std;

struct xampl {

  int x;

};

int main()

  xampl structure;

  xampl *ptr;

  structure.x = 12;

  ptr = &structure; // Yes, you need the & when dealing with structures

                    //  and using pointers to them

  cout<< ptr->x;    // The -> acts somewhat like the * when used with pointers

                    //  It says, get whatever is at that memory address

                    //  Not "get what that memory address is"

  cin.get();

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Lesson 8: Arrays in C and C++

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Arrays are useful critters because they can be used in many ways to store large amounts of data in a structured way. For example, a tic-tac-toe board can be held in an array. Arrays are essentially a way to store many values under the same name. You can make an array out of any data-type including structures and classes.

Think about arrays like this:

[][][][][][]

Each of the bracket pairs is a slot(element) in the array, and you can put information into each one of them. It is almost like having a group of variables side by side.

Let's look at the syntax for declaring an array.

int examplearray[100]; // This declares an array

This would make an integer array with 100 slots, or places to store values(also called elements). To access a specific part element of the array, you merely put the array name and, in brackets, an index number. This corresponds to a specific element of the array. The one trick is that the first index number, and thus the first element, is zero, and the last is the number of elements minus one. 0-99 in a 100 element array, for example.

What can you do with this simple knowledge? Let's say you want to store a string, because C had no built-in datatype for strings, it was common to use arrays of characters to simulate strings. (C++ now has a string type as part of the standard library.)

For example:

char astring[100];

will allow you to declare a char array of 100 elements, or slots. Then you can receive input into it from the user, and if the user types in a long string, it will go in the array. The neat thing is that it is very easy to work with strings in this way, and there is even a header file called cstring. There is another lesson on the uses of strings, so it's not necessary to discuss here.

The most useful aspect of arrays is multidimensional arrays. How I think about multi-dimensional arrays:

[][][][][]

[][][][][]

[][][][][]

[][][][][]

[][][][][]

This is a graphic of what a two-dimensional array looks like when I visualize it.

For example:

int twodimensionalarray[8][8];

declares an array that has two dimensions. Think of it as a chessboard. You can easily use this to store information about some kind of game or to write something like tic-tac-toe. To access it, all you need are two variables, one that goes in the first slot and one that goes in the second slot. You can even make a three dimensional array, though you probably won't need to. In fact, you could make a four-hundred dimensional array. It would be confusing to visualize, however. Arrays are treated like any other variable in most ways. You can modify one value in it by putting:

arrayname[arrayindexnumber] = whatever;

or, for two dimensional arrays

arrayname[arrayindexnumber1][arrayindexnumber2] = whatever;

However, you should never attempt to write data past the last element of the array, such as when you have a 10 element array, and you try to write to the [10] element. The memory for the array that was allocated for it will only be ten locations in memory, but the next location could be anything, which could crash your computer.

You will find lots of useful things to do with arrays, from storing information about certain things under one name, to making games like tic-tac-toe. One suggestion I have is to use for loops when access arrays.

#include <iostream>

using namespace std;

int main()

  int x;

  int y;

  int array[8][8]; // Declares an array like a chessboard

  for ( x = 0; x < 8; x++ ) {

    for ( y = 0; y < 8; y++ )

      array[x][y] = x * y; // Set each element to a value

  cout<<"Array Indices:\n";

  for ( x = 0; x < 8;x++ ) {

    for ( y = 0; y < 8; y++ )

      cout<<"["<<x<<"]["<<y<<"]="<< array[x][y] <<" ";

    cout<<"\n";

  cin.get();

Here you see that the loops work well because they increment the variable for you, and you only need to increment by one. It's the easiest loop to read, and you access the entire array.

One thing that arrays don't require that other variables do, is a reference operator when you want to have a pointer to the string. For example:

char *ptr;

char str[40];

ptr = str;  // Gives the memory address without a reference operator(&)

As opposed to

int *ptr;

int num;

ptr = &num; // Requires & to give the memory address to the ptr

The reason for this is that when an array name is used as an expression, it refers to a pointer to the first element, not the entire array. This rule causes a great deal of confusion, for more information please see our Frequently Asked Questions.

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Lesson 9: C Strings

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In C++ there are two types of strings, C-style strings, and C++-style strings. This lesson will discuss C-style strings. C-style strings are really arrays, but there are some different functions that are used for strings, like adding to strings, finding the length of strings, and also of checking to see if strings match. The definition of a string would be anything that contains more than one character strung together. For example, "This" is a string. However, single characters will not be strings, though they can be used as strings.

Strings are arrays of chars. String literals are words surrounded by double quotation marks.

"This is a static string"

To declare a string of 49 letters, you would want to say:

char string[50];

This would declare a string with a length of 50 characters. Do not forget that arrays begin at zero, not 1 for the index number. In addition, a string ends with a null character, literally a '\0' character. However, just remember that there will be an extra character on the end on a string. It is like a period at the end of a sentence, it is not counted as a letter, but it still takes up a space. Technically, in a fifty char array you could only hold 49 letters and one null character at the end to terminate the string.

TAKE NOTE: char *arry; Can also be used as a string. If you have read the tutorial on pointers, you can do something such as:

arry = new char[256];

which allows you to access arry just as if it were an array. Keep in mind that to use delete you must put [] between delete and arry to tell it to free all 256 bytes of memory allocated.

For example:

delete [] arry.

Strings are useful for holding all types of long input. If you want the user to input his or her name, you must use a string. Using cin>> to input a string works, but it will terminate the string after it reads the first space. The best way to handle this situation is to use the function cin.getline. Technically cin is a class (a beast similar to a structure), and you are calling one of its member functions. The most important thing is to understand how to use the function however.

The prototype for that function is:

istream& getline(char *buffer, int length, char terminal_char);

The char *buffer is a pointer to the first element of the character array, so that it can actually be used to access the array. The int length is simply how long the string to be input can be at its maximum (how big the array is). The char terminal_char means that the string will terminate if the user inputs whatever that character is. Keep in mind that it will discard whatever the terminal character is.

It is possible to make a function call of cin.getline(arry, 50); without the terminal character. Note that '\n' is the way of actually telling the compiler you mean a new line, i.e. someone hitting the enter key.

For a example:

#include <iostream>

using namespace std;

int main()

  char string[256];                               // A nice long string

  cout<<"Please enter a long string: ";

  cin.getline ( string, 256, '\n' );              // Input goes into string

  cout<<"Your long string was: "<< string <<endl;

  cin.get();

Remember that you are actually passing the address of the array when you pass string because arrays do not require an address operator (&) to be used to pass their address. Other than that, you could make '\n' any character you want (make sure to enclose it with single quotes to inform the compiler of its character status) to have the getline terminate on that character.

cstring is a header file that contains many functions for manipulating strings. One of these is the string comparison function.

int strcmp ( const char *s1, const char *s2 );

strcmp will accept two strings. It will return an integer. This integer will either be:

Negative if s1 is less than s2.

Zero if s1 and s2 are equal.

Positive if s1 is greater than s2.

Strcmp is case sensitive. Strcmp also passes the address of the character array to the function to allow it to be accessed.

char *strcat ( char *dest, const char *src );

strcat is short for string concatenate, which means to add to the end, or append. It adds the second string to the first string. It returns a pointer to the concatenated string. Beware this function, it assumes that dest is large enough to hold the entire contents of src as well as its own contents.

char *strcpy ( char *dest, const char *src );

strcpy is short for string copy, which means it copies the entire contents of src into dest. The contents of dest after strcpy will be exactly the same as src such that strcmp ( dest, src ) will return 0.

size_t strlen ( const char *s );

strlen will return the length of a string, minus the terminating character ('\0'). The size_t is nothing to worry about. Just treat it as an integer that cannot be negative, which it is.

Here is a small program using many of the previously described functions:

#include <iostream> //For cout

#include <cstring>  //For the string functions

using namespace std;

int main()

  char name[50];

  char lastname[50];

  char fullname[100]; // Big enough to hold both name and lastname

  cout<<"Please enter your name: ";

  cin.getline ( name, 50 );

  if ( strcmp ( name, "Julienne" ) == 0 ) // Equal strings

    cout<<"That's my name too.\n";

  else                                    // Not equal

    cout<<"That's not my name.\n";

  // Find the length of your name

  cout<<"Your name is "<< strlen ( name ) <<" letters long\n";

  cout<<"Enter your last name: ";

  cin.getline ( lastname, 50 );

  fullname[0] = '\0';            // strcat searches for '\0' to cat after

  strcat ( fullname, name );     // Copy name into full name

  strcat ( fullname, " " );      // We want to separate the names by a space

  strcat ( fullname, lastname ); // Copy lastname onto the end of fullname

  cout<<"Your full name is "<< fullname <<"\n";

  cin.get();

Safe Programming

The above string functions all rely on the existence of a null terminator at the end of a string. This isn't always a safe bet. Moreover, some of them, noticeably strcat, rely on the fact that the destination string can hold the entire string being appended onto the end. Although it might seem like you'll never make that sort of mistake, historically, problems based on accidentally writing off the end of an array in a function like strcat, have been a major problem.

Fortunately, in their infinite wisdom, the designers of C have included functions designed to help you avoid these issues. Similar to the way that fgets takes the maximum number of characters that fit into the buffer, there are string functions that take an additional argument to indicate the length of the destination buffer. For instance, the strcpy function has an analogous strncpy function

char *strncpy ( char *dest, const char *src, size_t len );

which will only copy len bytes from src to dest (len should be less than the size of dest or the write could still go beyond the bounds of the array). Unfortunately, strncpy can lead to one niggling issue: it doesn't guarantee that dest will have a null terminator attached to it (this might happen if the string src is longer than dest). You can avoid this problem by using strlen to get the length of src and make sure it will fit in dest. Of course, if you were going to do that, then you probably don't need strncpy in the first place, right? Wrong. Now it forces you to pay attention to this issue, which is a big part of the battle.

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Lesson 10: C++ File I/O

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This is a slightly more advanced topic than what I have covered so far, but I think that it is useful. File I/O is reading from and writing to files. This lesson will only cover text files, that is, files that are composed only of ASCII text.

C++ has two basic classes to handle files, ifstream and ofstream. To use them, include the header file fstream. Ifstream handles file input (reading from files), and ofstream handles file output (writing to files). The way to declare an instance of the ifstream or ofstream class is:

ifstream a_file;

ifstream a_file ( "filename" );

The constructor for both classes will actually open the file if you pass the name as an argument. As well, both classes have an open command (a_file.open()) and a close command (a_file.close()). You aren't required to use the close command as it will automatically be called when the program terminates, but if you need to close the file long before the program ends, it is useful.

The beauty of the C++ method of handling files rests in the simplicity of the actual functions used in basic input and output operations. Because C++ supports overloading operators, it is possible to use << and >> in front of the instance of the class as if it were cout or cin. In fact, file streams can be used exactly the same as cout and cin after they are opened.

For example:

#include <fstream>

#include <iostream>

using namespace std;

int main()

  char str[10];

  //Creates an instance of ofstream, and opens example.txt

  ofstream a_file ( "example.txt" );

  // Outputs to example.txt through a_file

  a_file<<"This text will now be inside of example.txt";

  // Close the file stream explicitly

  a_file.close();

  //Opens for reading the file

  ifstream b_file ( "example.txt" );

  //Reads one string from the file

  b_file>> str;

  //Should output 'this'

  cout<< str <<"\n";

  cin.get();    // wait for a keypress

  // b_file is closed implicitly here

The default mode for opening a file with ofstream's constructor is to create it if it does not exist, or delete everything in it if something does exist in it. If necessary, you can give a second argument that specifies how the file should be handled. They are listed below:

ios::app   -- Append to the file

ios::ate   -- Set the current position to the end

ios::trunc -- Delete everything in the file

For example:

ofstream a_file ( "test.txt", ios::app );

This will open the file without destroying the current contents and allow you to append new data. When opening files, be very careful not to use them if the file could not be opened. This can be tested for very easily:

ifstream a_file ( "example.txt" );

if ( !a_file.is_open() ) {

  // The file could not be opened

else {

  // Safely use the file stream

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Lesson 11: Typecasting in C and C++

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Typecasting is making a variable of one type, such as an int, act like another type, a char, for one single operation. To typecast something, simply put the type of variable you want the actual variable to act as inside parentheses in front of the actual variable. (char)a will make 'a' function as a char.

For example:

#include <iostream>

using namespace std;

int main()

  cout<< (char)65 <<"\n";

  // The (char) is a typecast, telling the computer to interpret the 65 as a

  //  character, not as a number.  It is going to give the character output of

  //  the equivalent of the number 65 (It should be the letter A for ASCII).

  cin.get();

One use for typecasting for is when you want to use the ASCII characters. For example, what if you want to create your own chart of all 256 ASCII characters. To do this, you will need to use to typecast to allow you to print out the integer as its character equivalent.

