调整原子矢量的大小？

Question

如何调整原子矢量的大小？

例如,以下代码无法编译:

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

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

错误:

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

你不能......

A std::atomic<T>既不是复制/移动可构造的,也不能分配std::atomic<T>给另一个; 这意味着它没有使用要求.std::vector<...>::resize(size_type)

23.3.6.2 vector构造函数,复制和赋值 [vector.const]

void resize (size_type sz);

要求:T应为CopyInsertable *this.

^{注意:std::vector::resize (size_type sz, T const& init)也不适用,因为它T也需要MoveInsertable.}

提议的决议

在修改已存储在内部的元素时,您将需要使用一些其他容器类型,该容器类型不需要移动,复制或复制/移动分配已经构造的数据.

您还可以在std::atomic伪造的副本/移动/分配周围定义一个包装器,但实际上只是浅读取/写入底层原子的值.