Resize a vector of atomic?

Question

How to resize a vector of atomics ?

As an example, the following code does not compile:

#include <iostream>
#include <vector>
#include <atomic>

int main()
{
    std::vector<std::atomic<int>> v;
    v.resize(1000); // Problem here!
    v[0] = 1;
    return 0;
}

Error:

In file included from /usr/local/gcc-4.8.1/include/c++/4.8.1/vector:62:0,
                 from main.cpp:2:
/usr/local/gcc-4.8.1/include/c++/4.8.1/bits/stl_construct.h: In instantiation of ‘void std::_Construct(_T1*, _Args&& ...) [with _T1 = std::atomic<int>; _Args = {std::atomic<int>}]’:
/usr/local/gcc-4.8.1/include/c++/4.8.1/bits/stl_uninitialized.h:75:53:   required from ‘static _ForwardIterator std::__uninitialized_copy<_TrivialValueTypes>::__uninit_copy(_InputIterator, _InputIterator, _ForwardIterator) [with _InputIterator = std::move_iterator<std::atomic<int>*>; _ForwardIterator = std::atomic<int>*; bool _TrivialValueTypes = false]’
/usr/local/gcc-4.8.1/include/c++/4.8.1/bits/stl_uninitialized.h:117:41:   required from ‘_ForwardIterator std::uninitialized_copy(_InputIterator, _InputIterator, _ForwardIterator) [with _InputIterator = std::move_iterator<std::atomic<int>*>; _ForwardIterator = std::atomic<int>*]’
/usr/local/gcc-4.8.1/include/c++/4.8.1/bits/stl_uninitialized.h:258:63:   required from ‘_ForwardIterator std::__uninitialized_copy_a(_InputIterator, _InputIterator, _ForwardIterator, std::allocator<_Tp>&) [with _InputIterator = std::move_iterator<std::atomic<int>*>; _ForwardIterator = std::atomic<int>*; _Tp = std::atomic<int>]’
/usr/local/gcc-4.8.1/include/c++/4.8.1/bits/stl_uninitialized.h:281:69:   required from ‘_ForwardIterator std::__uninitialized_move_if_noexcept_a(_InputIterator, _InputIterator, _ForwardIterator, _Allocator&) [with _InputIterator = std::atomic<int>*; _ForwardIterator = std::atomic<int>*; _Allocator = std::allocator<std::atomic<int> >]’
/usr/local/gcc-4.8.1/include/c++/4.8.1/bits/vector.tcc:556:42:   required from ‘void std::vector<_Tp, _Alloc>::_M_default_append(std::vector<_Tp, _Alloc>::size_type) [with _Tp = std::atomic<int>; _Alloc = std::allocator<std::atomic<int> >; std::vector<_Tp, _Alloc>::size_type = long unsigned int]’
/usr/local/gcc-4.8.1/include/c++/4.8.1/bits/stl_vector.h:667:41:   required from ‘void std::vector<_Tp, _Alloc>::resize(std::vector<_Tp, _Alloc>::size_type) [with _Tp = std::atomic<int>; _Alloc = std::allocator<std::atomic<int> >; std::vector<_Tp, _Alloc>::size_type = long unsigned int]’
main.cpp:8:17:   required from here
/usr/local/gcc-4.8.1/include/c++/4.8.1/bits/stl_construct.h:75:7: error: use of deleted function ‘std::atomic<int>::atomic(const std::atomic<int>&)’
     { ::new(static_cast<void*>(__p)) _T1(std::forward<_Args>(__args)...); }
       ^
In file included from main.cpp:3:0:
/usr/local/gcc-4.8.1/include/c++/4.8.1/atomic:601:7: error: declared here
       atomic(const atomic&) = delete;
       ^

Answer 1

You can't...

A std::atomic<T> is neither copy/move-constructible, nor can you assign one std::atomic<T> to another; this means that it doesn't have the requirements to use std::vector<...>::resize(size_type).

23.3.6.2 vector constructors, copy, and assignment [vector.const]

void resize (size_type sz);

Requires: T shall be CopyInsertable into *this.

^{Note: std::vector::resize (size_type sz, T const& init) isn't applicable either since that requires T to also be MoveInsertable.}

---

Proposed resolution

You will need to use some other container-type which doesn't require already constructed data to be moved, copied, or copy/move assigned, upon modifying the elements already stored inside.

You could also define a wrapper around your std::atomic that fakes copy/moves/assigns, but actually just shallow read/writes the value of the underlying atomic.