#include <iostream>

using namespace std;

int main()

  for ( int x = 0; x < 256; x++ ) {

    cout<< x <<". "<< (char)x <<" ";

    //Note the use of the int version of x to

    // output a number and the use of (char) to

    // typecast the x into a character

    // which outputs the ASCII character that

    // corresponds to the current number

  cin.get();

The typecast described above is a C-style cast, C++ supports two other types. First is the function-style cast:

int main()

  cout<< char ( 65 ) <<"\n";

  cin.get();

This is more like a function call than a cast as the type to be cast to is like the name of the function and the value to be cast is like the argument to the function. Next is the named cast, of which there are four:

int main()

  cout<< static_cast<char> ( 65 ) <<"\n";

  cin.get();

static_cast is similar in function to the other casts described above, but the name makes it easier to spot and less tempting to use since it tends to be ugly. Typecasting should be avoided whenever possible. The other three types of named casts are const_cast, reinterpret_cast, and dynamic_cast. They are of no use to us at this time.

Typecasts in practice

So when exactly would a typecast come in handy? One use of typecasts is to force the correct type of mathematical operation to take place. It turns out that in C and C++ (and other programming languages), the result of the division of integers is itself treated as an integer: for instance, 3/5 becomes 0! Why? Well, 3/5 is less than 1, and integer division ignores the remainder.

On the other hand, it turns out that division between floating point numbers, or even between one floating point number and an integer, is sufficient to keep the result as a floating point number. So if we were performing some kind of fancy division where we didn't want truncated values, we'd have to cast one of the variables to a floating point type. For instance, static_cast<float>(3)/5 comes out to .6, as you would expect!

When might this come up? It's often reasonable to store two values in integers. For instance, if you were tracking heart patients, you might have a function to compute their age in years and the number of heart times they'd come in for heart pain. One operation you might conceivably want to perform is to compute the number of times per year of life someone has come in to see their physician about heart pain. What would this look like?

/* magical function returns the age in years */

int age = getAge();

/* magical function returns the number of visits */

int pain_visits = getVisits();

float visits_per_year = pain_visits / age;

The problem is that when this program is run, visits_per_year will be zero unless the patient had an awful lot of visits to the doc. The way to get around this problem is to cast one of the values being divided so it gets treated as a floating point number, which will cause the compiler to treat the expression as if it were to result in a floating point number:

float visits_per_year = pain_visits / static_cast<float>(age);

/* or */

float visits_per_year = static_cast<float>(pain_visits) / age;

This would cause the correct values to be stored in visits_per_year. Can you think of another solution to this problem (in this case)?

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Lesson 12: Introduction to Classes in C++

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C++ is a bunch of small additions to C, with a few major additions. One major addition is the object-oriented approach (the other addition is support for generic programming, which we'll cover later). As the name object-oriented programming suggests, this approach deals with objects. Of course, these are not real-life objects themselves. Instead, these objects are the essential definitions of real world objects. Classes are collections of data related to a single object type. Classes not only include information regarding the real world object, but also functions to access the data, and classes possess the ability to inherit from other classes. (Inheritance is covered in a later lesson.)

If a class is a house, then the functions will be the doors and the variables will be the items inside the house. The functions usually will be the only way to modify the variables in this structure, and they are usually the only way even to access the variables in this structure. This might seem silly at first, but the idea to make programs more modular - the principle itself is called "encapsulation". The key idea is that the outside world doesn't need to know exactly what data is stored inside the class--it just needs to know which functions it can use to access that data. This allows the implementation to change more easily because nobody should have to rely on it except the class itself.

The syntax for these classes is simple. First, you put the keyword 'class' then the name of the class. Our example will use the name Computer. Then you put an open bracket. Before putting down the different variables, it is necessary to put the degree of restriction on the variable. There are three levels of restriction. The first is public, the second protected, and the third private. For now, all you need to know is that the public restriction allows any part of the program, including parts outside the class, to access the functions and variables specified as public. The protected restriction prevents functions outside the class to access the variable. The private restriction is similar to protected (we'll see the difference later when we look at inheritance. The syntax for declaring these access restrictions is merely the restriction keyword (public, private, protected) and then a colon. Finally, you put the different variables and functions (You usually will only put the function prototype[s]) you want to be part of the class. Then you put a closing bracket and semicolon. Keep in mind that you still must end the function prototype(s) with a semi-colon.

Let's look at these different access restrictions for a moment. Why would you want to declare something private instead of public? The idea is that some parts of the class are intended to be internal to the class--only for the purpose of implementing features. On the other hand, some parts of the class are supposed to be available to anyone using the class--these are the public class functions. Think of a class as though it were an appliance like a microwave: the public parts of the class correspond to the parts of the microwave that you can use on an everyday basis--the keypad, the start button, and so forth. On the other hand, some parts of the microwave are not easily accessible, but they are no less important--it would be hard to get at the microwave generator. These would correspond to the protected or private parts of the class--the things that are necessary for the class to function, but that nobody who uses the class should need to know about. The great thing about this separation is that it makes the class easier to use (who would want to use a microwave where you had to know exactly how it works in order to use it?) The key idea is to separate the interface you use from the way the interface is supported and implemented.

Classes must always contain two functions: a constructor and a destructor. The syntax for them is simple: the class name denotes a constructor, a ~ before the class name is a destructor. The basic idea is to have the constructor initialize variables, and to have the destructor clean up after the class, which includes freeing any memory allocated. If it turns out that you don't need to actually perform any initialization, then you can allow the compiler to create a "default constructor" for you. Similarly, if you don't need to do anything special in the destructor, the compiler can write it for you too!

When the programmer declares an instance of the class, the constructor will be automatically called. The only time the destructor is called is when the instance of the class is no longer needed--either when the program ends, the class reaches the end of scope, or when its memory is deallocated using delete (if you don't understand all of that, don't worry; the key idea is that destructors are always called when the class is no longer usable). Keep in mind that neither constructors nor destructors return arguments! This means you do not want to (and cannot) return a value in them.

Note that you generally want your constructor and destructor to be made public so that your class can be created! The constructor is called when an object is created, but if the constructor is private, it cannot be called so the object cannot be constructed. This will cause the compiler to complain.

The syntax for defining a function that is a member of a class outside of the actual class definition is to put the return type, then put the class name, two colons, and then the function name. This tells the compiler that the function is a member of that class.

For example:

#include <iostream>

using namespace std;

class Computer // Standard way of defining the class

public:

  // This means that all of the functions below this(and any variables)

  //  are accessible to the rest of the program.

  //  NOTE: That is a colon, NOT a semicolon...

  Computer();

  // Constructor

  ~Computer();

  // Destructor

  void setspeed ( int p );

  int readspeed();

protected:

  // This means that all the variables under this, until a new type of

  //  restriction is placed, will only be accessible to other functions in the

  //  class.  NOTE: That is a colon, NOT a semicolon...

  int processorspeed;

};

// Do Not forget the trailing semi-colon

Computer::Computer()

  //Constructors can accept arguments, but this one does not

  processorspeed = 0;

Computer::~Computer()

  //Destructors do not accept arguments

void Computer::setspeed ( int p )

  // To define a function outside put the name of the class

  //  after the return type and then two colons, and then the name

  //  of the function.

  processorspeed = p;

int Computer::readspeed()

  // The two colons simply tell the compiler that the function is part

  //  of the class

  return processorspeed;

int main()

  Computer compute;

  // To create an 'instance' of the class, simply treat it like you would

  //  a structure.  (An instance is simply when you create an actual object

  //  from the class, as opposed to having the definition of the class)

  compute.setspeed ( 100 );

  // To call functions in the class, you put the name of the instance,

  //  a period, and then the function name.

  cout<< compute.readspeed();

  // See above note.

This introduction is far from exhaustive and, for the sake of simplicity, recommends practices that are not always the best option. For more detail, I suggest asking questions on our forums and getting a book recommended by our book reviews.

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Lesson 13: Inline Functions in C++

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Although you've already learned about basic functions in c++, there is more: the inline function. Inline functions are not always important, but it is good to understand them. The basic idea is to save time at a cost in space. Inline functions are a lot like a placeholder. Once you define an inline function, using the 'inline' keyword, whenever you call that function the compiler will replace the function call with the actual code from the function.

How does this make the program go faster? Simple, function calls are simply more time consuming than writing all of the code without functions. To go through your program and replace a function you have used 100 times with the code from the function would be time consuming not too bright. Of course, by using the inline function to replace the function calls with code you will also greatly increase the size of your program.

Using the inline keyword is simple, just put it before the name of a function. Then, when you use that function, pretend it is a non-inline function.

Example Inline Function

#include <iostream>

using namespace std;

inline void hello()

  cout<<"hello";

int main()

  hello(); //Call it like a normal function...

  cin.get();

However, once the program is compiled, the call to hello(); will be replaced by the code making up the function.

A WORD OF WARNING: Inline functions are very good for saving time, but if you use them too often or with large functions you will have a tremendously large program. Sometimes large programs are actually less efficient, and therefore they will run more slowly than before. Inline functions are best for small functions that are called often.

Finally, note that the compiler may choose, in its infinite wisdom, to ignore your attempt to inline a function. So if you do make a mistake and inline a monster fifty-line function that gets called thousands of times, the compiler may ignore you.

In the future, we will discuss inline functions in terms of C++ classes. Now that you understand the concept I will feel more comfortable using inline functions in later tutorials.

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Lesson 14: Accepting command line arguments in C++ using argc and argv

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In C++ it is possible to accept command line arguments. Command-line arguments are given after the name of a program in command-line operating systems like DOS or Linux, and are passed in to the program from the operating system. To use command line arguments in your program, you must first understand the full declaration of the main function, which previously has accepted no arguments. In fact, main can actually accept two arguments: one argument is number of command line arguments, and the other argument is a full list of all of the command line arguments.

The full declaration of main looks like this:

int main ( int argc, char *argv[] )

The integer, argc is the ARGument Count (hence argc). It is the number of arguments passed into the program from the command line, including the name of the program.

The array of character pointers is the listing of all the arguments. argv[0] is the name of the program, or an empty string if the name is not available. After that, every element number less than argc is a command line argument. You can use each argv element just like a string, or use argv as a two dimensional array. argv[argc] is a null pointer.

How could this be used? Almost any program that wants its parameters to be set when it is executed would use this. One common use is to write a function that takes the name of a file and outputs the entire text of it onto the screen.

#include <fstream>

#include <iostream>

using namespace std;

int main ( int argc, char *argv[] )

  if ( argc != 2 ) // argc should be 2 for correct execution

    // We print argv[0] assuming it is the program name

    cout<<"usage: "<< argv[0] <<" <filename>\n";

  else {

    // We assume argv[1] is a filename to open

    ifstream the_file ( argv[1] );

    // Always check to see if file opening succeeded

    if ( !the_file.is_open() )

      cout<<"Could not open file\n";

    else {

      char x;

      // the_file.get ( x ) returns false if the end of the file

      //  is reached or an error occurs

      while ( the_file.get ( x ) )

        cout<< x;

    // the_file is closed implicitly here

This program is fairly simple. It incorporates the full version of main. Then it first checks to ensure the user added the second argument, theoretically a file name. The program then checks to see if the file is valid by trying to open it. This is a standard operation that is effective and easy. If the file is valid, it gets opened in the process. The code is self-explanatory, but is littered with comments, you should have no trouble understanding its operation this far into the tutorial. :-)

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Lesson 15: Singly linked lists in C++

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Linked lists are a way to store data with structures so that the programmer can automatically create a new place to store data whenever necessary. Specifically, the programmer writes a struct or class definition that contains variables holding information about something, and then has a pointer to a struct of its type. Each of these individual struct or classes in the list is commonly known as a node.

Think of it like a train. The programmer always stores the first node of the list. This would be the engine of the train. The pointer is the connector between cars of the train. Every time the train adds a car, it uses the connectors to add a new car. This is like a programmer using the keyword new to create a pointer to a new struct or class.

In memory it is often described as looking like this:

----------        ----------

- Data   -        - Data   -

----------        ----------

- Pointer- - - -> - Pointer-

----------        ----------

The representation isn't completely accurate, but it will suffice for our purposes. Each of the big blocks is a struct (or class) that has a pointer to another one. Remember that the pointer only stores the memory location of something, it is not that thing, so the arrow goes to the next one. At the end, there is nothing for the pointer to point to, so it does not point to anything, it should be a null pointer or a dummy node to prevent it from accidentally pointing to a totally arbitrary and random location in memory (which is very bad).

