Portable UNUSED parameter macro used on function s

问题:

I'm interested in creating a macro for eliminating the unused variable warning.

This question describes a way to suppress the unused parameter warning by writing a macro inside the function code:

Universally compiler independent way of implementing an UNUSED macro in C/C++

But I'm interested in a macro that can be used in the function signature:

void callback(int UNUSED(some_useless_stuff)) {}

This is what I dug out using Google (source)

#ifdef UNUSED
#elif defined(__GNUC__)
# define UNUSED(x) UNUSED_ ## x __attribute__((unused))
#elif defined(__LCLINT__)
# define UNUSED(x) /*@unused@*/ x
#elif defined(__cplusplus)
# define UNUSED(x)
#else
# define UNUSED(x) x
#endif

Can this be further expanded for other compilers?

Edit: For those who can't understand how tagging works: I want a solution for both C and C++. That is why this question is tagged both C and C++ and that is why a C++ only solution is not acceptable.

回答1:

The way I do it is like this:

#define UNUSED(x) (void)(x)
void foo(const int i) {
    UNUSED(i);
}

I've not had a problem with that in Visual Studio, Intel, gcc and clang.

The other option is to just comment out the parameter:

void foo(const int /*i*/) {
  // When we need to use `i` we can just uncomment it.
}

回答2:

After testing and following the comments, the original version mentioned in the question turned out to be good enough.

Using: #define UNUSED(x) __pragma(warning(suppress:4100)) x (mentioned in comments), might be necessary for compiling C on MSVC, but that's such a weird combination, that I didn't include it in the end.

回答3:

Just one small thing, better using __attribute__((__unused__)) as __attribute__((unused)), because unused could be somewhere defined as macro, personally I had a few issues with this situation.

But the trick I'm using is, which I found more readable is:

#define UNUSED(x) (void)x;

It works however only for the variables, and arguments of the methods, but not for the function itself.

回答4:

Across many compilers I have used the following, excluding support for lint.

#if (__GNUC__ > 2) || (__GNUC__ == 2 && __GNUC_MINOR__ > 4)
#       define PGM_GNUC_UNUSED         __attribute__((__unused__))
#else
#       define PGM_GNUC_UNUSED
#endif

Tested compilers: GCC, Clang, EKOPath, Intel C Compiler / Composer XE, MinGW32 on Cygwin / Linux / MSYS, MinGW-w64 on Cygwin / Linux, Sun ONE Studio / Oracle Solaris Studio, Visual Studio 2008 / 2010.

Example usage:

pgm_tsc_init (
        PGM_GNUC_UNUSED pgm_error_t**   error
        )
{
...
}

PGM is the standard prefix for this C based project. GNUC is the convention from GLib for this attribute.

I think one compile warns about __attribute__ in certain circumstances but certainly no error.