Will the c++ compiler optimize away unused return value?

If the ReturnValue class has a non-trivial copy constructor, the compiler must not eliminate the call to the copy constructor - it is mandated by the language that it is invoked.

If the copy constructor is inline, the compiler might be able to inline the call, which in turn might cause a elimination of much of its code (also depending on whether FunctionThatAltersMembersAndNeverFails is inline).