Directive #undef Foundation
Undefines a #define directive
The directive #undef revokes a definition created via the #define directive. It can be used in the context of conditional compilation, by which the preprocessor translates different parts of a source code depending on the presence or absence of symbolic constants. This directive is also useful when it is helpful to redefine an existing constant. Without #undef, redefining an existing constant raises a compiler warning. The #undef directive makes it possible to first undefine, then redefine a particular constant. Note that this directive is recognized for #define only, not other directives.
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