我打算声明一个原子变量 vector ，用作多线程程序中的计数器。这是我尝试过的:

#include <atomic>
#include <vector>

int main(void)
{
  std::vector<std::atomic<int>> v_a;
  std::atomic<int> a_i(1);
  v_a.push_back(a_i);
  return 0;
}

这是 gcc 4.6.3 令人讨厌的冗长错误消息:

In file included from /usr/include/c++/4.6/x86_64-linux-gnu/./bits/c++allocator.h:34:0,
             from /usr/include/c++/4.6/bits/allocator.h:48,
             from /usr/include/c++/4.6/vector:62,
             from test_atomic_vec.h:2,
             from test_atomic_vec.cc:1:
/usr/include/c++/4.6/ext/new_allocator.h: In member function ‘void __gnu_cxx::new_allocator<_Tp>::construct(__gnu_cxx::new_allocator<_Tp>::pointer, const _Tp&) [with _Tp = std::atomic<int>, __gnu_cxx::new_allocator<_Tp>::pointer = std::atomic<int>*]’:
/usr/include/c++/4.6/bits/stl_vector.h:830:6:   instantiated from ‘void std::vector<_Tp, _Alloc>::push_back(const value_type&) [with _Tp = std::atomic<int>, _Alloc = std::allocator<std::atomic<int> >, std::vector<_Tp, _Alloc>::value_type = std::atomic<int>]’
test_atomic_vec.cc:10:20:   instantiated from here
/usr/include/c++/4.6/ext/new_allocator.h:108:9: error: use of deleted function ‘std::atomic<int>::atomic(const std::atomic<int>&)’
/usr/include/c++/4.6/atomic:538:7: error: declared here
In file included from /usr/include/c++/4.6/vector:70:0,
             from test_atomic_vec.h:2,
             from test_atomic_vec.cc:1:
/usr/include/c++/4.6/bits/vector.tcc: In member function ‘void std::vector<_Tp, _Alloc>::_M_insert_aux(std::vector<_Tp, _Alloc>::iterator, _Args&& ...) [with _Args = {const std::atomic<int>&}, _Tp = std::atomic<int>, _Alloc = std::allocator<std::atomic<int> >, std::vector<_Tp, _Alloc>::iterator = __gnu_cxx::__normal_iterator<std::atomic<int>*, std::vector<std::atomic<int> > >, typename std::_Vector_base<_Tp, _Alloc>::_Tp_alloc_type::pointer = std::atomic<int>*]’:
/usr/include/c++/4.6/bits/stl_vector.h:834:4:   instantiated from ‘void std::vector<_Tp, _Alloc>::push_back(const value_type&) [with _Tp = std::atomic<int>, _Alloc = std::allocator<std::atomic<int> >, std::vector<_Tp, _Alloc>::value_type = std::atomic<int>]’
test_atomic_vec.cc:10:20:   instantiated from here
/usr/include/c++/4.6/bits/vector.tcc:319:4: error: use of deleted function ‘std::atomic<int>::atomic(const std::atomic<int>&)’
/usr/include/c++/4.6/atomic:538:7: error: declared here
/usr/include/c++/4.6/bits/stl_vector.h:834:4:   instantiated from ‘void std::vector<_Tp, _Alloc>::push_back(const value_type&) [with _Tp = std::atomic<int>, _Alloc = std::allocator<std::atomic<int> >, std::vector<_Tp, _Alloc>::value_type = std::atomic<int>]’
test_atomic_vec.cc:10:20:   instantiated from here
/usr/include/c++/4.6/bits/vector.tcc:319:4: error: use of deleted function ‘std::atomic<int>& std::atomic<int>::operator=(const std::atomic<int>&)’
/usr/include/c++/4.6/atomic:539:15: error: declared here
In file included from /usr/include/c++/4.6/x86_64-linux-gnu/./bits/c++allocator.h:34:0,
             from /usr/include/c++/4.6/bits/allocator.h:48,
             from /usr/include/c++/4.6/vector:62,
             from test_atomic_vec.h:2,
             from test_atomic_vec.cc:1:
/usr/include/c++/4.6/ext/new_allocator.h: In member function ‘void __gnu_cxx::new_allocator<_Tp>::construct(__gnu_cxx::new_allocator<_Tp>::pointer, _Args&& ...) [with _Args = {std::atomic<int>}, _Tp = std::atomic<int>, __gnu_cxx::new_allocator<_Tp>::pointer = std::atomic<int>*]’:
/usr/include/c++/4.6/bits/vector.tcc:306:4:   instantiated from ‘void std::vector<_Tp, _Alloc>::_M_insert_aux(std::vector<_Tp, _Alloc>::iterator, _Args&& ...) [with _Args = {const std::atomic<int>&}, _Tp = std::atomic<int>, _Alloc = std::allocator<std::atomic<int> >, std::vector<_Tp, _Alloc>::iterator = __gnu_cxx::__normal_iterator<std::atomic<int>*, std::vector<std::atomic<int> > >, typename std::_Vector_base<_Tp, _Alloc>::_Tp_alloc_type::pointer = std::atomic<int>*]’
/usr/include/c++/4.6/bits/stl_vector.h:834:4:   instantiated from ‘void std::vector<_Tp, _Alloc>::push_back(const value_type&) [with _Tp = std::atomic<int>, _Alloc = std::allocator<std::atomic<int> >, std::vector<_Tp, _Alloc>::value_type = std::atomic<int>]’
test_atomic_vec.cc:10:20:   instantiated from here
