Practical C++ Programming, 2nd Edition

Chapter 10. The C++ Preprocessor

The speech of man is like embroidered tapestries, since like them this has to be extended in order to display its patterns, but when it is rolled up it conceals and distorts them.

—Themistocles

The first C compilers had no constants or inline functions. When C was still being developed, it soon became apparent that it needed a facility for handling named constants, macros, and include files. The solution was to create a preprocessor that is run on the programs before they are passed to the C compiler. The preprocessor is nothing more than a specialized text editor. Its syntax is completely different from C’s, and it has no understanding of C constructs. It is merely a dumb text editor.

The preprocessor was soon merged into the main C compiler. The C++ compiler kept this preprocessor. On some systems, such as Unix, it is still a separate program, automatically executed by the compiler wrapper cc. Some of the newer compilers, such as Borland-C++ Builder, have the preprocessor built in.

#define Statement

The #define statement can be used to define a constant. For example, the following two lines perform similar functions:

#define SIZE 20       // The array size is 20
const int SIZE = 20;  // The array size is 20

Actually the line #define SIZE 20 acts as a command to the preprocessor to globallychangeSIZEto20. This takes the drudgery and guesswork out of making changes.

All preprocessor commands begin with a hash mark (#) as the first character of the line. ...