How to deprecate a macro in GCC?

Question

Hi,

i Know how to use attribute deprecated to deprcate a function like this:

int old_fn () __attribute__ ((deprecated));

But how to deprecate a Macro like this:

#define OLD_MACRO 1

Thank you in advance.

Eric

Answer 1

Since Macros are not part of the compiler (they are a pre-processor function), there is no clean way of doing this. At best, place your deprecated macros in a new header file filled with #warnings. This will of course break existing code, but its one way to guarantee drawing attention to the deprecation.

Answer 2

I think the best you can do is something like this:

#ifdef USE_DEPRECATED_MACROS
#warning using deprecated macros
#define OLD_MACRO 1
...
#endif

This way you force the user to e.g. add -DUSE_DEPRECATED_MACROS to their compiler options and they get a warning.

Answer 3

You could make sure that these macros would expand to something that would include a expression that would trigger the attribut((deprecated)) warning.

For function-like macros this is pretty easy (especially with the comma operator), but for constant defines or nonstandard macros this may be more complicated, since the context in which these expand is different. I think you could do:

#define DEPRECATE(name) static inline void __attribute__((deprecated)) deprecate_ ## name (void) { ; }
...
#define MAX(x, y) (DEPRECATE(MAX), x>y?x:y)
// yeah, yeah, it repeats args in the body, but it's just an example

For a constant define you probably want to assume that the body has to evaluate without having to generate code, such as outside of a function body, in the targets of a switch/case, or as the initial value of a static variable within a function.

This is tricky, but you may be able to do it for many things.

I wish that C had a __builtin_warn(const char *) that would work at the compiler level (not-preprocessor) and make things like this easier.