#undef
The #undef directive removes the current definition of the specified name. All subsequent occurrences of the name are processed without replacement.
#undef name
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name
-
Name to be removed. This value is any combination of letters, digits, and punctuation that is valid for the C/C++ preprocessor.
Example
This example removes the definitions for the names nonzero and USERCLASS:
#undef nonzero
#undef USERCLASS
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