So far we know what the node struct should look like:

struct node {

  int x;

  node *next;

};

int main()

  node *root;      // This will be the unchanging first node

  root = new node; // Now root points to a node struct

  root->next = 0;  // The node root points to has its next pointer

                   //  set equal to a null pointer

  root->x = 5;     // By using the -> operator, you can modify the node

                   //  a pointer (root in this case) points to.

This so far is not very useful for doing anything. It is necessary to understand how to traverse (go through) the linked list before going further.

Think back to the train. Lets imagine a conductor who can only enter the train through the engine, and can walk through the train down the line as long as the connector connects to another car. This is how the program will traverse the linked list. The conductor will be a pointer to node, and it will first point to root, and then, if the root's pointer to the next node is pointing to something, the "conductor" (not a technical term) will be set to point to the next node. In this fashion, the list can be traversed. Now, as long as there is a pointer to something, the traversal will continue. Once it reaches a null pointer (or dummy node), meaning there are no more nodes (train cars) then it will be at the end of the list, and a new node can subsequently be added if so desired.

Here's what that looks like:

struct node {

  int x;

  node *next;

};

int main()

  node *root;       // This won't change, or we would lose the list in memory

  node *conductor;  // This will point to each node as it traverses the list

  root = new node;  // Sets it to actually point to something

  root->next = 0;   //  Otherwise it would not work well

  root->x = 12;

  conductor = root; // The conductor points to the first node

  if ( conductor != 0 ) {

    while ( conductor->next != 0)

      conductor = conductor->next;

  conductor->next = new node;  // Creates a node at the end of the list

  conductor = conductor->next; // Points to that node

  conductor->next = 0;         // Prevents it from going any further

  conductor->x = 42;

That is the basic code for traversing a list. The if statement ensures that there is something to begin with (a first node). In the example it will always be so, but if it was changed, it might not be true. If the if statement is true, then it is okay to try and access the node pointed to by conductor. The while loop will continue as long as there is another pointer in the next. The conductor simply moves along. It changes what it points to by getting the address of conductor->next.

Finally, the code at the end can be used to add a new node to the end. Once the while loop as finished, the conductor will point to the last node in the array. (Remember the conductor of the train will move on until there is nothing to move on to? It works the same way in the while loop.) Therefore, conductor->next is set to null, so it is okay to allocate a new area of memory for it to point to. Then the conductor traverses one more element (like a train conductor moving on to the newly added car) and makes sure that it has its pointer to next set to 0 so that the list has an end. The 0 functions like a period, it means there is no more beyond. Finally, the new node has its x value set. (It can be set through user input. I simply wrote in the '=42' as an example.)

To print a linked list, the traversal function is almost the same. It is necessary to ensure that the last element is printed after the while loop terminates.

For example:

conductor = root;

if ( conductor != 0 ) { //Makes sure there is a place to start

  while ( conductor->next != 0 ) {

    cout<< conductor->x;

    conductor = conductor->next;

  cout<< conductor->x;

The final output is necessary because the while loop will not run once it reaches the last node, but it will still be necessary to output the contents of the next node. Consequently, the last output deals with this. Because we have a pointer to the beginning of the list (root), we can avoid this redundancy by allowing the conductor to walk off of the back of the train. Bad for the conductor (if it were a real person), but the code is simpler as it also allows us to remove the initial check for null (if root is null, then conductor will be immediately set to null and the loop will never begin):

conductor = root;

while ( conductor != NULL ) {

  cout<< conductor->x;

  conductor = conductor->next;

}

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Lesson 16: Recursion in C and C++

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Recursion is a programming technique that allows the programmer to express operations in terms of themselves. In C++, this takes the form of a function that calls itself. A useful way to think of recursive functions is to imagine them as a process being performed where one of the instructions is to "repeat the process". This makes it sound very similar to a loop because it repeats the same code, and in some ways it is similar to looping. On the other hand, recursion makes it easier to express ideas in which the result of the recursive call is necessary to complete the task. Of course, it must be possible for the "process" to sometimes be completed without the recursive call. One simple example is the idea of building a wall that is ten feet high; if I want to build a ten foot high wall, then I will first build a 9 foot high wall, and then add an extra foot of bricks. Conceptually, this is like saying the "build wall" function takes a height and if that height is greater than one, first calls itself to build a lower wall, and then adds one a foot of bricks.

A simple example of recursion would be:

void recurse()

  recurse(); //Function calls itself

int main()

  recurse(); //Sets off the recursion

This program will not continue forever, however. The computer keeps function calls on a stack and once too many are called without ending, the program will crash. Why not write a program to see how many times the function is called before the program terminates?

#include <iostream>

using namespace std;

void recurse ( int count ) // Each call gets its own count

  cout<< count <<"\n";

  // It is not necessary to increment count since each function's

  //  variables are separate (so each count will be initialized one greater)

  recurse ( count + 1 );

int main()

  recurse ( 1 ); //First function call, so it starts at one

This simple program will show the number of times the recurse function has been called by initializing each individual function call's count variable one greater than it was previous by passing in count + 1. Keep in mind, it is not a function restarting itself, it is hundreds of functions that are each unfinished with the last one calling a new recurse function.

It can be thought of like the Russian dolls that always have a smaller doll inside. Each doll calls another doll, and you can think of the size being a counter variable that is being decremented by one.

Think of a really tiny doll, the size of a few atoms. You can't get any smaller than that, so there are no more dolls. Normally, a recursive function will have a variable that performs a similar action; one that controls when the function will finally exit. The condition where the function will not call itself is termed the base case of the function. Basically, it is an if-statement that checks some variable for a condition (such as a number being less than zero, or greater than some other number) and if that condition is true, it will not allow the function to call itself again. (Or, it could check if a certain condition is true and only then allow the function to call itself).

A quick example:

void doll ( int size )

  if ( size == 0 )   // No doll can be smaller than 1 atom (10^0==1) so doesn't call itself

    return;          // Return does not have to return something, it can be used

                     //  to exit a function

  doll ( size - 1 ); // Decrements the size variable so the next doll will be smaller.

int main()

  doll ( 10 ); //Starts off with a large doll (it's a logarithmic scale)

This program ends when size equals one. This is a good base case, but if it is not properly set up, it is possible to have an base case that is always true (or always false).

Once a function has called itself, it will be ready to go to the next line after the call. It can still perform operations. One function you could write could print out the numbers 123456789987654321. How can you use recursion to write a function to do this? Simply have it keep incrementing a variable passed in, and then output the variable...twice, once before the function recurses, and once after...

void printnum ( int begin )

  cout<< begin;

  if ( begin < 9 )         // The base case is when begin is greater than 9

  {                           //  for it will not recurse after the if-statement

      printnum ( begin + 1 );

  cout<< begin;         // Outputs the second begin, after the program has

                              //  gone through and output

This function works because it will go through and print the numbers begin to 9, and then as each printnum function terminates it will continue printing the value of begin in each function from 9 to begin.

This is just the beginning of the usefulness of recursion. Here's a little challenge, use recursion to write a program that returns the factorial of any number greater than 0. (Factorial is number * (number - 1) * (number - 2) ... * 1).

Hint: Recursively find the factorial of the smaller numbers first, i.e., it takes a number, finds the factorial of the previous number, and multiplies the number times that factorial...have fun. :-)
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Lesson 17: Functions with Variable Argument Lists in C and C++ using va_list

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Perhaps you would like to have a function that will accept any number of values and then return the average. You don't know how many arguments will be passed in to the function. One way you could make the function would be to accept a pointer to an array. Another way would be to write a function that can take any number of arguments. So you could write avg(4, 12.2, 23.3, 33.3, 12.1); or you could write avg(2, 2.3, 34.4); Some library functions can accept a variable list of arguments (such as the venerable printf).

To use a function with variable number of arguments, or more precisely, a function without a set number of arguments, you would use the cstdarg header file. There are four parts needed: va_list, which stores the list of arguments, va_start, which initializes the list, va_arg, which returns the next argument in the list, and va_end, which cleans up the variable argument list. Whenever a function is declared to have an indeterminate number of arguments, in place of the last argument you should place an ellipsis (which looks like '...'), so, int a_function ( int x, ... ); would tell the compiler the function should accept however many arguments that the programmer uses, as long as it is equal to at least one, the one being the first, x.

va_list is like any other variable. For example,

va_list a_list;

va_start is a macro which accepts two arguments, a va_list and the name of the variable that directly precedes the ellipsis (...). So, in the function a_function, to initialize a_list with va_start, you would write va_start ( a_list, x );

va_arg takes a va_list and a variable type, and returns the next argument in the list in the form of whatever variable type it is told. It then moves down the list to the next argument. For example, va_arg ( a_list, double ) will return the next argument, assuming it exists, in the form of a double. The next time it is called, it will return the argument following the last returned number, if one exists.

To show how each of the parts works, take an example function:

#include <cstdarg>

#include <iostream>

using namespace std;

// this function will take the number of values to average

// followed by all of the numbers to average

double average ( int num, ... )

  va_list arguments;                     // A place to store the list of arguments

  double sum = 0;

  va_start ( arguments, num );           // Initializing arguments to store all values after num

  for ( int x = 0; x < num; x++ )        // Loop until all numbers are added

    sum += va_arg ( arguments, double ); // Adds the next value in argument list to sum.

  va_end ( arguments );                  // Cleans up the list

  return sum / num;                      // Returns the average

int main()

    // this computes the average of 13.2, 22.3 and 4.5 (3 indicates the number of values to average)

  cout<< average ( 3, 12.2, 22.3, 4.5 ) <<endl;

    // here it computes the average of the 5 values 3.3, 2.2, 1.1, 5.5 and 3.3

  cout<< average ( 5, 3.3, 2.2, 1.1, 5.5, 3.3 ) <<endl;

It isn't necessarily a good idea to use a variable argument list at all times, because the potential exists for assuming a value is of one type, while it is in fact another, such as a null pointer being assumed to be an integer. Consequently, variable argument lists should be used sparingly.

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Binary Trees in C++: Part 1

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The binary tree is a fundamental data structure used in computer science. The binary tree is a useful data structure for rapidly storing sorted data and rapidly retrieving stored data. A binary tree is composed of parent nodes, or leaves, each of which stores data and also links to up to two other child nodes (leaves) which can be visualized spatially as below the first node with one placed to the left and with one placed to the right. It is the relationship between the leaves linked to and the linking leaf, also known as the parent node, which makes the binary tree such an efficient data structure. It is the leaf on the left which has a lesser key value (i.e., the value used to search for a leaf in the tree), and it is the leaf on the right which has an equal or greater key value. As a result, the leaves on the farthest left of the tree have the lowest values, whereas the leaves on the right of the tree have the greatest values. More importantly, as each leaf connects to two other leaves, it is the beginning of a new, smaller, binary tree. Due to this nature, it is possible to easily access and insert data in a binary tree using search and insert functions recursively called on successive leaves.

The typical graphical representation of a binary tree is essentially that of an upside down tree. It begins with a root node, which contains the original key value. The root node has two child nodes; each child node might have its own child nodes. Ideally, the tree would be structured so that it is a perfectly balanced tree, with each node having the same number of child nodes to its left and to its right. A perfectly balanced tree allows for the fastest average insertion of data or retrieval of data. The worst case scenario is a tree in which each node only has one child node, so it becomes as if it were a linked list in terms of speed. The typical representation of a binary tree looks like the following:

                                                   /    \

                                                  6      14

                                                 / \    /  \

                                                5   8  11  18

The node storing the 10, represented here merely as 10, is the root node, linking to the left and right child nodes, with the left node storing a lower value than the parent node, and the node on the right storing a greater value than the parent node. Notice that if one removed the root node and the right child nodes, that the node storing the value 6 would be the equivalent a new, smaller, binary tree.
The structure of a binary tree makes the insertion and search functions simple to implement using recursion. In fact, the two insertion and search functions are also both very similar. To insert data into a binary tree involves a function searching for an unused node in the proper position in the tree in which to insert the key value. The insert function is generally a recursive function that continues moving down the levels of a binary tree until there is an unused leaf in a position which follows the rules of placing nodes. The rules are that a lower value should be to the left of the node, and a greater or equal value should be to the right. Following the rules, an insert function should check each node to see if it is empty, if so, it would insert the data to be stored along with the key value (in most implementations, an empty node will simply be a NULL pointer from a parent node, so the function would also have to create the node). If the node is filled already, the insert function should check to see if the key value to be inserted is less than the key value of the current node, and if so, the insert function should be recursively called on the left child node, or if the key value to be inserted is greater than or equal to the key value of the current node the insert function should be recursively called on the right child node. The search function works along a similar fashion. It should check to see if the key value of the current node is the value to be searched. If not, it should check to see if the value to be searched for is less than the value of the node, in which case it should be recursively called on the left child node, or if it is greater than the value of the node, it should be recursively called on the right child node. Of course, it is also necessary to check to ensure that the left or right child node actually exists before calling the function on the node.
Because binary trees have log (base 2) n layers, the average search time for a binary tree is log (base 2) n. To fill an entire binary tree, sorted, takes roughly log (base 2) n * n. Let's take a look at the necessary code for a simple implementation of a binary tree. First, it is necessary to have a struct, or class, defined as a node.

