#undef directive (C/C++)
Removes (undefines) a name previously created with
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
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