Question

I recently read (and unfortunately forgot where), that the best way to write operator= is like this:

foo &operator=(foo other)
{
    swap(*this, other);
    return *this;
}

instead of this:

foo &operator=(const foo &other)
{
    foo copy(other);
    swap(*this, copy);
    return *this;
}

The idea is that if operator= is called with an rvalue, the first version can optimize away construction of a copy. So when called with a rvalue, the first version is faster and when called with an lvalue the two are equivalent.

I’m curious as to what other people think about this? Would people avoid the first version because of lack of explicitness? Am I correct that the first version can be better and can never be worse?

Answer 1

You probably read it from: http://cpp-next.com/archive/2009/08/want-speed-pass-by-value/

I don’t have much to say since I think the link explains the rationale pretty well. Anecdotally I can confirm that the first form results in fewer copies in my builds with MSVC, which makes sense since compilers might not be able to do copy-elision on the second form. I agree that the first form is a strict improvement and is never worse than the second.

Edit:
The first form might be a bit less idiomatic, but I don’t think it’s much less clear. (IMO, it’s not any more surprising than seeing the copy-and-swap implementation of the assignment operator for the first time.)

Edit #2: Oops, I meant to write copy-elision, not RVO.