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cc-preprocessorself-reference

Defining something to itself in C preprocessor

I ran into these lines:

#define bool  bool
#define false false
#define true  true

I don't think I need to say more than "wtf?", but just to be clear: What is the point of defining something to itself?

The lines come from clang stdbool.h

Solution

  • The C and C++ standards explicitly allow that (and requires that there is no infinite expansion)

    BTW, function-like recursive (or self-refential) macros are even more useful:

    #define puts(X) (nblines++,puts(X))

    (the inner puts is a call to the standard puts function; the macro "overloads" such further calls by counting nblines)

    Your define could be useful, e.g. with later constructs like #ifdef true, and it can't be a simple #define true because that would "erase" every further use of true, so it has to be exactly#define true true.