C++ Primer Plus

The terminating semicolon in the prototype identifies it as a statement and thus makes it a prototype instead of a function header. If you omit the semicolon, the compiler interprets the line as a function header and expects you to follow it with a function body that defines the function.

When you use sqrt() in a program, you must also provide the prototype. You can do this in either of two ways:

• You can type the function prototype into your source code file yourself.

• You can include the cmath (math.h on older systems) header file, which has the prototype in it.

The second way is better because the header file is even more likely than you to get the prototype right. Every function in the C++ library has a prototype in one or more header files. Just check the function description in your manual or with online help, if you have it, and the description tells you which header file to use. For example, the description of the sqrt() function should tell you to use the cmath header file. (Again, you might have to use the older math.h header file, which works for both C and C++ programs.)

Don’t confuse the function prototype with the function definition. The prototype, as you’ve seen, only describes the function interface. That is, it describes the information sent to the function and the information sent back. The definition, however, includes the code for the function’s workings—for example, the code for calculating the square root of a number. C and C++ divide these two features—prototype and definition—for library functions. The library files contain the compiled code for the functions, whereas the header files contain the prototypes.

You should place a function prototype ahead of where you first use the function. The usual practice is to place prototypes just before the definition of the main() function. Listing 2.4 demonstrates the use of the library function sqrt(); it provides a prototype by including the cmath file.

Listing 2.4. sqrt.cpp

// sqrt.cpp -- using the sqrt() function

#include

#include     // or math.h

int main()

{

    using namespace std;

    double area;

    cout << "Enter the floor area, in square feet, of your home: ";

    cin >> area;

    double side;

    side = sqrt(area);

    cout << "That's the equivalent of a square " << side

         << " feet to the side." << endl;

    cout << "How fascinating!" << endl;

    return 0;

}

Using Library Functions

C++ library functions are stored in library files. When the compiler compiles a program, it must search the library files for the functions you’ve used. Compilers differ on which library files they search automatically. If you try to run Listing 2.4 and get a message that _sqrt is an undefined external (sounds like a condition to avoid!), chances are that your compiler doesn’t automatically search the math library. (Compilers like to add an underscore prefix to function names—another subtle reminder that they have the last say about your program.) If you get such a message, check your compiler documentation to see how to have the compiler search the correct library. If you get such a complaint on a Unix implementation, for example, it may require that you use the -lm option (for library math) at the end of the command line:

CC sqrt.C -lm

Some versions of the Gnu compiler under Linux behave similarly:

g++ sqrt.C –lm

Merely including the cmath header file provides the prototype but does not necessarily cause the compiler to search the correct library file.

Here’s a sample run of the program in Listing 2.4:

Enter the floor area, in square feet, of your home: 1536

That's the equivalent of a square 39.1918 feet to the side.

How fascinating!

Because sqrt() works with type double values, the example makes the variables that type. Note that you declare a type double variable by using the same form, or syntax, as when you declare a type int variable:

type-name variable-name;

Type double allows the variables area and side to hold values with decimal fractions, such as 1536.0 and 39.1918. An apparent integer, such as 1536, is stored as a real value with a decimal fraction part of .0 when stored in a type double variable. As you’ll see in Chapter 3, type double encompasses a much greater range of values than type int.

C++ allows you to declare new variables anywhere in a program, so sqrt.cpp didn’t declare side until just before using it. C++ also allows you to assign a value to a variable when you create it, so you could also have done this:

double side = sqrt(area);

You’ll learn more about this process, called initialization, in Chapter 3.

Note that cin knows how to convert information from the input stream to type double, and cout knows how to insert type double into the output stream. As noted earlier, these objects are smart.

Function Variations