c++ - Move semantics == custom swap function obsolete?

Question

Recently, many questions pop up on how to provide your own swap function. With C++11, std::swap will use std::move and move semantics to swap the given values as fast as possible. This, of course, only works if you provide a move constructor and a move assignment operator (or one that uses pass-by-value).

Now, with that given, is it actually necessary to write your own swap functions in C++11? I could only think of non-movable types, but then again, the custom swaps usually work through some kind of "pointer exchange" (aka moving). Maybe with certain reference variables? Hm...

Answer 1

It is a matter of judgment. I will typically let std::swap do the job for prototyping code, but for release code write a custom swap. I can usually write a custom swap that is about twice as fast as 1 move construction + 2 move assignments + 1 resourceless destruction. However one may want to wait until std::swap actually proves to be a performance problem before going to the bother.

Update for Alf P. Steinbach:

20.2.2 [utility.swap] specifies that std::swap(T&, T&) has a noexcept equivalent to:

template <class T>
void
swap(T& a, T& b) noexcept
                 (
                    is_nothrow_move_constructible<T>::value &&
                    is_nothrow_move_assignable<T>::value
                 );

I.e. if move operations on T are noexcept, then std::swap on T is noexcept.

Note that this spec doesn't require move members. It only requires that construction and assignment from rvalues exists, and if it is noexcept, then swap will be noexcept. E.g.:

class A
{
public:
    A(const A&) noexcept;
    A& operator=(const A&) noexcept;
};

std::swap<A> is noexcept, even without move members.