struct node

  int key_value;

  node *left;

  node *right;

};

The struct has the ability to store the key_value and contains the two child nodes which define the node as part of a tree. In fact, the node itself is very similar to the node in a linked list. A basic knowledge of the code for a linked list will be very helpful in understanding the techniques of binary trees. Essentially, pointers are necessary to allow the arbitrary creation of new nodes in the tree.
It is most logical to create a binary tree class to encapsulate the workings of the tree into a single area, and also making it reusable. The class will contain functions to insert data into the tree and to search for data. Due to the use of pointers, it will be necessary to include a function to delete the tree in order to conserve memory after the program has finished.

class btree

    public:

        btree();

        ~btree();

        void insert(int key);

        node *search(int key);

        void destroy_tree();

    private:

        void destroy_tree(node *leaf);

        void insert(int key, node *leaf);

        node *search(int key, node *leaf);

        node *root;

};

The insert and search functions that are public members of the class are designed to allow the user of the class to use the class without dealing with the underlying design. The insert and search functions which will be called recursively are the ones which contain two parameters, allowing them to travel down the tree. The destroy_tree function without arguments is a front for the destroy_tree function which will recursively destroy the tree, node by node, from the bottom up.
The code for the class would look similar to the following:

btree::btree()

  root=NULL;

It is necessary to initialize root to NULL for the later functions to be able to recognize that it does not exist.

btree::~btree()

  destroy_tree();

The destroy_tree function will set off the recursive function destroy_tree shown below which will actually delete all nodes of the tree.

void btree::destroy_tree(node *leaf)

  if(leaf!=NULL)

    destroy_tree(leaf->left);

    destroy_tree(leaf->right);

    delete leaf;

The function destroy_tree goes to the bottom of each part of the tree, that is, searching while there is a non-null node, deletes that leaf, and then it works its way back up. The function deletes the leftmost node, then the right child node from the leftmost node's parent node, then it deletes the parent node, then works its way back to deleting the other child node of the parent of the node it just deleted, and it continues this deletion working its way up to the node of the tree upon which delete_tree was originally called. In the example tree above, the order of deletion of nodes would be 5 8 6 11 18 14 10. Note that it is necessary to delete all the child nodes to avoid wasting memory.

void btree::insert(int key, node *leaf)

  if(key< leaf->key_value)

    if(leaf->left!=NULL)

     insert(key, leaf->left);

    else

      leaf->left=new node;

      leaf->left->key_value=key;

      leaf->left->left=NULL;    //Sets the left child of the child node to null

      leaf->left->right=NULL;   //Sets the right child of the child node to null

  else if(key>=leaf->key_value)

    if(leaf->right!=NULL)

      insert(key, leaf->right);

    else

      leaf->right=new node;

      leaf->right->key_value=key;

      leaf->right->left=NULL;  //Sets the left child of the child node to null

      leaf->right->right=NULL; //Sets the right child of the child node to null

The case where the root node is still NULL will be taken care of by the insert function that is nonrecursive and available to non-members of the class. The insert function searches, moving down the tree of children nodes, following the prescribed rules, left for a lower value to be inserted and right for a greater value, until it finds an empty node which it creates using the 'new' keyword and initializes with the key value while setting the new node's child node pointers to NULL. After creating the new node, the insert function will no longer call itself.

node *btree::search(int key, node *leaf)

  if(leaf!=NULL)

    if(key==leaf->key_value)

      return leaf;

    if(key<leaf->key_value)

      return search(key, leaf->left);

    else

      return search(key, leaf->right);

  else return NULL;

The search function shown above recursively moves down the tree until it either reaches a node with a key value equal to the value for which the function is searching or until the function reaches an uninitialized node, meaning that the value being searched for is not stored in the binary tree. It returns a pointer to the node to the previous instance of the function which called it, handing the pointer back up to the search function accessible outside the class.

void btree::insert(int key)

  if(root!=NULL)

    insert(key, root);

  else

    root=new node;

    root->key_value=key;

    root->left=NULL;

    root->right=NULL;

The public version of the insert function takes care of the case where the root has not been initialized by allocating the memory for it and setting both child nodes to NULL and setting the key_value to the value to be inserted. If the root node already exists, insert is called with the root node as the initial node of the function, and the recursive insert function takes over.

node *btree::search(int key)

  return search(key, root);

The public version of the search function is used to set off the search recursion at the root node, keeping it from being necessary for the user to have access to the root node.

void btree::destroy_tree()

  destroy_tree(root);

The public version of the destroy tree function is merely used to initialize the recursive destroy_tree function which then deletes all the nodes of the tree.

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Lesson 19: Inheritance in C++

(Printable Version)

The ability to use the object-oriented programming is an important feature of C++. Lesson 12: classes in C++ introduced the idea of the class; if you have not read it and do not know the basic details of classes, you should read it before continuing this tutorial.

Inheritance is an important feature of classes; in fact, it is integral to the idea of object oriented programming. Inheritance allows you to create a hierarchy of classes, with various classes of more specific natures inheriting the general aspects of more generalized classes. In this way, it is possible to structure a program starting with abstract ideas that are then implemented by specific classes. For example, you might have a class Animal from which class dog and cat inherent the traits that are general to all animals; at the same time, each of those classes will have attributes specific to the animal dog or cat.

Inheritance offers many useful features to programmers. The ability, for example, of a variable of a more general class to function as any of the more specific classes which inherit from it, called polymorphism, is handy. For now, we will concentrate on the basic syntax of inheritance. Polymorphism will be covered in its own tutorial.

Any class can inherit from any other class, but it is not necessarily good practice to use inheritance (put it in the bank rather than go on a vacation). Inheritance should be used when you have a more general class of objects that describes a set of objects. The features of every element of that set (of every object that is also of the more general type) should be reflected in the more general class. This class is called the base class. base classes usually contain functions that all the classes inheriting from it, known as derived classes, will need. base classes should also have all the variables that every derived class would otherwise contain.

Let us look at an example of how to structure a program with several classes. Take a program used to simulate the interaction between types of organisms, trees, birds, bears, and other creatures coinhabiting a forest. There would likely be several base classes that would then have derived classes specific to individual animal types. In fact, if you know anything about biology, you might wish to structure your classes to take advantage of the biological classification from Kingdom to species, although it would probably be overly complex. Instead, you might have base classes for the animals and the plants. If you wanted to use more base classes (a class can be both a derived of one class and a base of another), you might have classes for flying animals and land animals, and perhaps trees and scrub. Then you would want classes for specific types of animals: pigeons and vultures, bears and lions, and specific types of plants: oak and pine, grass and flower. These are unlikely to live together in the same area, but the idea is essentially there: more specific classes ought to inherit from less specific classes.

Classes, of course, share data. A derived class has access to most of the functions and variables of the base class. There are, however, ways to keep a derived class from accessing some attributes of its base class. The keywords public, protected, and private are used to control access to information within a class. It is important to remember that public, protected, and private control information both for specific instances of classes and for classes as general data types. Variables and functions designated public are both inheritable by derived classes and accessible to outside functions and code when they are elements of a specific instance of a class. Protected variables are not accessible by functions and code outside the class, but derived classes inherit these functions and variables as part of their own class. Private variables are neither accessible outside the class when it is a specific class nor are available to derived classes. Private variables are useful when you have variables that make sense in the context of large idea.

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Lesson 20: C++ Inheritance - Syntax

(Printable Version)

Before beginning this lesson, you should have an understanding of the idea of inheritance. If you do not, please read lesson 19. This lesson will consist of an overview of the syntax of inheritance, the use of the keywords public, private, and protected, and then an example program following to demonstrate each.

The syntax to denote one class as inheriting from another is simple. It looks like the following: class Bear : public Animal, in place of simply the keyword class and then the class name. The ": public base_class_name" is the essential syntax of inheritance; the function of this syntax is that the class will contain all public and protected variables of the base class. Do not confuse the idea of a derived class having access to data members of a base class and specific instances of the derived class possessing data. The data members - variables and functions - possessed by the derived class are specific to the type of class, not to each individual object of that type. So, two different Bear objects, while having the same member variables and functions, may have different information stored in their variables; furthermore, if there is a class Animal with an object, say object BigAnimal, of that type, and not of a more specific type inherited from that class, those two bears will not have access to the data within BigAnimal. They will simply possess variables and functions with the same name and of the same type.

A quick example of inheritance:

class Animal

  public:

  Animal();

  ~Animal();

  void eat();

  void sleep();

  void drink();

private:

  int legs;

  int arms;

  int age;

};

//The class Animal contains information and functions

//related to all animals (at least, all animals this lesson uses)

class Cat : public Animal

  public:

  int fur_color;

  void purr();

  void fish();

  void markTerritory();

};

//each of the above operations is unique

//to your friendly furry friends

//(or enemies, as the case may be)

A discussion of the keywords public, private, and protected is useful when discussing inheritance. The three keywords are used to control access to functions and variables stored within a class.

public:

The most open level of data hiding is public. Anything that is public is available to all derived classes of a base class, and the public variables and data for each object of both the base and derived class is accessible by code outside the class. Functions marked public are generally those the class uses to give information to and take information from the outside world; they are typically the interface with the class. The rest of the class should be hidden from the user using private or protected data (This hidden nature and the highly focused nature of classes is known collectively as encapsulation). The syntax for public is:

public:

Everything following is public until the end of the class or another data hiding keyword is used.

In general, a well-designed class will have no public fields--everything should go through the class's functions. Functions that retrieve variables are known as 'getters' and those that change values are known as 'setters'. Since the public part of the class is intended for use by others, it is often sensible to put the public section at the top of the class.

protected:

Variables and functions marked protected are inherited by derived classes; however, these derived classes hide the data from code outside of any instance of the object. Keep in mind, even if you have another object of the same type as your first object, the second object cannot access a protected variable in the first object. Instead, the second object will have its own variable with the same name - but not necessarily the same data. Protected is a useful level of access control for important aspects to a class that must be passed on without allowing it to be accessed. The syntax is the same as that of public. specifically,

 protected:

private:

Private is the highest level of data-hiding. Not only are the functions and variables marked private not accessible by code outside the specific object in which that data appears, but private variables and functions are not inherited (in the sense that the derived class cannot directly access these variables or functions). The level of data protection afforded by protected is generally more flexible than that of the private level. On the other hand, if you do not wish derived classes to access a method, declaring it private is sensible.

 private: 



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Class Design in C++

Understanding Interfaces

When you're designing a class in C++, the first thing you should decide is the public interface for the class. The public interface determines how your class will be used by other programmers (or you), and once designed and implemented it should generally stay pretty constant. You may decide to add to the interface, but once you've started using the class, it will be hard to remove functions from the public interface (unless they aren't used and weren't necessary in the first place).

But that doesn't mean that you should include more functionality in your class than necessary just so that you can later decide what to remove from the interface. If you do this, you'll just make the class harder to use. People will ask questions like, "why are there four ways of doing this? Which one is better? How can I choose between them?" It's usually easier to keep things simple and provide one way of doing each thing unless there's a compelling reason why your class should offer multiple methods with the same basic functionality.

At the same time, just because adding methods to the public interface (probably) won't break anything that doesn't mean that you should start off with a tiny interface. First of all, if anybody decides to inherit from your class and you then choose a function with the same name, you're in for a boatload of confusion. First, if you don't declare the function virtual, then an object of the subclass will have the function chosen depending on the static type of the pointer. This can be messy. Moreover, if you do declare it virtual, then you have the issue that it might provide a different type of functionality than was intended by the original implementation of that function. Finally, you just can't add a pure virtual function to a class that's already in use because nobody who has inherited from it will have implemented that function.

The public interface, then, should remain as constant as possible. In fact, a good approach to designing classes is to write the interface before the implementation because it's what determines how your class interacts with the rest of the world (which is more important for the program as a whole than how the class is actually implemented). Moreover, if you write the interface first, you can get a feel for how the class will work with other classes before you actually dive into the implementation details.

Inheritance and Class Design

The second issue of your class design is what should be available to programmers who wish to create subclasses. This interface is primarily determined by virtual functions, but you can also include protected methods that are designed for use by the class or its subclasses (remember that protected methods are visible to subclasses while private methods are not).

A key consideration is whether it makes sense for a function to be virtual. A function should be virtual when the implementation is likely to differ from subclass to subclass. Vice-versa, whenever a function should not change, then it should be made non-virtual. The key idea is to think about whether to make a function virtual by asking if the function should always be the same for every class.

