Explicit move constructor needed in container with no compiler-defined move constructor?

Question

This is a modification of my earlier question Explicit move constructor needed in container?.

I have a templated container class:

template<class Stuff>
class Bag{
     public: 
        ~Bag() {//Do some stuff here so that the compiler doesn't implement move semantics}
     private:
        std::vector<Stuff> mData;
 };

I want to do

void InPlace(Bag<Array>& Left){
  Bag<Array> temp;
  Transform(Left, temp); //fills temp with desirable output
  Left = std::move(temp);
}

Suppose Array has user-defined move semantics, but Bag does not. Would mData in this case be moved or copied?

Are you planning to ask a question about every row in that grid? — T.C., Jun 15 '17 at 19:44
[Similar question](https://stackoverflow.com/q/44573222/7359094) you asked 2 hours ago. — François Andrieux, Jun 15 '17 at 19:46
Bag does not have a move constructor, so temp gets cast to an lvalue (by the assignment operator). If I had to guess I would think this means that the std::vector assignment (not move) operator gets called, since mData is also an lvalue, but I'm not totally sure, hence the question. Sorry? — AGML, Jun 15 '17 at 19:48
If an object is not movable then move falls back to copy. That means `Left = std::move(temp);` is copy assignment. — NathanOliver, Jun 15 '17 at 19:50

score 1 · Accepted Answer · answered Jun 16 '17 at 07:58

1

If Bag supports no move semantics then there is no move operations applicable. Copy assignment/construction will take place accordingly.

answered Jun 16 '17 at 07:58

Navie

164
1
8

Btw. temp is an lvalue so it wouldn't result in a move here anyway. – Navie Jun 16 '17 at 08:03

Explicit move constructor needed in container with no compiler-defined move constructor?

1 Answers1