ZFX + Developia

struct a
{
    a()             { std::cout << "cons" << std::endl; }
    a(a const& o)   { std::cout << "copy cons" << std::endl; }
    a(a&& o)        { std::cout << "move cons" << std::endl; }
};

a f(a b)
{
    return b;
}

auto b = a();
b = f(std::move(b));

cons
move cons
move cons

Reality Bites hat geschrieben:Second, I’ve yet to find a compiler that will elide the copy when a function parameter is returned, as in our implementation of sorted. When you think about how these elisions are done, it makes sense: without some form of inter-procedural optimization, the caller of sorted can’t know that the argument (and not some other object) will eventually be returned, so the compiler must allocate separate space on the stack for the argument and the return value.

If you need to return a function parameter, you can still get near-optimal performance by swapping into a default-constructed return value (provided default construction and swap are cheap, as they should be):