Defining something to itself in C preprocessor

Question

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

Answer 1

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.

Answer 2

It allows the user code to conditionally compile based on whether those macros are or aren't defined:

#if defined(bool)
    /*...*/
#else
    /*...*/
#endif

It basically saves you from having to pollute the global namespace with yet another name (like HAVE_BOOL), provided that the implementation lets its users know that iff it provides a bool, it will also provide a macro with the same name that expands to it (or the implementation may simply use this internally for its own preprocessor conditionals).