C++ Primer Plus

Figure 2.8. Function definitions occur sequentially in a file.

Function Headers

The simon() function in Listing 2.5 has this header:

void simon(int n)

The initial void means that simon() has no return value. So calling simon() doesn’t produce a number that you can assign to a variable in main(). Thus, the first function call looks like this:

simon(3);            // ok for void functions

Because poor simon() lacks a return value, you can’t use it this way:

simple = simon(3);   // not allowed for void functions

The int n within the parentheses means that you are expected to use simon() with a single argument of type int. The n is a new variable assigned the value passed during a function call. Thus, the following function call assigns the value 3 to the n variable defined in the simon() header:

simon(3);

When the cout statement in the function body uses n, it uses the value passed in the function call. That’s why simon(3) displays a 3 in its output. The call to simon(count) in the sample run causes the function to display 512 because that was the value entered for count. In short, the header for simon() tells you that this function takes a single type int argument and that it doesn’t have a return value.

Let’s review main()’s function header:

int main()

The initial int means that main() returns an integer value. The empty parentheses (which optionally could contain void) means that main() has no arguments. Functions that have return values should use the keyword return to provide the return value and to terminate the function. That’s why you’ve been using the following statement at the end of main():

return 0;

This is logically consistent: main() is supposed to return a type int value, and you have it return the integer 0. But, you might wonder, to what are you returning a value? After all, nowhere in any of your programs have you seen anything calling main():

squeeze = main();   // absent from our programs

The answer is that you can think of your computer’s operating system (Unix, say, or Windows) as calling your program. So main()’s return value is returned not to another part of the program but to the operating system. Many operating systems can use the program’s return value. For example, Unix shell scripts and Window’s command-line interface batch files can be designed to run programs and test their return values, usually called exit values. The normal convention is that an exit value of zero means the program ran successfully, whereas a nonzero value means there was a problem. Thus, you can design a C++ program to return a nonzero value if, say, it fails to open a file. You can then design a shell script or batch file to run that program and to take some alternative action if the program signals failure.

Keywords

Keywords are the vocabulary of a computer language. This chapter has used four C++ keywords: int, void, return, and double. Because these keywords are special to C++, you can’t use them for other purposes. That is, you can’t use return as the name for a variable or double as the name of a function. But you can use them as part of a name, as in painter (with its hidden int) or return_aces. Appendix B, “C++ Reserved Words,” provides a complete list of C++ keywords. Incidentally, main is not a keyword because it’s not part of the language. Instead, it is the name of a required function. You can use main as a variable name. (That can cause a problem in circumstances too esoteric to describe here, and because it is confusing in any case, you’d best not.) Similarly, other function names and object names are not keywords. However, using the same name, say cout, for both an object and a variable in a program confuses the compiler. That is, you can use cout as a variable name in a function that doesn’t use the cout object for output, but you can’t use cout both ways in the same function.

Using a User-Defined Function That Has a Return Value

Let’s go one step further and write a function that uses the return statement. The main() function already illustrates the plan for a function with a return value: Give the return type in the function header and use return at the end of the function body. You can use this form to solve a weighty problem for those visiting the United Kingdom. In the United Kingdom, many bathroom scales are calibrated in stone instead of in U.S. pounds or international kilograms. The word stone is both singular and plural in this context. (The English language does lack the internal consistency of, say, C++.) One stone is 14 pounds, and the program in Listing 2.6 uses a function to make this conversion.

Listing 2.6. convert.cpp

// convert.cpp -- converts stone to pounds

#include

int stonetolb(int);     // function prototype

int main()

{

    using namespace std;

    int stone;

    cout << "Enter the weight in stone: ";

    cin >> stone;