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26.5.8 Undefining and Redefining Macros

You can undefine a macro 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.

You can define a macro again without first undefining it only if the new definition is 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)

but these are not:

#define FOUR (2 + 2)
#define FOUR ( 2+2 )
#define FOUR (2 * 2)
#define FOUR(score,and,seven,years,ago) (2 + 2)

This allows two different header files to define a common macro.

You can redefine an existing macro with #define, but redefining an existing macro name with a different definition results in a warning.

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