How can I hide "defined but not used" warnings in GCC?

Question

I have a bunch of compile time asserts, such as:

CASSERT(isTrue) or CASSERT2(isTrue, prefix_)

When compiling with GCC I get many warnings like 'prefix_LineNumber' defined but not used. Is there a way I can hide warnings for compile time asserts? I had no luck searching the GCC documentation. I thought I might have the var automatically used globally inside the same macro but I couldn't think of any way to do it.

Does anyone know of a way to hide that warning in GCC?

Answer 1

Just saw this thread while searching for solutions to this problem. I post here for completeness the solution I found...

The GCC compiler flags that control unused warnings include:

-Wunused-function
-Wunused-label
-Wunused-parameter
-Wunused-value
-Wunused-variable
-Wunused (=all of the above)

Each of these has a corresponding negative form with "no-" inserted after the W which turns off the warning (in case it was turned on by -Wall, for example). Thus, in your case you should use

-Wno-unused-function

Of course this works for the whole code, not just compile-time asserts. For function-specific behaviour, have a look at Function attributes.

Answer 2

Solution for GCC not causing conflicts with other compilers

#ifdef __GNUC__
#define VARIABLE_IS_NOT_USED __attribute__ ((unused))
#else
#define VARIABLE_IS_NOT_USED
#endif

int VARIABLE_IS_NOT_USED your_variable;

Answer 3

This is one of the most anoying warnings, although I undestand that it may useful (sometimes) to check dead code. But I usually have static functions for debugging, or functions that maybe useful sometime in the future, or that are only used temporaly, and I want to keep them in the code.

Fortunately this warning does not care about inline functions.

inline static foo()
{
}

Answer 4

You can create a null statement and cast the result to void. This is portable across compilers, and gcc will not give you any warnings, even with -Wall and -Wextra enabled. For example:

int var;    // var is not used
(void)var;  // null statement, cast to void -- suppresses warning

A common technique is to create a macro for this:

#define UNUSED(x) ((void)(x))

int var;
UNUSED(var);

Answer 5

#define UNUSED_VAR     __attribute__ ((unused))

for any variable just use the above macro before its type for example:

UNUSED_VAR int a = 2;

Answer 6

Wrap this functions by the following directives All the code that will be placed between push and pop will not warn you about unused functions. All the rest of the code (outside push and pop) will not be affected.

#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Wunused-function"

... your code

#pragma GCC diagnostic pop

Answer 7

This is hard to answer without knowing the details of your static assert macros. Perhaps you could change to a different macro to avoid this problem? You could either add the 'unused' attribute to the macro as was suggested, or you could use a different form of CASSERT().

Here are descriptions of a few alternatives:

http://www.jaggersoft.com/pubs/CVu11_3.html

http://blog.kowalczyk.info/kb/compile-time-asserts-in-c.html

http://www.pixelbeat.org/programming/gcc/static_assert.html

Answer 8

How about -Wunused-label ?

Answer 9

As of C++ 17 you can use [[maybe_unused]]. For example,

[[maybe_unused]] int foo = 42;

which has the same effect as the old(er) __attribute__((unused)).