c++ - In what scenarios should I expect to explicitly need to implement a move constructor and move assignment operator?

Question

Given that a class actually is moveable, manually implementing the move constructor and move assignment operator for a class quickly become tedious.

I was wondering when doing so is actually a heavy, heavy, premature optimization?

For instance, if a class only has trivial POD data or members that themselves have move constructor and move assignment operator defined, then I'd guess that the compiler will either just optimize the shit out of the lot (in the case of PODs) and otherwise use the members' move constructor and move assignment operator.

But is that guaranteed? In what scenarios should I expect to explicitly need to implement a move constructor and move assignment operator?

EDIT: As mentioned below by Nicol Bolas in a comment to his answer at https://stackoverflow.com/a/9966105/6345, with Visual Studio 11 Beta (and before) no move constructor or move assignment operator is ever automatically generated. Reference: http://blogs.msdn.com/b/vcblog/archive/2011/09/12/10209291.aspx

Answer 1

If you find yourself implementing, any of:

• destructor
• copy constructor
• copy assignment

Then you should be asking yourself if you need to implement move construction. If you "= default" any of the above, you should be asking yourself if you should then also "= default" the move members.

Even more importantly, you should be documenting and testing your assumptions, for example:

static_assert(std::is_nothrow_default_constructible<A>::value, "");
static_assert(std::is_copy_constructible<A>::value, "");
static_assert(std::is_copy_assignable<A>::value, "");
static_assert(std::is_nothrow_move_constructible<A>::value, "");
static_assert(std::is_nothrow_move_assignable<A>::value, "");
static_assert(std::is_nothrow_destructible<A>::value, "");