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

Question

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

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

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Answer 1

The simplest thing is to only declare/assign those variables if the asserts will exist. The NDEBUG macro is specifically defined if asserts won't be effected (done that way round just because -DNDEBUG is a convenient way to disable debugging, I think), so this tweaked copy of @Jardel's answer should work (cf. comment by @AdamPeterson on that answer):

#ifndef NDEBUG
int Result =
#endif
Func();
assert(Result == 1);

or, if that doesn't suit your tastes, all sorts of variants are possible, e.g. this:

#ifndef NDEBUG
int Result = Func();
assert(Result == 1);
#else
Func();
#endif

In general with this stuff, be careful that there's never a possibility for different translation units to be build with different NDEBUG macro states -- especially re. asserts or other conditional content in public header files. The danger is that you, or users of your library might accidentally instantiate a different definition of an inline function from the one used inside the compiled part of the library, quietly violating the one definition rule and making the runtime behaviour undefined.

Answer 2

I'd use the following:

#ifdef _DEBUG
#define ASSERT(FUNC, CHECK) assert(FUNC == CHECK)
#else
#define ASSERT(FUNC, CHECK)
#endif

...

ASSERT(Func(), 1);

This way, for release build, the compiler don't even need to produce any code for assert.

Answer 3

If this code is inside a function, then act on and return the result:

bool bigPicture() {

   //Check the results
   bool success = 1 != Func();
   assert(success == NO, "Bad times");

   //Success is given, so...
   actOnIt();

   //and
   return success;
}

Answer 4

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

This will make the compiler stop complaining about Result not being used.

But you should think about using a version of assert that does something useful at run-time, like log descriptive errors to a file that can be retrieved from the production environment.