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Language Reference - Preprocessor - Undefining and Redefining Macros

Undefining and Redefining Macros

If a macro ceases to be useful, it may be undefined with the ‘#undef’ directive. ‘#undef’ takes a single argument, the name of the macro to undefine. You use the bare macro name, even if the macro is function-like. It is an error if anything appears on the line after the macro name. ‘#undef’ has no effect if the name is not a macro.

#define FOO 4
$x = FOO; // x = 4
#undef FOO
$x = FOO; // x = FOO

Once a macro has been undefined, that identifier may be redefined as a macro by a subsequent ‘#define’ directive. The new definition need not have any resemblance to the old definition.

However, if an identifier which is currently a macro is redefined, then the new definition must be effectively the same as the old one. Two macro definitions are effectively the same if:

These definitions are effectively the same:

#define FOUR (2 + 2)
#define FOUR         (2      +      2)
#define FOUR (2 /* two */ + 2)

 


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