c++warningsassertions

Avoiding unused variables warnings when using assert() in a Release build

Sometimes a local variable is used for the sole purpose of checking it in an assert(), like so -

int Result = Func();
assert( Result == 1 );

When compiling code in a Release build, assert()s are usually disabled, so this code may produce a warning about Result being set but never read.

A possible workaround is -

int Result = Func();
if ( Result == 1 )
{
    assert( 0 );
}

But it requires too much typing, isn't easy on the eyes and causes the condition to be always checked (yes, the compiler may optimize the check away, but still).

I'm looking for an alternative way to express this assert() in a way that wouldn't cause the warning, but still be simple to use and avoid changing the semantics of assert().

(disabling the warning using a #pragma in this region of code isn't an option, and lowering warning levels to make it go away isn't an option either...).

Solution

  • We use a macro to specifically indicate when something is unused:

    #define _unused(x) ((void)(x))

    Then in your example, you'd have:

    int Result = Func();
assert( Result == 1 );
_unused( Result ); // make production build happy

    That way (a) the production build succeeds, and (b) it is obvious in the code that the variable is unused by design, not that it's just been forgotten about. This is especially helpful when parameters to a function are not used.