E.4. #define Preprocessor Directive: Macros

[Note: This section is included for the benefit of C++ programmers who will need to work with C legacy code. In C++, macros can often be replaced by templates and inline functions.] A macro is an operation defined in a #define preprocessor directive. As with symbolic constants, the macro-identifier is replaced with the replacement-text before the program is compiled. Macros may be defined with or without arguments. A macro without arguments is processed like a symbolic constant. In a macro with arguments, the arguments are substituted in the replacement-text, then the macro is expanded—i.e., the replacement-text replaces the macro-identifier and argument list in the program. There is no data type checking for macro arguments. A macro is used simply for text substitution.

Consider the following macro definition with one argument for the area of a circle: