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?

good catch, fixed

AGML
– AGML

2017-06-15 17:14:33 +00:00
Commented Jun 15, 2017 at 17:14 — AGML
– AGML, Commented Jun 15, 2017 at 17:14

Hatted Rooster · Accepted Answer · 2017-06-15 17:15:47Z

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, 2017 at 17:15

Hatted Rooster

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4 Comments

zett42 Over a year ago

Where is this nice table from?

Hatted Rooster Over a year ago

@zett42 I forgot, I just have it laying around. However, I did a quick search and it's from a presentation : stackoverflow.com/a/24512883/1870760

Hatted Rooster Over a year ago

The actual presentation : accu.org/content/conf2014/Howard_Hinnant_Accu_2014.pdf

AGML Over a year ago

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?

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