For example, if you have a class is designed to allow users to monitor network traffic and you want to allow subclasses that implement different ways of analyzing the traffic, you might use the following interface:

class TrafficWatch

        public:

        // Packet is some class that implements information about network

        // packets

        void addPacket (const Packet& network_packet);

        int getAveragePacketSize ();

        int getMaxPacket ();

        virtual bool isOverloaded ();

};

In this class, some methods will not change from implementation to implementation; adding a packet should always be handled the same way, and the average packet size isn't going to change either. On the other hand, someone might have a very different idea of what it means to have an overloaded network. This will change from situation to situation and we don't want to prevent someone from changing how this is computed--for some, anything over 10 Mbits/sec of traffic might be an overloaded network, and for others, it would require 100 Mbits/sec on some specific network cables.

Finally, when publicly inheriting from any class or designing for inheritance, remember that you should strive for it to be clear that inheritance models is-a. At heart, the is-a relationship means that the subclass should be able to appear anywhere the parent class could appear. From the standpoint of the user of the class, it should not matter whether a class is the parent class or a subclass.

To design an is-a relationship, make sure that it makes sense for the class to include certain functions to be sure that it doesn't include that subclasses might not actually need. One example of having an extra function is that of a Bird class that implements a fly function. The problem is that not all birds can fly--penguins and emus, for instance. This suggests that a more prudent design choice might be to have two subclasses of birds, one for birds that can fly and one for flightless birds. Of course, it might be overkill to have two subclasses of bird depending on how complex your class hierarchy will be. If you know that nobody would ever expect use your class for a flightless bird, then it's not so bad. Of course, you won't always know what someone will use your class for and it's much easier to think carefully before you start to implement an entire class hierarchy than it will be to go back and change it once people are using it.

Enumerated Types - enums

Sometimes as programmers we want to express the idea that a variable will be used for a specific purpose and should only be able to have a small number of values--for instance, a variable that stores the current direction of the wind might only need to store values corresponding to north, south, east, and west. One solution to this problem might be to use an int and some #define'd values:

#define NORTH_WIND        0

#define SOUTH_WIND        1

#define EAST_WIND         2

#define WEST_WIND         3

#define NO_WIND           4

int wind_direction = NO_WIND;

The problem with this approach is that it doesn't really prevent someone from assigning a nonsensical value to wind_direction; for instance, I could set wind_direction to 453 without any complaints from my compiler. And if I looked at the type of wind_direction, i would see that it's just a plain old integer. there's just no way to know that something is wrong.

The idea behind enumerated types is to create new data types that can take on only a restricted range of values. Moreover, these values are all expressed as constants rather than magic numbers--in fact, there should be no need to know the underlying values. The names of the constants should be sufficient for the purposes of comparing values.

When you declare an enumerated type, you specify the name of the new type, and the possible values it can take on:

enum wind_directions_t {NO_WIND, NORTH_WIND, SOUTH_WIND, EAST_WIND, WEST_WIND};

Note the _t at the end of the name of the type: this stands for "type" and is a way to visually distinguish the name of the type from the name of variables. Your text editor may also have the ability to use syntax highlighting to make the new type look like other built-in types, such as int, for you.

Now we can declare a wind_directions_t variable that can only take on five values:

wind_directions_t wind_direction = NO_WIND;

wind_direction = 453; // doesn't work, we get a compiler error!

Note to C Programmers: If you're planning on using enums in C, however, you don't get this type safety. The above assignment will compile without giving you an error. By the way, if you're using enums in C, you will also need to prefix the declaration with the keyword enum: enum wind_directions_t wind_direction = NO_WIND;

You might be wondering exactly what values the constants take on--what if you wanted to compare then using < or >? You actually have a choice: if you want to set the values yourself, you may, or you can choose to use default values, which start at zero for the first constant and increase by one. In our example, NO_WIND has the value 0, and WEST_WIND has the value 4 (just like our #define'd constants).

On the other hand, we could reverse this by giving explicit values:

enum wind_directions_t {NO_WIND = 4, NORTH_WIND = 3, SOUTH_WIND = 2, EAST_WIND = 1, WEST_WIND = 0};

Why would you ever want to give explicit values to elements of an enumerated type? Isn't the whole point of constants so that you don't need to know what the values are? The answer is that if the values of the constant are never used outside of comparisons between elements of the enumeration, then there's almost no reason to define the values to be anything in particular (one exception is if you want one value to have multiple names, you'd have to set at least one value explicitly). But if you need the values for communicating with the outside world, you might need to give specific values. For example, if you decided to use an enum to store all of the possible text colors you could pass into a function to set the text colors, you'd probably need to make sure that the enum names, such a RED or BLUE, matched up to the values corresponding to those colors.

Printing Enums

You might wonder what happens when you print out an enum: by default, you'll get the integer value of the enum. If you want to do something fancier than that, you'll have to handle it specially.

Naming Enums

One issue with enums is that the name of the enumerated type doesn't show up along with the enum. When you use the enum constant, it could really mean anything. The problem is that if you give your enums names that are too general, you can run into problems. First, it becomes hard to tell which enumeration a constant belongs to if you have several enumerated lists of values. A related problem is that sometimes you really want to use the same name. For instance, what if you had two color schemes, each of which included the color red, but for which the value of the RED constant needed to be different?

The solution to both of these problems is to include part of the name of the enum in the names of the constants. Notice that in the above example, I included "WIND" in the name of each enumerated constant. (Perhaps this wasn't entirely necessary--why not just have an enum for each ordinal direction? The answer is that it depends on whether someone else is already using the name. In this case, we avoid the problem by making the names specific enough that it's unlikely someone else will have a WEST_WIND constant.

Type Correctness

Because enums are "integer-like" types, they can safely be assigned into an integer without a cast. For instance, both of the following assignments are totally valid:

int my_wind = EAST_WIND;

wind_directions_t wind_direction = NO_WIND;

int my_wind = wind_direction;

As already mentioned, you can't make the reverse assignment in C++ without using a typecast. There might be times when you do need to do this, but you'd like to avoid it as best you can. For instance, if you need to convert a user's input into an enumerated type, it would be a bad idea to just typecast the variable to an int:

wind_directions_t wind_direction = NO_WIND;

std::cin >> static_cast( wind_direction );

This would let the user input any value at all and, almost as bad, force the user to know the range of values that the enum could take on. A much better solution would be to shield the user from the enumeration by asking for a string and then validating the input by comparing it to the possible input strings to choose which constant to assign the enum. For instance,

std::cout << "Please enter NORTH, SOUTH, EAST, WEST, or NONE for our wind direction";

std::cout << std::endl;

string input_wind_dir;

cin >>

wind_directions_t wind_dir;

if ( user_wind_dir == "NORTH" )

        wind_dir = NORTH_WIND;

else if ( user_wind_dir == "SOUTH" )

        wind_dir = SOUTH_WIND;

else if ( user_wind_dir == "EAST" )

        wind_dir = EAST_WIND;

else if ( user_wind_dir == "WEST" )

        wind_dir = WEST_WIND;

else if ( user_wind_dir == "NONE" )

        wind_dir = NO_WIND;

else

        std::cout << "That's not a valid direction!" << std::endl;

Polymorphic Enums?

In C++, we often use polymorphism to allow old code to handle new code--for instance, as long as we subclass the interface expected by a function, we can pass in the new class and expect it to work correctly with the code that was written before the new class ever existed. Unfortunately, with enums, you can't really do this, even though there are occasional times you'd like to. (For instance, if you were managing the settings for your program and you stored all of them as enum values, then it might be nice to have an enum, settings_t, from which all of your other enums inherited so that you could store every new enum in the settings list. Note that since the list contains values of different types, you can't use templates.)

If you need this kind of behavior, you're forced to store the enums as integers and then retrieve them using typecasts to assign the particular value to the setting of interest. And you won't even get the benefit of dynamic_cast to help you ensure that the cast is safe--you'll have to rely on the fact that incorrect values cannot be stored in the list.

Summary

The Good

Enums allow you to constrain the values a variable takes on
Enums can be used to make your program more readable by eliminating magic numbers are specifying exact what range of values a variable expects to take on
Enums can be used to quickly declare a range of constant values without using #define

The Gotchas

Enum constants must be carefully named to avoid name collisions
Enums don't work "polymorphically" except from the int type, which can be inconvenient

Formatting Cout Output in C++ using iomanip

Creating cleanly formatted output is a common programming requirement--it improves your user interface and makes it easier to read any debugging messages that you might print to the screen. In C, formatted output works via the printf statement, but in C++, you can create nicely formatted output to streams such as cout. This tutorial covers a set of basic I/O manipulations possible in C++ from the iomanip header file. Note that all of the functions in the iomanip header are inside the std namespace, so you will need to either prefix your calls with "std::" or put "using namespace std;" before using the functions.

Dealing with Spacing Issues using iomanip

A principle aspect of nicely formatted output is that the spacing looks right. There aren't columns of text that are too long or too short, and everything is appropriately aligned. This section deals with ways of spacing output correctly.

Setting the field width with setw

The std::setw function allows you to set the minimum width of the next output via the insertion operator. setw takes, one argument, the width of the next output (insertion), an integer. if the next output is too short, then spaces will be used for padding. There is no effect if the output is longer than the width--note that the output won't be truncated. The only strange thing about setw is that its return value must be inserted into the stream. The setw function has no effect if it is called without reference to a stream. A simple example is

using namespace std;

cout<<setw(10)<<"ten"<<"four"<<"four";

The output from the above would look like this:

ten       fourfour

Note that since setw takes an argument, at runtime it would be possible to specify the width of a column of output so that it is slightly wider than the longest element of the column.

You might wonder whether it is possible to change the padding character. It turns out that yes, you can, by using the setfill function, which takes a character to use for the padding. Note that setfill should also be used as a stream manipulator only, so it must be inserted into the stream:

cout<<setfill('-')<<setw(80)<<"-"<<endl;

The above code sets the padding character to a dash, the width of the next output to be at least 80 characters, and then outputs a dash. This results in the rest of the line being filled with dashes too. The output would look like this:

--------------------------------------------------------------------------------

Note that the pad character is changed until the next time you call setfill to change it again.

Aligning text with iomanip

It's possible to specify whether output is left or right aligned by using the manipulator flags that are part of ios_bas. In particular, it is possible to specify that output should be either left or right aligned by passing in the stream manipulators std::left and std::right.

Putting Your Knowledge of iomanip Together

Now that we know how to space and align text, we can correctly print formatted data in columns. For instance, if you had a struct containing the names of individuals:

using namespace std;

struct person

    string firstname;

    string lastname;

};

If you then had a vector of persons, then you could output them in a nice way with evenly spaced columns for the first and last name as follows:

// given the above code, we could write this

vector<person> people;

// fill the vector somehow

int field_one_width = 0, field_two_width = 0;

// get the max widths

for ( vector<person>::iterator iter = people.begin();

      iter != people.end();

      ++iter )

    if ( iter->firstname.length() > field_one_width )

        field_one_width = iter->firstname.length();

    if ( iter->lastname.length() > field_two_width )

        field_two_width = iter->lastname.length();

// print the elements of the vector

for ( vector<person>::iterator iter = people.begin();

      iter != people.end();

      ++iter )

    cout<<setw(field_one_width)<<left<<iter->firstname;

    cout<<" ";

    cout<<setw(field_two_width)<<left<<iter->lastname;

Note that the space output between the two fields wasn't strictly necessary because we could have added it by changing the first call to setw to set the width to one more than the longest first name (since it would use a space as the padding for the extra character).

Printing Numbers

Another challenge in creating nice output is correctly formatting numbers; for instance, when printing out a hexadecimal value, it would be nice if it were preceded by the "0x" prefix. More generally, it's nice to correctly set the number of trailing zeros after a decimal place.

Setting the precision of numerical output with setprecision

The setprecision function can be used to set the maximum number of digits that are displayed for a number. Like setw, it should be inserted into the stream. In fact, its usage is very similar to setw in all respects. For instance, to print the number 2.71828 to 3 decimal places:

std::cout << setprecision(3) << 2.71828;

Note that setprecision will change the precision until the next time it is passed into a given stream. So changing the above example to also print out 1.412 would result in the output of

2.71 1.41

Output in different bases

In computer science, frequently numbers need to be printed in octal or hexadecimal. The setbase function returns a value that can be passed into a stream to set the base of numbers to either base 8, 10, or 16. The input number is still read as a number in base ten, but it is printed in the given base. For instance,

std::cout << setbase(16) << 32;

will print out "20", which is 32 written in base 16. Note that you can use dec, oct, and hex as shorthand for setbase(10), setbase(8), and setbase(16) respectively when inserting into a stream. If you wish to include an indication of the base along with the printed number, you can use the setiosflags function, again passed into a stream, with an input of ios_base::showbase. Using the ios_base::showbase flag will append a "0x" in front of hexadecimal numbers and a 0 in front of octal numbers. Decimal numbers will be printed as normal.

std::cout << setbase(16) << 32;

This should get you started with the ability to create nicely formatted output in C++ without having to resort to returning to printf!

