casting unused return values to void

Question

int fn();

void whatever()
{
    (void) fn();
}

Is there any reason for casting an unused return value to void, or am I right in thinking it's a complete waste of time?

Follow up:

Well that seems pretty comprehensive. I suppose it's better than commenting an unused return value since self documenting code is better than comments. Personally, I'll turn these warnings off since it's unnecessary noise.

I'll eat my words if a bug escapes because of it...

Answer 1

At work we use that to acknowledge that the function has a return value but the developer has asserted that it is safe to ignore it. Since you tagged the question as C++ you should be using *static_cast*:

static_cast<void>(fn());

As far as the compiler goes casting the return value to void has little meaning.

Answer 2

Cast to void is costless. It is only information for compiler how to treat it.

Answer 3

David's answer pretty covers the motivation for this, to explicitly show other "developers" that you know this function returns but you're explicitly ignoring it.

This is a way to ensure that where necessary error codes are always handled.

I think for C++ this is probably the only place that I prefer to use C-style casts too, since using the full static cast notation just feels like overkill here. Finally, if you're reviewing a coding standard or writing one, then it's also a good idea to explicitly state that calls to overloaded operators (not using function call notation) should be exempt from this too:

class A {};
A operator+(A const &, A const &);

int main () {
  A a;
  a + a;                 // Not a problem
  (void)operator+(a,a);  // Using function call notation - so add the cast.

Answer 4

The true reason for doing this dates back to a tool used on C code, called lint.

It analyzes code looking for possible problems and issuing warnings and suggestions. If a function returned a value which was then not checked, lint would warn in case this was accidental. To silence lint on this warning, you cast the call to (void).

Answer 5

For the functionality of you program casting to void is meaningless. I would also argue that you should not use it to signal something to the person that is reading the code, as suggested in the answer by David. If you want to communicate something about your intentions, it is better to use a comment. Adding a cast like this will only look strange and raise questions about the possible reason. Just my opinion...