/usr/include/c++/4.6/ext/new_allocator.h:114:4: error: use of deleted function ‘std::atomic<int>::atomic(const std::atomic<int>&)’
/usr/include/c++/4.6/atomic:538:7: error: declared here
In file included from /usr/include/c++/4.6/vector:61:0,
             from test_atomic_vec.h:2,
             from test_atomic_vec.cc:1:
/usr/include/c++/4.6/bits/stl_algobase.h: In static member function ‘static _BI2 std::__copy_move_backward<true, false, std::random_access_iterator_tag>::__copy_move_b(_BI1, _BI1, _BI2) [with _BI1 = std::atomic<int>*, _BI2 = std::atomic<int>*]’:
/usr/include/c++/4.6/bits/stl_algobase.h:581:18:   instantiated from ‘_BI2 std::__copy_move_backward_a(_BI1, _BI1, _BI2) [with bool _IsMove = true, _BI1 = std::atomic<int>*, _BI2 = std::atomic<int>*]’
/usr/include/c++/4.6/bits/stl_algobase.h:590:34:   instantiated from ‘_BI2 std::__copy_move_backward_a2(_BI1, _BI1, _BI2) [with bool _IsMove = true, _BI1 = std::atomic<int>*, _BI2 = std::atomic<int>*]’
/usr/include/c++/4.6/bits/stl_algobase.h:661:15:   instantiated from ‘_BI2 std::move_backward(_BI1, _BI1, _BI2) [with _BI1 = std::atomic<int>*, _BI2 = std::atomic<int>*]’
/usr/include/c++/4.6/bits/vector.tcc:313:4:   instantiated from ‘void std::vector<_Tp, _Alloc>::_M_insert_aux(std::vector<_Tp, _Alloc>::iterator, _Args&& ...) [with _Args = {const std::atomic<int>&}, _Tp = std::atomic<int>, _Alloc = std::allocator<std::atomic<int> >, std::vector<_Tp, _Alloc>::iterator = __gnu_cxx::__normal_iterator<std::atomic<int>*, std::vector<std::atomic<int> > >, typename std::_Vector_base<_Tp, _Alloc>::_Tp_alloc_type::pointer = std::atomic<int>*]’
/usr/include/c++/4.6/bits/stl_vector.h:834:4:   instantiated from ‘void std::vector<_Tp, _Alloc>::push_back(const value_type&) [with _Tp = std::atomic<int>, _Alloc = std::allocator<std::atomic<int> >, std::vector<_Tp, _Alloc>::value_type = std::atomic<int>]’
test_atomic_vec.cc:10:20:   instantiated from here
/usr/include/c++/4.6/bits/stl_algobase.h:546:6: error: use of deleted function ‘std::atomic<int>& std::atomic<int>::operator=(const std::atomic<int>&)’
/usr/include/c++/4.6/atomic:539:15: error: declared here
In file included from /usr/include/c++/4.6/vector:63:0,
             from test_atomic_vec.h:2,
             from test_atomic_vec.cc:1:
/usr/include/c++/4.6/bits/stl_construct.h: In function ‘void std::_Construct(_T1*, _Args&& ...) [with _T1 = std::atomic<int>, _Args = {std::atomic<int>}]’:
/usr/include/c++/4.6/bits/stl_uninitialized.h:77:3:   instantiated 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/include/c++/4.6/bits/stl_uninitialized.h:119:41:   instantiated from ‘_ForwardIterator std::uninitialized_copy(_InputIterator, _InputIterator, _ForwardIterator) [with _InputIterator = std::move_iterator<std::atomic<int>*>, _ForwardIterator = std::atomic<int>*]’
/usr/include/c++/4.6/bits/stl_uninitialized.h:259:63:   instantiated 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/include/c++/4.6/bits/stl_uninitialized.h:269:24:   instantiated from ‘_ForwardIterator std::__uninitialized_move_a(_InputIterator, _InputIterator, _ForwardIterator, _Allocator&) [with _InputIterator = std::atomic<int>*, _ForwardIterator = std::atomic<int>*, _Allocator = std::allocator<std::atomic<int> >]’
/usr/include/c++/4.6/bits/vector.tcc:343:8:   instantiated from ‘void std::vector<_Tp, _Alloc>::_M_insert_aux(std::vector<_Tp, _Alloc>::iterator, _Args&& ...) [with _Args = {const std::atomic<int>&}, _Tp = std::atomic<int>, _Alloc = std::allocator<std::atomic<int> >, std::vector<_Tp, _Alloc>::iterator = __gnu_cxx::__normal_iterator<std::atomic<int>*, std::vector<std::atomic<int> > >, typename std::_Vector_base<_Tp, _Alloc>::_Tp_alloc_type::pointer = std::atomic<int>*]’
/usr/include/c++/4.6/bits/stl_vector.h:834:4:   instantiated from ‘void std::vector<_Tp, _Alloc>::push_back(const value_type&) [with _Tp = std::atomic<int>, _Alloc = std::allocator<std::atomic<int> >, std::vector<_Tp, _Alloc>::value_type = std::atomic<int>]’
test_atomic_vec.cc:10:20:   instantiated from here
/usr/include/c++/4.6/bits/stl_construct.h:76:7: error: use of deleted function ‘std::atomic<int>::atomic(const std::atomic<int>&)’
/usr/include/c++/4.6/atomic:538:7: error: declared here

