Is std::array movable?

Question

Is std::array movable?

In Bjarne Native 2012 presentation slides (slide 41) it lists std::array as one of the only containers that isn't movable.

A quick look on gcc 4.8 libraries source code seems to confirm that std::array is not movable:

std::vector:

/* @brief  %Vector move constructor.
   ...       */
  vector(vector&& __x) noexcept
  : _Base(std::move(__x)) { }

while in std::array the only method that receives a rvalue reference parameter is the random element access, which avoids a return by copy:

get(array<_Tp, _Nm>&& __arr) noexcept
    { /*...*/ return std::move(get<_Int>(__arr)); }

Is move-constructor and move-assignment for std::array defaulted created, or is std::array unmovable? If it is unmovable, why std::array cannot be moved while std::vector can?

As a point of interest, the presence or absence alone of a move constructor is not enough to establish that a type is or isn't move constructible (if using the expression/concepts-oriented view on the matter, as is most common). Similarly for move assignment. — Luc Danton, Jan 18 '13 at 01:39

Potatoswatter · Accepted Answer · 2013-01-17T01:29:18.847

59

std::array is movable only if its contained objects are movable.

std::array is quite different from the other containers because the container object contains the storage, not just pointers into the heap. Moving a std::vector only copies some pointers, and the contained objects are none the wiser.

Yes, std::array uses the default move constructor and assignment operator. As an aggregate class, it's not allowed to define any constructors.

edited Jan 17 '13 at 01:29

answered Jan 17 '13 at 01:20

Potatoswatter

126,977
21
238
404

1

And the default move constructor for an array moves each of the elements? – Seth Carnegie Jan 17 '13 at 01:23
12

@SethCarnegie It move-constructs the member array, which moves each of its elements. – Potatoswatter Jan 17 '13 at 01:24
5

Thanks! So a move operation in an std::array is O(n), since move occurs in reality on its elements, while in a std::vector it would be O(1)? (since in the std::vector case only the pointer needs to be moved) – Alessandro Stamatto Jan 17 '13 at 02:17
1

If the elements of `std::array` are PODs, Won't it just do one `memcpy`? Will it be still O(n)? – balki Jan 17 '13 at 07:45
3

@balki `memcpy` is still O(n). – Potatoswatter Jan 17 '13 at 08:06
7

@AlessandroStamatto Though even if O(n) you can still profit from the movement of the individual elements. So in general moving a `std::array` should still be better than copying it. – Christian Rau Jan 17 '13 at 08:52

Is std::array movable?

1 Answers1

Linked