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