Function macro that evaluates to zero and can be used as a statement

c, c-preprocessor

Solution

I guess it's a little bit compiler dependent but this should work:

#ifndef NDEBUG
    #define FOO(arg) foo(arg)
#else
    #define FOO(arg) ((int)0)
#endif

It prevents the "expression has no effect" warning, it does nothing and its value when used is still 0.

EDITED It seems it's something not so portable so (now) you have these conditions:

- `(0)` or `((int)0)` work at least on VC 2010.

- `__noop` should work on any version of VC after 2003.

VC6 is not a problem because it doesn't emit the C4555 warning at all. For other compilers you may use:

- `((void)0, 0)` It may work on a lot of compilers (maybe it's the more portable one?).

- `inline int foo(const char* bar) { return 0; }` works with any other C99 compiler (as you wrote you may need to declare it as `static` on gcc).

For any other prehistoric C compiler use the solution pointed by @Jobs: `abs(0)`

Problem

We have a function macro `#define FOO(arg) foo(arg)` with `int foo(const char* bar);`. When NDEBUG is defined FOO is defined as `#define FOO(arg) 0`, however this causes many compiler warnings because in many cases FOO's return value is not used. The solution should work with with ANSI C compilers and cause no warnings. I've tried: `(void)0`: can't be assigend to variable `static int foo(const char* bar) { return 0; }` : causes unused static function warning in some modules `static inline int foo(const char* bar) { return 0; }` : only works with C99 compilers Thanks for your help! edit1: It's somewhat like a trace macro and used all over the project. Mostly it's just used as a statement like `FOO("function x called");`, but in a few cases I saw `if (FOO("condition a")) { /* some more debug output */ }`. With NDEBUG defined and optimization enabled nothing should be left of FOO. I didn't come up with this, but I have to clean up this mess :). edit2: I should add that for gcc release builds these flags are used: -O3 -Wall -ansi edit3: For now I'm going with `__inline int dummy() { return 0; }`. __inline works with both VisualC and GCC in ansi mode.

Original source