C++ By Example: UnderC Learning Edition

Macros

The preprocessor allows you to define macros, which are used rather like functions, except that they directly insert text into the source that the compiler sees.

The #define Directive

The #define directive changes the meaning of a word, or token, so that it is replaced by substitute text. This token is called a C macro. In this example, I have created three macros, PI, IF, and THEN:

#define PI 3.1412
#define IF if(
#define THEN )
#define TWOPI 2*PI

After these definitions, you can type IF x > PI THEN, and the preprocessor replaces the macros with their substitute text; this is called macro expansion. The compiler then actually sees if (PI > 3.1312). If the expanded text itself contains a macro, that will be further expanded. The preprocessor ...