Explicit move constructor needed in container?

Question

I have a templated container class:

 template<class Stuff>
 class Bag{
     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?

score 9 · Accepted Answer · answered Jun 15 '17 at 17:15

9

It would be moved, not copied.

I would suggest looking at the following image:

This clearly shows that the compiler implicitly generates a move constructor as long as the user doesn't define his/her own :

destructor
copy constructor
copy assignment
move assignment

Since your class has none of these user defined constructors the compiler generated move constructor will be called, that constructor will move mData.

answered Jun 15 '17 at 17:15

Hatted Rooster

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2

Where is this nice table from? – zett42 Jun 15 '17 at 17:35
1

@zett42 I forgot, I just have it laying around. However, I did a quick search and it's from a presentation : https://stackoverflow.com/a/24512883/1870760 – Hatted Rooster Jun 15 '17 at 17:36
4

The actual presentation : https://accu.org/content/conf2014/Howard_Hinnant_Accu_2014.pdf – Hatted Rooster Jun 15 '17 at 17:37
This very usefully answers the question I asked, but I was actually interested in the case when Bag does *not* have an implicitly generated move constructor. Should I ask a new one? – AGML Jun 15 '17 at 19:34

Explicit move constructor needed in container?

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