C++ Primer Plus

Some functions require more than one item of information. These functions use multiple arguments separated by commas. For example, the math function pow() takes two arguments and returns a value equal to the first argument raised to the power given by the second argument. It has this prototype:

double pow(double, double);  // prototype of a function with two arguments

If, say, you wanted to find 5⁸ (5 to the eighth power), you would use the function like this:

answer = pow(5.0, 8.0);      // function call with a list of arguments

Other functions take no arguments. For example, one of the C libraries (the one associated with the cstdlib or the stdlib.h header file) has a rand() function that has no arguments and that returns a random integer. Its prototype looks like this:

int rand(void);        // prototype of a function that takes no arguments

The keyword void explicitly indicates that the function takes no arguments. If you omit void and leave the parentheses empty, C++ interprets this as an implicit declaration that there are no arguments. You could use the function this way:

myGuess = rand();       // function call with no arguments

Note that unlike some computer languages, in C++ you must use the parentheses in the function call even if there are no arguments.

There also are functions that have no return value. For example, suppose you wrote a function that displayed a number in dollars-and-cents format. You could send to it an argument of, say, 23.5, and it would display $23.50 onscreen. Because this function sends a value to the screen instead of to the calling program, it doesn’t require a return value. You indicate this in the prototype by using the keyword void for the return type:

void bucks(double);  // prototype for function with no return value

Because bucks() doesn’t return a value, you can’t use this function as part of an assignment statement or of some other expression. Instead, you have a pure function call statement:

bucks(1234.56);      // function call, no return value

Some languages reserve the term function for functions with return values and use the terms procedure or subroutine for those without return values, but C++, like C, uses the term function for both variations.

User-Defined Functions

The standard C library provides more than 140 predefined functions. If one fits your needs, by all means use it. But often you have to write your own, particularly when you design classes. Anyway, it’s fun to design your own functions, so now let’s examine that process. You’ve already used several user-defined functions, and they have all been named main(). Every C++ program must have a main() function, which the user must define. Suppose you want to add a second user-defined function. Just as with a library function, you can call a user-defined function by using its name. And, as with a library function, you must provide a function prototype before using the function, which you typically do by placing the prototype above the main() definition. But now you, not the library vendor, must provide source code for the new function. The simplest way is to place the code in the same file after the code for main(). Listing 2.5 illustrates these elements.

Listing 2.5. ourfunc.cpp

// ourfunc.cpp -- defining your own function

#include

void simon(int);    // function prototype for simon()

int main()

{

    using namespace std;

    simon(3);       // call the simon() function

    cout << "Pick an integer: ";

    int count;

    cin >> count;

    simon(count);   // call it again

    cout << "Done!" << endl;

    return 0;

}

void simon(int n)   // define the simon() function

{

    using namespace std;

    cout << "Simon says touch your toes " << n << " times." << endl;

}                   // void functions don't need return statements

The main() function calls the simon() function twice, once with an argument of 3 and once with a variable argument count. In between, the user enters an integer that’s used to set the value of count. The example doesn’t use a newline character in the cout prompting message. This results in the user input appearing on the same line as the prompt. Here is a sample run of the program in Listing 2.5:

Simon says touch your toes 3 times.

Pick an integer: 512

Simon says touch your toes 512 times.

Done!

Function Form

The definition for the simon() function in Listing 2.5 follows the same general form as the definition for main(). First, there is a function header. Then, enclosed in braces, comes the function body. You can generalize the form for a function definition as follows:

type functionname(argumentlist)

{

     statements

}

Note that the source code that defines simon() follows the closing brace of main(). Like C, and unlike Pascal, C++ does not allow you to embed one function definition inside another. Each function definition stands separately from all others; all functions are created equal (see Figure 2.8).