Should a move constructor take a const or non-const rvalue reference?

Question

In several places I've seen the recommended signatures of copy and move constructors given as:

struct T {     T();     T(const T& other);     T(T&& other); }; 

Where the copy constructor takes a const reference, and the move constructor takes a non-const rvalue reference.

As far as I can see though, this prevents me taking advantage of move semantics when returning const objects from a function, such as in the case below:

T generate_t() {     const T t;     return t; } 

Testing this with VC11 Beta, T's copy constructor is called, and not the move constructor. Even using return std::move(t); the copy constructor is still called.

I can see how this makes sense, since t is const so shouldn't bind to T&&. Using const T&& in the move constructor signature works fine, and makes sense, but then you have the problem that because other is const, you can't null its members out if they need to be nulled out - it'll only work when all members are scalars or have move constructors with the right signature.

It looks like the only way to make sure the move constructor is called in the general case to have made t non-const in the first place, but I don't like doing that - consting things is good form and I wouldn't expect the client of T to know that they had to go against that form in order to increase performance.

So, I guess my question is twofold; first, should a move constructor take a const or non-const rvalue reference? And second: am I right in this line of reasoning? That I should stop returning things that are const?

Answer 1

It should be a non-const rvalue reference.

If an object is placed in read-only memory, you can't steal resources from it, even if its formal lifetime is ending shortly. Objects created as const in C++ are allowed to live in read-only memory (using const_cast to try to change them results in undefined behavior).

Answer 2

A move constructor should normally take a non-const reference.

If it were possible to move from a const object it would usually imply that it was as efficient to copy an object as it was to "move" from it. At this point there is normally no benefit to having a move constructor.

You are also correct that if you have a variable that you are potentially going to want to move from then it will need to be non-const.

As I understand it this is the reason that Scott Meyers has changed his advice on returning objects of class type by value from functions for C++11. Returning objects by const qualified value does prevent unintentionally modification of a temporary object but it also inhibits moving from the return value.