The important points about the undef directive are as follows:
1. If the identifier specified with the
undef directive is not currently defined as a macro
name, it is ignored.
2. The identifier specified with the
undef directive cannot be the name of a predefined
macro.
3. A macro can be redefined anywhere in the program. The most recent definition of the
macro is considered while expanding the macro. If the redefinition of the macro is not
identical (i.e. the redefined macro definition is not exactly the same as the first defini-
tion of the macro), the compiler will issue a warning message ‘
Redefinition of ‘macroname’ ...
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