__attribute__((unused)) indeed would be the best option when using gcc. I believe when using C++11, this can be replaced by [[gnu::unused]].
Another one that I've seen quite often is casting to void.
Another one that I've seen quite often is casting to void.
E.g. in older GCC you would get int x; switch (!!foo) { case 0:x=0;break; case 1:x=1;break; /no default/ } print(..x)
Or many other cases and the compiler will give an erroneous uninitialized warning. you can assign x to itself at the stop to eliminate the warning without changing the generated code.