Defining something to itself in C preprocessor

问题

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

回答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.

回答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).

回答3:

It is called self referential Macros.

According to gcv reference :

A self-referential macro is one whose name appears in its definition. Recall that all macro definitions are rescanned for more macros to replace. If the self-reference were considered a use of the macro, it would produce an infinitely large expansion. To prevent this, the self-reference is not considered a macro call. It is passed into the preprocessor output unchanged.

Reference example :

One common, useful use of self-reference is to create a macro which expands to itself. If you write

#define EPERM EPERM

then the macro EPERM expands to EPERM. Effectively, it is left alone by the preprocessor whenever it’s used in running text. You can tell that it’s a macro with ‘#ifdef’. You might do this if you want to define numeric constants with an enum, but have ‘#ifdef’ be true for each constant.

来源：https://stackoverflow.com/questions/46797804/defining-something-to-itself-in-c-preprocessor

标签

c-preprocessor

self-reference