As its name implies, the #undef directive removes (undefines) a name previously created with #define.
#undefidentifier
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; do not give 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 is not 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 (see “The #undef Directive”) to control conditional compilation of the source program.
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)
.
.
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#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.¨