Cancelling macro substitution (#undef)

Substitutions registered with #define can be undone if they are no longer needed after a particular piece of code. For these purposes, the #undef directive is used.

In particular, it is useful if you need to define the same macro in different ways in different parts of the code. If the identifier specified in #define has already been registered somewhere in earlier lines of code (by another #define directive), then the old definition is replaced with the new one, and the preprocessor generates the "macro redefinition" warning. The use of #undef avoids the warning while explicitly indicating the programmer's intention not to use a particular macro further down the code.

#undef cannot undefine predefined macros.