Read more similar articles

Learn to interpret and use sophisticated printf format strings

Templates and Template Classes in C++

What's better than having several classes that do the same thing to different datatypes? One class that lets you choose which datatype it acts on.
Templates are a way of making your classes more abstract by letting you define the behavior of the class without actually knowing what datatype will be handled by the operations of the class. In essence, this is what is known as generic programming; this term is a useful way to think about templates because it helps remind the programmer that a templated class does not depend on the datatype (or types) it deals with. To a large degree, a templated class is more focused on the algorithmic thought rather than the specific nuances of a single datatype. Templates can be used in conjunction with abstract datatypes in order to allow them to handle any type of data. For example, you could make a templated stack class that can handle a stack of any datatype, rather than having to create a stack class for every different datatype for which you want the stack to function. The ability to have a single class that can handle several different datatypes means the code is easier to maintain, and it makes classes more reusable.
The basic syntax for declaring a templated class is as follows:

template <class a_type> class a_class {...};

The keyword 'class' above simply means that the identifier a_type will stand for a datatype. NB: a_type is not a keyword; it is an identifier that during the execution of the program will represent a single datatype. For example, you could, when defining variables in the class, use the following line:

a_type a_var;

and when the programmer defines which datatype 'a_type' is to be when the program instantiates a particular instance of a_class, a_var will be of that type.
When defining a function as a member of a templated class, it is necessary to define it as a templated function:

template<class a_type> void a_class<a_type>::a_function(){...}

When declaring an instance of a templated class, the syntax is as follows:

a_class<int> an_example_class;

An instantiated object of a templated class is called a specialization; the term specialization is useful to remember because it reminds us that the original class is a generic class, whereas a specific instantiation of a class is specialized for a single datatype (although it is possible to template multiple types).
Usually when writing code it is easiest to precede from concrete to abstract; therefore, it is easier to write a class for a specific datatype and then proceed to a templated - generic - class. For that brevity is the soul of wit, this example will be brief and therefore of little practical application.
We will define the first class to act only on integers.

class calc

  public:

    int multiply(int x, int y);

    int add(int x, int y);

};

int calc::multiply(int x, int y)

  return x*y;

int calc::add(int x, int y)

  return x+y;

We now have a perfectly harmless little class that functions perfectly well for integers; but what if we decided we wanted a generic class that would work equally well for floating point numbers? We would use a template.

template <class A_Type> class calc

  public:

    A_Type multiply(A_Type x, A_Type y);

    A_Type add(A_Type x, A_Type y);

};

template <class A_Type> A_Type calc<A_Type>::multiply(A_Type x,A_Type y)

  return x*y;

template <class A_Type> A_Type calc<A_Type>::add(A_Type x, A_Type y)

  return x+y;

To understand the templated class, just think about replacing the identifier A_Type everywhere it appears, except as part of the template or class definition, with the keyword int. It would be the same as the above class; now when you instantiate an
object of class calc you can choose which datatype the class will handle.

calc <double> a_calc_class;

Templates are handy for making your programs more generic and allowing your code to be reused later.

Related articles

Templated Functions

Template Specialization and Partial Specialization in C++

Intro to C++ classes

Initialization lists: Initialization lists are important for allowing template types to work with both primitive and user-defined types

Standard Template Library Introduction

Understanding Initialization Lists in C++

Understanding the Start of an Object's Lifetime

In C++, whenever an object of a class is created, its constructor is called. But that's not all--its parent class constructor is called, as are the constructors for all objects that belong to the class. By default, the constructors invoked are the default ("no-argument") constructors. Moreover, all of these constructors are called before the class's own constructor is called.

For instance, take the following code:

#include <iostream>

class Foo

        public:

        Foo() { std::cout << "Foo's constructor" << std::endl; }

};

class Bar : public Foo

        public:

        Bar() { std::cout << "Bar's constructor" << std::endl; }

};

int main()

        // a lovely elephant ;)

        Bar bar;

The object bar is constructed in two stages: first, the Foo constructor is invoked and then the Bar constructor is invoked. The output of the above program will be to indicate that Foo's constructor is called first, followed by Bar's constructor.

Why do this? There are a few reasons. First, each class should need to initialize things that belong to it, not things that belong to other classes. So a child class should hand off the work of constructing the portion of it that belongs to the parent class. Second, the child class may depend on these fields when initializing its own fields; therefore, the constructor needs to be called before the child class's constructor runs. In addition, all of the objects that belong to the class should be initialized so that the constructor can use them if it needs to.

But what if you have a parent class that needs to take arguments to its constructor? This is where initialization lists come into play. An initialization list immediately follows the constructor's signature, separated by a colon:

class Foo : public parent_class

        Foo() : parent_class( "arg" ) // sample initialization list

                // you must include a body, even if it's merely empty

};

Note that to call a particular parent class constructor, you just need to use the name of the class (it's as though you're making a function call to the constructor).

For instance, in our above example, if Foo's constructor took an integer as an argument, we could do this:

#include <iostream>

class Foo

        public:

        Foo( int x )

                std::cout << "Foo's constructor "

                          << "called with "

                          << x

                          << std::endl;

};

class Bar : public Foo

        public:

        Bar() : Foo( 10 )  // construct the Foo part of Bar

                std::cout << "Bar's constructor" << std::endl;

};

int main()

        Bar stool;

Using Initialization Lists to Initialize Fields

In addition to letting you pick which constructor of the parent class gets called, the initialization list also lets you specify which constructor gets called for the objects that are fields of the class. For instance, if you have a string inside your class:

class Qux

        public:

                Qux() : _foo( "initialize foo to this!" ) { }

                // This is nearly equivalent to

                // Qux() { _foo = "initialize foo to this!"; }

                // but without the extra call to construct an empty string

        private:

        std::string _foo;

};

Here, the constructor is invoked by giving the name of the object to be constructed rather than the name of the class (as in the case of using initialization lists to call the parent class's constructor).

If you have multiple fields of a class, then the names of the objects being initialized should appear in the order they are declared in the class (and after any parent class constructor call):

class Baz

        public:

                Baz() : _foo( "initialize foo first" ), _bar( "then bar" ) { }

        private:

        std::string _foo;

        std::string _bar;

};

Initialization Lists and Scope Issues

If you have a field of your class that is the same name as the argument to your constructor, then the initialization list "does the right thing." For instance,

class Baz

        public:

                Baz( std::string foo ) : foo( foo ) { }

        private:

            std::string foo;

};

is roughly equivalent to

class Baz

        public:

                Baz( std::string foo )

                    this->foo = foo;

        private:

            std::string foo;

};

That is, the compiler knows which foo belongs to the object, and which foo belongs to the function.

Initialization Lists and Primitive Types

It turns out that initialization lists work to initialize both user-defined types (objects of classes) and primitive types (e.g., int). When the field is a primitive type, giving it an argument is equivalent to assignment. For instance,

class Quux

        public:

                Quux() : _my_int( 5 )  // sets _my_int to 5

{ }

        private:

                int _my_int;

};

This behavior allows you to specify templates where the templated type can be either a class or a primitive type (otherwise, you would have to have different ways of handling initializing fields of the templated type for the case of classes and objects).

template <class T>

class my_template

        public:

                // works as long as T has a copy constructor

                my_template( T bar ) : _bar( bar ) { }

        private:

                T _bar;

};

Initialization Lists and Const Fields

Using initialization lists to initialize fields is not always necessary (although it is probably more convenient than other approaches). But it is necessary for const fields. If you have a const field, then it can be initialized only once, so it must be initialized in the initialization list.

class const_field

        public:

                const_field() : _constant( 1 ) { }

                // this is an error: const_field() { _constant = 1; }

        private:

                const int _constant;

};

When Else do you Need Initialization Lists?

No Default Constructor

If you have a field that has no default constructor (or a parent class with no default constructor), you must specify which constructor you wish to use.

References

If you have a field that is a reference, you also must initialize it in the initialization list; since references are immutable they can be initialized only once.

Initialization Lists and Exceptions

Since constructors can throw exceptions, it's possible that you might want to be able to handle exceptions that are thrown by constructors invoked as part of the initialization list.

First, you should know that even if you catch the exception, it will get rethrown because it cannot be guaranteed that your object is in a valid state because one of its fields (or parts of its parent class) couldn't be initialized. That said, one reason you'd want to catch an exception here is that there's some kind of translation of error messages that needs to be done.

The syntax for catching an exception in an initialization list is somewhat awkward: the 'try' goes right before the colon, and the catch goes after the body of the function:

class Foo

        Foo() try : _str( "text of string" )

        catch ( ... )

                std::cerr << "Couldn't create _str";

                // now, the exception is rethrown as if we'd written

                // "throw;" here

};

Initialization Lists: Summary

Before the body of the constructor is run, all of the constructors for its parent class and then for its fields are invoked. By default, the no-argument constructors are invoked. Initialization lists allow you to choose which constructor is called and what arguments that constructor receives.

If you have a reference or a const field, or if one of the classes used does not have a default constructor, you must use an initialization list.

Related articles

Inheritance in C++
Constructors and Destructors in C++

C++ Class Design Tips

Templated Functions

C++ templates can be used both for classes and for functions in C++. Templated functions are actually a bit easier to use than templated classes, as the compiler can often deduce the desired type from the function's argument list.

The syntax for declaring a templated function is similar to that for a templated class:

template <class type> type func_name(type arg1, ...);

For instance, to declare a templated function to add two values together, you could use the following syntax:

template <class type> type add(type a, type b)

    return a + b;

Now, when you actually use the add function, you can simply treat it like any other function because the desired type is also the type given for the arguments. This means that upon compiling the code, the compiler will know what type is desired:

int x = add(1, 2);

will correctly deduce that "type" should be int. This would be the equivalent of saying:

int x = add<int>(1, 2);

where the template is explicitly instantiated by giving the type as a template parameter.

On the other hand, type inference of this sort isn't always possible because it's not always feasible to guess the desired types from the arguments to the function. For instance, if you wanted a function that performed some kind of cast on the arguments, you might have a template with multiple parameters:

template <class type1, class type2> type2 cast(type1 x)

    return (type2)x;

Using this function without specifying the correct type for type2 would be impossible. On the other hand, it is possible to take advantage of some type inference if the template parameters are correctly ordered. In particular, if the first argument must be specified and the second deduced, it is only necessary to specify the first, and the second parameter can be deduced.

For instance, given the following declaration

template <class rettype, class argtype> rettype cast(argtype x)

    return (rettype)x;

this function call specifies everything that is necessary to allow the compiler deduce the correct type:

cast<double>(10);

which will cast an int to a double. Note that arguments to be deduced must always follow arguments to be specified. (This is similar to the way that default arguments to functions work.)

You might wonder why you cannot use type inference for classes in C++. The problem is that it would be a much more complex process with classes, especially as constructors may have multiple versions that take different numbers of parameters, and not all of the necessary template parameters may be used in any given constructor.

Templated Classes with Templated Functions

It is also possible to have a templated class that has a member function that is itself a template, separate from the class template. For instance,

template <class type> class TClass

    // constructors, etc

    template <class type2> type2 myFunc(type2 arg);

};

The function myFunc is a templated function inside of a templated class, and when you actually define the function, you must respect this by using the template keyword twice:

template <class type>  // For the class

    template <class type2>  // For the function

    type2 TClass<type>::myFunc(type2 arg)

        // code

The following attempt to combine the two is wrong and will not work:

// bad code!

template <class type, class type2> type2 TClass<type>::myFunc(type2 arg)

    // ...

because it suggests that the template is entirely the class template and not a function template at all.

Template Specialization and Partial Template Specialization

Template Specialization

In many cases when working with templates, you'll write one generic version for all possible data types and leave it at that--every vector may be implemented in exactly the same way. The idea of template specialization is to override the default template implementation to handle a particular type in a different way.

For instance, while most vectors might be implemented as arrays of the given type, you might decide to save some memory and implement vectors of bools as a vector of integers with each bit corresponding to one entry in the vector. So you might have two separate vector classes. The first class would look like this.

template <typename T>

class vector

    // accessor functions and so forth

    private:

    T* vec_data;   // we'll store the data as block of dynamically allocated

                   // memory

    int length;    // number of elements used

    int vec_size;  // actual size of vec_data

};

But when it comes to bools, you might not really want to do this because most systems are going to use 16 or 32 bits for each boolean type even though all that's required is a single bit. So we might make our boolean vector look a little bit different by representing the data as an array of integers whose bits we manually manipulate. (For more on manipulating bits directly, see bitwise operators and bit manipulations in C and C++.)