我该如何解决这个问题？

当我用 push_back() 注释掉该行时，错误消失。

编辑:我编辑了帖子...对于那些看到第一篇帖子的人来说，错误是我使用 gcc 而不是 g++ :\

最佳答案

如 closely related question 中所述评论中提到的，std::atomic<T>不是可复制构造的，也不是可复制分配的。

不具备这些属性的对象类型不能用作 std::vector 的元素.

但是，应该可以在 std::atomic<T> 周围创建一个包装器。可复制构造和可复制分配的元素。它必须使用 load()和 store() std::atomic<T> 的成员函数提供构造和分配(这是上述问题的公认答案所描述的想法):

#include <atomic>
#include <vector>

template <typename T>
struct atomwrapper
{
  std::atomic<T> _a;

  atomwrapper()
    :_a()
  {}

  atomwrapper(const std::atomic<T> &a)
    :_a(a.load())
  {}

  atomwrapper(const atomwrapper &other)
    :_a(other._a.load())
  {}

  atomwrapper &operator=(const atomwrapper &other)
  {
    _a.store(other._a.load());
  }
};

int main(void)
{
  std::vector<atomwrapper<int>> v_a;
  std::atomic<int> a_i(1);
  v_a.push_back(a_i);
  return 0;
}

编辑:正如 Bo Persson 正确指出的那样，包装器执行的复制操作不是原子的。它使您能够复制原子对象，但拷贝本身不是原子的。这意味着对原子的任何并发访问都不能使用复制操作。这意味着 vector 本身的操作(例如添加或删除元素)不得同时执行。

示例:如果一个线程修改了其中一个原子中存储的值，而另一个线程将新元素添加到 vector 中，则可能会发生 vector 重新分配，并且第一个线程修改的对象可能会从 vector 到另一个。在这种情况下，第一个线程执行的元素访问和第二个线程触发的复制操作之间将存在数据竞争。

关于c++ - 如何在 C++ 中声明原子 vector ，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/13193484/