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The
#undef
directive removes the current definition of
identifier
. Consequently, subsequent occurrences of
identifier
are ignored by the preprocessor. To remove a macro definition using
#undef
, give only the macro
identifier
, not a parameter list.
You can also apply the
#undef
directive to an identifier that has no previous definition. This ensures that the identifier is undefined. Macro replacement isn't performed within
#undef
statements.
The
#undef
directive is typically paired with a
#define
directive to create a region in a source program in which an identifier has a special meaning. For example, a specific function of the source program can use manifest constants to define environment-specific values that do not affect the rest of the program. The
#undef
directive also works with the
#if
directive to control conditional compilation of the source program. For more information, see
The #if, #elif, #else, and #endif directives
.
In the following example, the
#undef
directive removes definitions of a symbolic constant and a macro. Note that only the identifier of the macro is given.
#define WIDTH 80
#define ADD( X, Y ) ((X) + (Y))
#undef WIDTH
#undef ADD
Microsoft Specific
Macros can be undefined from the command line using the /U option, followed by the macro names to be undefined. The effect of issuing this command is equivalent to a sequence of #undef macro-name statements at the beginning of the file.
END Microsoft Specific
See also
Preprocessor directives