To do this, we still need to specify that we're working with something akin to a template, but this time the list of template parameters will be empty:

template <>

and the class name is followed by the specialized type: class className<type>. In this case, the template would look like this:

template <>

class vector <bool>

    // interface

    private:

    unsigned int *vector_data;

    int length;

    int size;

};

Note that it would be perfectly reasonable if the specialized version of the vector class had a different interface (set of public methods) than the generic vector class--although they're both vector templates, they don't share any interface or any code.

It's worth pointing out that the salient reason for the specialization in this case was to allow for a more space-efficient implementation, but you could think of other reasons why this might come in handy--for instance, if you wanted to add extra methods to one templated class based on its type, but not to other templates. For instance, you might have a vector of doubles with a method that returns the non-integer component of each element although you might think prefer inheritance in this case. There isn't a particular reason to prevent the existence of a vector of doubles without those extra features. If, however, you felt strongly about the issue and wanted to prevent it, you could do so using template specialization.

Another time when you might want to specialize certain templates could be if you have a template type that relies on some behavior that was not implemented in a collection of classes you'd like to store in that template. For example, if you had a templated sortedVector type that required the > operator to be defined, and a set of classes written by someone else that didn't include any overloaded operators but did include a function for comparison, you might specialize your template to handle these classes separately.

Template Partial Specialization

Partial template specialization stems from similar motives as full specialization as described above. This time, however, instead of implementing a class for one specific type, you end up implementing a template that still allows some parameterization. That is, you write a template that specializes on one feature but still lets the class user choose other features as part of the template. Let's make this more concrete with an example.

Going back to the idea of extending the concept of vectors so that we can have a sortedVector, let's think about how this might look: we'll need a way of making comparisons. Fine; we can just use > if it's been implemented, or specialize if it hasn't. But now let's say that we wanted to have pointers to objects in our sorted vector. We could sort them by the value of the pointers, just doing a standard > comparison (we'll have a vector sorted from low to high):

template <typename T>

class sortedVector

    public:

    void insert (T val)

        if ( length == vec_size )   // length is the number of elements

            vec_size *= 2;    // we'll just ignore overflow possibility!

            vec_data = new T[vec_size];

        ++length;  // we are about to add an element

        // we'll start at the end, sliding elements back until we find the

        // place to insert the new element

        int pos;

        for( pos = length; pos > 0 && val > vec_data[pos - 1]; --pos )

            vec_data[pos] = vec_data[pos - 1];

        vec_data[pos] = val;

    // other functions...

    private:

    T *vec_data;

    int length;

    int size;

};

Now, notice that in the above for loop, we're making a direct comparison between elements of type T. That's OK for most things, but it would probably make more sense to have sorted on the actual object type instead of the pointer address. To do that, we'd need to write code that had this line:

for( pos = length; pos > 0 && *val > *vec_data[pos - 1]; --pos )

Of course, that would break for any non-pointer type. What we want to do here is use a partial specialization based on whether the type is a pointer or a non-pointer (you could get fancy and have multiple levels of pointers, but we'll stay simple).

To declare a partially specialized template that handles any pointer types, we'd add this class declaration:

template <typename T>

class sortedVector<T *>

    public:

    // same functions as before.  Now the insert function looks like this:

    insert( T *val )

        if ( length == vec_size )   // length is the number of elements

            vec_size *= 2;    // we'll just ignore overflow possibility!

            vec_data = new T[vec_size];

        ++length;  // we are about to add an element

        // we'll start at the end, sliding elements back until we find the

        // place to insert the new element

        int pos;

        for( pos = length; pos > 0 && *val > *vec_data[pos - 1]; --pos )

            vec_data[pos] = vec_data[pos - 1];

        vec_data[pos] = val;

    private:

    T** vec_data;

    int length;

    int size;

};

There are a couple of syntax points to notice here. First, our template parameter list still names T as the parameter, but the declaration now has a T * after the name of the class; this tells the compiler to match a pointer of any type with this template instead of the more general template. The second thing to note is that T is now the type pointed to; it is not itself a pointer. For instance, when you declare a sortedVector<int *>, T will refer to the int type! This makes some sense if you think of it as a form of pattern matching where T matches the type if that type is followed by an asterisk. This does mean that you have to be a tad bit more careful in your implementation: note that vec_data is a T** because we need a dynamically sized array made up of pointers.

You might wonder if you really want your sortedVector type to work like this--after all, if you're putting them in an array of pointers, you'd expect them to be sorted by pointer type. But there's a practical reason for doing this: when you allocate memory for an array of objects, the default constructor must be called to construct each object. If no default constructor exists (for instance, if every object needs some data to be created), you're stuck needing a list of pointers to objects, but you probably want them to be sorted the same way the actual objects themselves would be!

Note, by the way, that you can also partially specialize on template arguments--for instance, if you had a fixedVector type that allowed the user of the class to specify both a type to store and the length of the vector (possibly to avoid the cost of dynamic memory allocations), it might look something like this:

template <typename T, unsigned length>

class fixedVector { ... };

Then you could partially specialize for booleans with the following syntax

template <unsigned length>

class fixedVector<bool, length> {...}

Note that since T is no longer a template parameter, it's left out of the template parameter list, leaving only length. Also note that length now shows up as part of fixedVector's name (unlike when you have a generic template declaration, where you specify nothing after the name). (By the way, don't be surprised to see a template parameter that's a non-type: it's perfectly valid, and sometimes useful, to have template arguments that are integer types such as unsigned.)

A final implementation detail comes up with partial specializations: how does the compiler pick which specialization to use if there are a combination of completely generic types, some partial specializations, and maybe even some full specializations? The general rule of thumb is that the compiler will pick the most specific template specialization--the most specific template specialization is the one whose template arguments would be accepted by the other template declarations, but which would not accept all possible arguments that other templates with the same name would accept.

For instance, if you decided that you wanted a sortedVector<int *> that sorted by memory location, you could create a full specialization of sortedVector and if you declared a sortedVector<int *>, then the compiler would pick that implementation over the less-specific partial specialization for pointers. It's the most specialized since only an int * matches the full specialization, not any other pointer type such as a double *, whereas int * certainly could be a parameter to either of the other templates.

The C Preprocessor

The C preprocessor modifies a source code file before handing it over to the compiler. You're most likely used to using the preprocessor to include files directly into other files, or #define constants, but the preprocessor can also be used to create "inlined" code using macros expanded at compile time and to prevent code from being compiled twice.

There are essentially three uses of the preprocessor--directives, constants, and macros. Directives are commands that tell the preprocessor to skip part of a file, include another file, or define a constant or macro. Directives always begin with a sharp sign (#) and for readability should be placed flush to the left of the page. All other uses of the preprocessor involve processing #define'd constants or macros. Typically, constants and macros are written in ALL CAPS to indicate they are special (as we will see).

Header Files

The #include directive tells the preprocessor to grab the text of a file and place it directly into the current file. Typically, such statements are placed at the top of a program--hence the name "header file" for files thus included.

Constants

If we write

#define [identifier name] [value]

whenever [identifier name] shows up in the file, it will be replaced by [value].

If you are defining a constant in terms of a mathematical expression, it is wise to surround the entire value in parentheses:

#define PI_PLUS_ONE (3.14 + 1)

By doing so, you avoid the possibility that an order of operations issue will destroy the meaning of your constant:

x = PI_PLUS_ONE * 5;

Without parentheses, the above would be converted to

x = 3.14 + 1 * 5;

which would result in 1 * 5 being evaluated before the addition, not after. Oops!

It is also possible to write simply

#define [identifier name]

which defines [identifier name] without giving it a value. This can be useful in conjunction with another set of directives that allow conditional compilation.

Conditional Compilation

There are a whole set of options that can be used to determine whether the preprocessor will remove lines of code before handing the file to the compiler. They include #if, #elif, #else, #ifdef, and #ifndef. An #if or #if/#elif/#else block or a #ifdef or #ifndef block must be terminated with a closing #endif.

The #if directive takes a numerical argument that evaluates to true if it's non-zero. If its argument is false, then code until the closing #else, #elif, of #endif will be excluded.

Commenting out Code

Conditional compilation is a particularly useful way to comment out a block of code that contains multi-line comments (which cannot be nested).

#if 0

/* comment ...

*/

// code

/* comment */

#endif

Avoiding Including Files Multiple Times (idempotency)

Another common problem is that a header file is required in multiple other header files that are later included into a source code file, with the result often being that variables, structs, classes or functions appear to be defined multiple times (once for each time the header file is included). This can result in a lot of compile-time headaches. Fortunately, the preprocessor provides an easy technique for ensuring that any given file is included once and only once.

By using the #ifndef directive, you can include a block of text only if a particular expression is undefined; then, within the header file, you can define the expression. This ensures that the code in the #ifndef is included only the first time the file is loaded.

#ifndef _FILE_NAME_H_

#define _FILE_NAME_H_

/* code */

#endif // #ifndef _FILE_NAME_H_

Notice that it's not necessary to actually give a value to the expression _FILE_NAME_H_. It's sufficient to include the line "#define _FILE_NAME_H_" to make it "defined". (Note that there is an n in #ifndef--it stands for "if not defined").

A similar tactic can be used for defining specific constants, such as NULL:

#ifndef NULL

#define NULL (void *)0

#endif // #ifndef NULL

Notice that it's useful to comment which conditional statement a particular #endif terminates. This is particularly true because preprocessor directives are rarely indented, so it can be hard to follow the flow of execution.

Macros

The other major use of the preprocessor is to define macros. The advantage of a macro is that it can be type-neutral (this can also be a disadvantage, of course), and it's inlined directly into the code, so there isn't any function call overhead. (Note that in C++, it's possible to get around both of these issues with templated functions and the inline keyword.)

A macro definition is usually of the following form:

#define MACRO_NAME(arg1, arg2, ...) [code to expand to]

For instance, a simple increment macro might look like this:

#define INCREMENT(x) x++

They look a lot like function calls, but they're not so simple. There are actually a couple of tricky points when it comes to working with macros. First, remember that the exact text of the macro argument is "pasted in" to the macro. For instance, if you wrote something like this:

#define MULT(x, y) x * y

and then wrote

int z = MULT(3 + 2, 4 + 2);

what value do you expect z to end up with? The obvious answer, 30, is wrong! That's because what happens when the macro MULT expands is that it looks like this:

int z = 3 + 2 * 4 + 2;    // 2 * 4 will be evaluated first!

So z would end up with the value 13! This is almost certainly not what you want to happen. The way to avoid it is to force the arguments themselves to be evaluated before the rest of the macro body. You can do this by surrounding them by parentheses in the macro definition:

#define MULT(x, y) (x) * (y)

// now MULT(3 + 2, 4 + 2) will expand to (3 + 2) * (4 + 2)

But this isn't the only gotcha! It is also generally a good idea to surround the macro's code in parentheses if you expect it to return a value. Otherwise, you can get similar problems as when you define a constant. For instance, the following macro, which adds 5 to a given argument, has problems when embedded within a larger statement:

#define ADD_FIVE(a) (a) + 5

int x = ADD_FIVE(3) * 3;

// this expands to (3) + 5 * 3, so 5 * 3 is evaluated first

// Now x is 18, not 24!

To fix this, you generally want to surround the whole macro body with parentheses to prevent the surrounding context from affecting the macro body.

#define ADD_FIVE(a) ((a) + 5)

int x = ADD_FIVE(3) * 3;

On the other hand, if you have a multiline macro that you are using for its side effects, rather than to compute a value, you probably want to wrap it within curly braces so you don't have problems when using it following an if statement.

// We use a trick involving exclusive-or to swap two variables

#define SWAP(a, b)  a ^= b; b ^= a; a ^= b;

int x = 10;

int y = 5;

// works OK

SWAP(x, y);

// What happens now?

if(x < 0)

    SWAP(x, y);

When SWAP is expanded in the second example, only the first statement, a ^= b, is governed by the conditional; the other two statements will always execute. What we really meant was that all of the statements should be grouped together, which we can enforce using curly braces:

#define SWAP(a, b)  {a ^= b; b ^= a; a ^= b;}

Now, there is still a bit more to our story! What if you write code like so:

#define SWAP(a, b)  { a ^= b; b ^= a; a ^= b; }

int x = 10;

int y = 5;

int z = 4;

// What happens now?

if(x < 0)

    SWAP(x, y);

else

    SWAP(x, z);

Then it will not compile because semicolon after the closing curly brace will break the flow between if and else. The solution? Use a do-while loop:

#define SWAP(a, b)  do { a ^= b; b ^= a; a ^= b; } while ( 0 )

int x = 10;

int y = 5;

int z = 4;

// What happens now?

if(x < 0)

    SWAP(x, y);

else

    SWAP(x, z);

Now the semi-colon doesn't break anything because it is part of the expression. (By the way, note that we didn't surround the arguments in parentheses because we don't expect anyone to pass an expression into swap!)

