The #undef directive removes the current definition of the specified name. All subsequent occurrences of the name are processed without replacement.
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
Build date: 11/16/2013