g++ How to get warning on ignoring function return value

Question

lint produces some warning like:

foo.c XXX Warning 534: Ignoring return value of function bar()

From the lint manual

Answer 1

The answers about using __attribute__((warn_unused_result)) are correct. GCC isn't so good at this functionality, though! Be aware: it will not warn for non-POD types. That means, for example, if you return a class with a destructor (or a class with instance variables with destructors) you'll never see a warning about ignoring the result.

Relevant bug: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66177

Example where it fails:

struct Error {
~Error();
};

__attribute__((warn_unused_result)) Error test();

int main()
{
    test();
    return 0;
}

So, don't rely on this for return types which aren't pretty simple.