More Gotchas

By now, you've probably realized why people don't really like using macros. They're dangerous, they're picky, and they're just not that safe. Perhaps the most irritating problem with macros is that you don't want to pass arguments with "side effects" to macros. By side effects, I mean any expression that does something besides evaluate to a value. For instance, ++x evaluates to x+1, but it also increments x. This increment operation is a side effect.

The problem with side effects is that macros don't evaluate their arguments; they just paste them into the macro text when performing the substitution. So something like

#define MAX(a, b) ((a) < (b) ? (b) : (a))

int x = 5, y = 10;

int z = MAX(x++, y++);

will end up looking like this:

int x = (x++ < y++ ? y++ : x++)

The problem here is that y++ ends up being evaluated twice! The nasty consequence is that after this expression, y will have a value of 12 rather than the expected 11. This can be a real pain to debug!

Multiline macros

Until now, we've seen only short, one line macros (possibly taking advantage of the semicolon to put multiple statements on one line.) It turns out that by using a the "\" to indicate a line continuation, we can write our macros across multiple lines to make them a bit more readable.

For instance, we could rewrite swap as

#define SWAP(a, b)  {                   \

                        a ^= b;         \

                        b ^= a;         \

                        a ^= b;         \

Notice that you do not need a slash at the end of the last line! The slash tells the preprocessor that the macro continues to the next line, not that the line is a continuation from a previous line.

Aside from readability, writing multi-line macros may make it more obvious that you need to use curly braces to surround the body because it's more clear that multiple effects are happening at once.

Advanced Macro Tricks

In addition to simple substitution, the preprocessor can also perform a bit of extra work on macro arguments, such as turning them into strings or pasting them together.

Pasting Tokens

Each argument passed to a macro is a token, and sometimes it might be expedient to paste arguments together to form a new token. This could come in handy if you have a complicated structure and you'd like to debug your program by printing out different fields. Instead of writing out the whole structure each time, you might use a macro to pass in the field of the structure to print.

To paste tokens in a macro, use ## between the two things to paste together.

For instance

#define BUILD_FIELD(field) my_struct.inner_struct.union_a.##field

Now, when used with a particular field name, it will expand to something like

my_struct.inner_struct.union_a.field1

The tokens are literally pasted together.

String-izing Tokens

Another potentially useful macro option is to turn a token into a string containing the literal text of the token. This might be useful for printing out the token. The syntax is simple--simply prefix the token with a pound sign (#).

#define PRINT_TOKEN(token) printf(#token " is %d", token)

For instance, PRINT_TOKEN(foo) would expand to

printf("<foo>" " is %d" <foo>)

(Note that in C, string literals next to each other are concatenated, so something like "token" " is " " this " will effectively become "token is this". This can be useful for formatting printf statements.)

For instance, you might use it to print the value of an expression as well as the expression itself (for debugging purposes).

PRINT_TOKEN(x + y);

Avoiding Macros in C++

In C++, you should generally avoid macros when possible. You won't be able to avoid them entirely if you need the ability to paste tokens together, but with templated classes and type inference for templated functions, you shouldn't need to use macros to create type-neutral code. Inline functions should also get rid of the need for macros for efficiency reasons. (Though you aren't guaranteed that the compiler will inline your code.)

Moreover, you should use const to declare typed constants rather than #define to create untyped (and therefore less safe) constants. Const should work in pretty much all contexts where you would want to use a #define, including declaring static sized arrays or as template parameters.

Getting Random Values in C and C++ with Rand

Written by RoD

At some point in any programmer's life, he or she must learn how to get a random value, or values, in their program. To some this seems involved, difficult, or even beyond their personal ability. This, however, is simply not the case.
Randomizing of values is, at its most basic form, one of the easier things a programmer can do with the C++ language. I have created this short tutorial for Cprogramming.com to aid you in learning, constructing, and using the functions available to you to randomize values.
I will first start with an introduction to the idea of randomizing values, followed by a simple example program that will output three random values. Once a secure understanding of these concepts is in place (hopefully it will be), I will include a short program that uses a range of values from which the random values can be taken.
Ok, now that you know why this tutorial was written, and what it includes, you are ready to learn how to randomize values! So without further ado, let's get started, shall we?
Many programs that you will write require the use of random numbers. For example, a game such as backgammon requires a roll of two dice on each move. Since there are 6 numbers on each die, you could calculate each roll by finding a random number from 1 to 6 for each die.
To make this task a little easier, C++ provides us with a library function, called rand that returns an integer between 0 and RAND_MAX. Let's take a break to explain what RAND_MAX is. RAND_MAX is a compiler-dependent constant, and it is inclusive. Inclusive means that the value of RAND_MAX is included in the range of values. The function, rand, and the constant, RAND_MAX, are included in the library header file stdlib.h.
The number returned by function rand is dependent on the initial value, called a seed that remains the same for each run of a program. This means that the sequence of random numbers that is generated by the program will be exactly the same on each run of the program.
How do you solve this problem you ask? Well I'll tell you! To help us combat this problem we will use another function, srand(seed), which is also declared in the stdlib.h header file. This function allows an application to specify the initial value used by rand at program startup.
Using this method of randomization, the program will use a different seed value on every run, causing a different set of random values every run, which is what we want in this case. The problem posed to us now, of course, is how to get an arbitrary seed value. Forcing the user or programmer to enter this value every time the program was run wouldn't be very efficient at all, so we need another way to do it.
So we turn to the perfect source for our always-changing value, the system clock. The C++ data type time_t and the function time, both declared in time.h, can be used to easily retrieve the time on the computers clock.
When converted to an unsigned integer, a positive whole number, the program time (at execution of program) can make a very nice seed value. This works nicely because no two program executions will occur at the same instant of the computers clock.
As promised, here is a very basic example program. The following code was written in Visual C++ 6.0, but should compile fine on most computers (given u have a compiler, which if your reading this I assume you do). The program outputs three random values.

/*Steven Billington

January 17, 2003

Ranexample.cpp

Program displays three random integers.

*/

/*

Header: iostream

Reason: Input/Output stream

Header: cstdlib

Reason: For functions rand and srand

Header: time.h

Reason: For function time, and for data type time_t

*/

#include <iostream>

#include <cstdlib>

#include <time.h>

using namespace std;

int main()

/*

Declare variable to hold seconds on clock.

*/

time_t seconds;

/*

Get value from system clock and

place in seconds variable.

*/

time(&seconds);

/*

Convert seconds to a unsigned

integer.

*/

srand((unsigned int) seconds);

/*

Output random values.

*/

cout<< rand() << endl;

cout<< rand() << endl;

cout<< rand() << endl;

return 0;

Users of a random number generator might wish to have a narrower or a wider range of numbers than provided by the rand function. Ideally, to solve this problem a user would specify the range with integer values representing the lower and the upper bounds. To understand how we might accomplish this with the rand function, consider how to generate a number between 0 and an arbitrary upper bound, referred to as high, inclusive.
For any two integers, say a and b, a % b is between 0 and b - 1, inclusive. With this in mind, the expression rand() % high + 1 would generate a number between 1 and high, inclusive, where high is less than or equal to RAND_MAX, a constant defined by the compiler. To place a lower bound in replacement of 1 on that result, we can have the program generate a random number between 0 and (high - low + 1) + low.
I realize how confused you might be right now, so take a look at the next sample program I promised, run it, toy with it, and alternate it to give you different values. It has been a pleasure to teach you another chapter in the world of C++, and you may feel free to email me at Silent_Death17@hotmail.com or to contact me on the message boards of this fine website, where I use the name RoD.

Enjoy, and happy programming!

/*

Steven Billington

January 17, 2003

exDice.cpp

Program rolls two dice with random

results.

*/

/*

Header: iostream

Reason: Input/Output stream

Header: stdlib

Reason: For functions rand and srand

Header: time.h

Reason: For function time, and for data type time_t

*/

#include <iostream>

#include <cstdlib>

#include <time.h>

/*

These constants define our upper

and our lower bounds. The random numbers

will always be between 1 and 6, inclusive.

*/

const int LOW = 1;

const int HIGH = 6;

using namespace std;

int main()

/*

Variables to hold random values

for the first and the second die on

each roll.

*/

int first_die, sec_die;

/*

Declare variable to hold seconds on clock.

*/

time_t seconds;

/*

Get value from system clock and

place in seconds variable.

*/

time(&seconds);

/*

Convert seconds to a unsigned

integer.

*/

srand((unsigned int) seconds);

/*

Get first and second random numbers.

*/

first_die = rand() % (HIGH - LOW + 1) + LOW;

sec_die = rand() % (HIGH - LOW + 1) + LOW;

/*

Output first roll results.

*/

cout<< "Your roll is (" << first_die << ", "

<< sec_die << "}" << endl << endl;

/*

Get two new random values.

*/

first_die = rand() % (HIGH - LOW + 1) + LOW;

sec_die = rand() % (HIGH - LOW + 1) + LOW;

/*

Output second roll results.

*/

cout<< "My roll is (" << first_die << ", "

<< sec_die << "}" << endl << endl;

return 0;

The C++ Modulus Operator

Take a simple arithmetic problem: what's left over when you divide 11 by 3? The answer is easy to compute: divide 11 by 3 and take the remainder: 2. But how would you compute this in a programming language like C or C++? It's not hard to come up with a formula, but the language provides a built-in mechanism, the modulus operator ('%'), that computes the remainder that results from performing integer division.

The modulus operator is useful in a variety of circumstances. It is commonly used to take a randomly generated number and reduce that number to a random number on a smaller range, and it can also quickly tell you if one number is a factor of another.

If you wanted to know if a number was odd or even, you could use modulus to quickly tell you by asking for the remainder of the number when divided by 2.

#include <iostream>

using namespace std;

int main()

    int num;

    cin >> num;

    // num % 2 computes the remainder when num is divided by 2

    if ( num % 2 == 0 )

        cout << num << " is even ";

    return 0;

The key line is the one that performs the modulus operation: "num % 2 == 0". A number is even if and only if it is divisible by two, and a number is divisible by another only if there is no remainder.

How could you use modulus to write a program that checks if a number is prime?

Pages

C++ program Toturial

C++ Made Easy: Learning to Program in C++

Lesson 3: Loops

Lesson 4: Functions

Lesson 5: switch case in C and C++

Lesson 6: Pointers in C++

What are pointers? Why should you care?

C++ Pointer Syntax

Pointing to Something: Retrieving an Address

Taking Stock of Pointers

Lesson 7: Structures in C++

Lesson 8: Arrays in C and C++

Lesson 9: C Strings

Safe Programming

Lesson 10: C++ File I/O

Lesson 11: Typecasting in C and C++

Typecasts in practice

Lesson 12: Introduction to Classes in C++

Lesson 13: Inline Functions in C++

Example Inline Function

Lesson 14: Accepting command line arguments in C++ using argc and argv

Lesson 15: Singly linked lists in C++

Lesson 16: Recursion in C and C++

Lesson 17: Functions with Variable Argument Lists in C and C++ using va_list

Binary Trees in C++: Part 1

Lesson 19: Inheritance in C++

Lesson 20: C++ Inheritance - Syntax

public:

protected:

private:

Class Design in C++

Understanding Interfaces

Inheritance and Class Design

Enumerated Types - enums

Printing Enums

Naming Enums

Type Correctness

Polymorphic Enums?

Summary

The Good

The Gotchas

Formatting Cout Output in C++ using iomanip

Dealing with Spacing Issues using iomanip

Setting the field width with setw

Aligning text with iomanip

Putting Your Knowledge of iomanip Together

Printing Numbers

Setting the precision of numerical output with setprecision

Output in different bases

Templates and Template Classes in C++

Understanding Initialization Lists in C++

Understanding the Start of an Object's Lifetime

Using Initialization Lists to Initialize Fields

Initialization Lists and Scope Issues

Initialization Lists and Primitive Types

Initialization Lists and Const Fields

When Else do you Need Initialization Lists?

No Default Constructor

References

Initialization Lists and Exceptions

Initialization Lists: Summary

Templated Functions

Templated Classes with Templated Functions

Template Specialization and Partial Template Specialization

Template Specialization

Template Partial Specialization

The C Preprocessor

Header Files

Constants

Conditional Compilation

Commenting out Code

Avoiding Including Files Multiple Times (idempotency)

Macros

More Gotchas

Multiline macros

Advanced Macro Tricks

Pasting Tokens

String-izing Tokens

Avoiding Macros in C++