如何在C ++中声明一个原子向量
我打算声明一个在多线程程序中用作计数器的原子变量向量。 这是我试过的：
#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的烦人的详细错误信息：
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，该错误消失。
编辑：我编辑帖子...对于那些看到第一篇文章的人，错误是令人尴尬的，我用gcc而不是g++ ：
正如在评论中提到的这个密切相关的问题中所描述的， std::atomic<T>不是可复制构建的，也不是可复制分配的。
没有这些属性的对象类型不能用作std::vector元素。
但是，应该可以在可复制和可复制分配的std::atomic<T>元素周围创建一个包装。 它必须使用std::atomic<T>的load()和store()成员函数来提供构造和赋值（这是上述问题的接受答案所描述的思想）：
#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所指出的，由包装执行的复制操作不是原子的。 它使您能够复制原子对象，但副本本身不是原子的。 这意味着任何对原子的并发访问都不能使用复制操作。 这意味着矢量本身的操作（例如添加或移除元素）不得同时执行。
例如：如果一个线程修改存储在其中一个原子中的值，而另一个线程向该向量添加新元素，则可能会发生向量重新分配，并且第一个线程修改的对象可能会从该向量中的一个位置复制到另一个位置。 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。
看起来像atomic<T>没有复制构造函数。 据我所知，也不是移动构造函数。
一个解决办法可能是使用vector<T>::emplace_back()来构造向量中的in-place原子。 唉，我现在还没有C ++ 11编译器，或者我会去测试它。
首先回答你的标题问题：你可以很容易地声明std::vector<std::atomic<...>> ，就像你在你的例子中所做的那样。
但是，由于缺少std::atomic<>对象的复制或移动构造函数，因此在使用push_back()发现编译错误时，将限制对vector的使用。 基本上你不能做任何会调用构造函数的东西。
这意味着您的向量的大小必须在构建时固定，并且您应该使用operator[]或.at()来操作元素。 对于您的示例代码，以下工作1：
std::vector<std::atomic<int>> v_a(1);
std::atomic<int> a_i(1);
v_a[0] = a_i;
如果“构建时的固定大小”限制过于繁重，则可以使用std::deque代替。 这使您可以放置对象，动态增加结构而不需要复制或移动构造函数，例如：
std::deque<std::atomic<int>> d;
d.emplace_back(1);
d.emplace_back(2);
d.pop_back();
但是，仍然有一些限制。 例如，您可以使用pop_back() ，但不能使用更一般的erase() 。 这些限制是有意义的： std::deque使用的连续存储块中间的erase()通常需要元素的移动，因此需要复制/移动构造函数或赋值运算符。
如果你不能忍受这些限制，你可以像其他答案中建议的那样创建一个包装类，但要注意底层的实现：在使用std::atomic<>对象后，移动std::atomic<>对象没什么意义：它会打破任何线程并发访问对象。 复制/移动构造函数的唯一合理使用通常在这些对象的集合初始设置为其他线程之前进行初始设置。
1除非，也许你使用的是英特尔的icc编译器，在编译这段代码时会失败并出现内部错误。