The C++ Preprocessor and the iostream File

Here’s the short version of what you need to know. If your program is to use the usual C++ input or output facilities, you provide these two lines:

#include <iostream>
using namespace std;

There are some alternatives to using the second line, but let’s keep things simple for now. (If your compiler doesn’t like these lines, it’s not C++98 compatible, and it will have many other problems with the examples in this book.) That’s all you really must know to make your programs work, but now let’s take a more in-depth look.

C++, like C, uses a preprocessor. This is a program that processes a source file before the main compilation takes place. (Some C++ implementations, as you might recall from Chapter 1, use a translator program to convert a C++ program to C. Although the translator is also a form of preprocessor, we’re not discussing that preprocessor; instead, we’re discussing the one that handles directives whose names begin with #.) You don’t have to do anything special to invoke this preprocessor. It automatically operates when you compile the program.

Listing 2.1 uses the #include directive:

#include <iostream>    // a PREPROCESSOR directive

This directive causes the preprocessor to add the contents of the iostream file to your program. This is a typical preprocessor action: adding or replacing text in the source code before it’s compiled.

This raises the question of why you should add the contents of the iostream file to the program. The answer concerns communication between the program and the outside world. The io in iostream refers to input, which is information brought into the program, and to output, which is information sent out from the program. C++’s input/output scheme involves several definitions found in the iostream file. Your first program needs these definitions to use the cout facility to display a message. The #include directive causes the contents of the iostream file to be sent along with the contents of your file to the compiler. In essence, the contents of the iostream file replace the #include <iostream> line in the program. Your original file is not altered, but a composite file formed from your file and iostream goes on to the next stage of compilation.