c++ - 不可 move -不可复制对象的 vector 的 move 分配不编译

Question

以下代码无法使用 Visual Studio 2013 编译:

#include <vector>

struct X {
    X() = default;
    X(const X&) = delete;
    X& operator=(const X&) = delete;
    X(X&&) = delete;
    X& operator=(X&&) = delete;
    ~X() = default;
};

void foo()
{
    std::vector<X> v;
    std::vector<X> w;
    w = std::move(v);
}

错误信息说

error C2280: 'X::X(X &&)' : attempting to reference a deleted function

这对我来说毫无意义。您应该不需要 X 的 move 构造函数为了 move vector<X> .这是编译器错误，还是我遗漏了什么？

这是完整的错误信息:

C:\Program Files (x86)\Microsoft Visual Studio 12.0\VC\include\xmemory0(600): error C2280: 'X::X(X &&)' : attempting to reference a deleted function
    Test.cpp(9) : see declaration of 'X::X'
    C:\Program Files (x86)\Microsoft Visual Studio 12.0\VC\include\xmemory0(723) : see reference to function template instantiation 'void std::allocator<_Ty>::construct<_Objty,_Ty>(_Objty *,_Ty &&)' being compiled
    with
    [
        _Ty=X
    ,   _Objty=X
    ]
    C:\Program Files (x86)\Microsoft Visual Studio 12.0\VC\include\xmemory0(723) : see reference to function template instantiation 'void std::allocator<_Ty>::construct<_Objty,_Ty>(_Objty *,_Ty &&)' being compiled
    with
    [
        _Ty=X
    ,   _Objty=X
    ]
    C:\Program Files (x86)\Microsoft Visual Studio 12.0\VC\include\xmemory0(872) : see reference to function template instantiation 'void std::allocator_traits<_Alloc>::construct<_Ty,_Ty>(std::allocator<_Ty> &,_Objty *,_Ty &&)' being compiled
    with
    [
        _Alloc=std::allocator<X>
    ,   _Ty=X
    ,   _Objty=X
    ]
    C:\Program Files (x86)\Microsoft Visual Studio 12.0\VC\include\xmemory0(872) : see reference to function template instantiation 'void std::allocator_traits<_Alloc>::construct<_Ty,_Ty>(std::allocator<_Ty> &,_Objty *,_Ty &&)' being compiled
    with
    [
        _Alloc=std::allocator<X>
    ,   _Ty=X
    ,   _Objty=X
    ]
    C:\Program Files (x86)\Microsoft Visual Studio 12.0\VC\include\xmemory(378) : see reference to function template instantiation 'void std::_Wrap_alloc<std::allocator<_Ty>>::construct<X,X>(_Ty *,X &&)' being compiled
    with
    [
        _Ty=X
    ]
    C:\Program Files (x86)\Microsoft Visual Studio 12.0\VC\include\xmemory(378) : see reference to function template instantiation 'void std::_Wrap_alloc<std::allocator<_Ty>>::construct<X,X>(_Ty *,X &&)' being compiled
    with
    [
        _Ty=X
    ]
    C:\Program Files (x86)\Microsoft Visual Studio 12.0\VC\include\xmemory(416) : see reference to function template instantiation '_FwdIt std::_Uninit_copy<_InIt,_FwdIt,std::allocator<_Ty>>(_InIt,_InIt,_FwdIt,std::_Wrap_alloc<std::allocator<_Ty>> &,std::_Nonscalar_ptr_iterator_tag)' being compiled
    with
    [
        _FwdIt=X *
    ,   _InIt=std::move_iterator<std::_Vector_iterator<std::_Vector_val<std::_Simple_types<X>>>>
    ,   _Ty=X
    ]
    C:\Program Files (x86)\Microsoft Visual Studio 12.0\VC\include\xmemory(427) : see reference to function template instantiation '_FwdIt std::_Uninit_copy<_Iter,X,_Alloc>(_InIt,_InIt,_FwdIt,_Alloc &)' being compiled
    with
    [
        _FwdIt=X *
    ,   _Iter=std::move_iterator<std::_Vector_iterator<std::_Vector_val<std::_Simple_types<X>>>>
    ,   _Alloc=std::_Wrap_alloc<std::allocator<X>>
    ,   _InIt=std::move_iterator<std::_Vector_iterator<std::_Vector_val<std::_Simple_types<X>>>>
    ]
    C:\Program Files (x86)\Microsoft Visual Studio 12.0\VC\include\vector(1640) : see reference to function template instantiation '_FwdIt std::_Uninitialized_copy<_Iter,X*,std::_Wrap_alloc<std::allocator<_Ty>>>(_InIt,_InIt,_FwdIt,_Alloc &)' being compiled
    with
    [
        _FwdIt=X *
    ,   _Iter=std::move_iterator<std::_Vector_iterator<std::_Vector_val<std::_Simple_types<X>>>>
    ,   _Ty=X
    ,   _InIt=std::move_iterator<std::_Vector_iterator<std::_Vector_val<std::_Simple_types<X>>>>
    ,   _Alloc=std::_Wrap_alloc<std::allocator<X>>
    ]
    C:\Program Files (x86)\Microsoft Visual Studio 12.0\VC\include\vector(789) : see reference to function template instantiation 'X *std::vector<X,std::allocator<_Ty>>::_Ucopy<_Iter>(_Iter,_Iter,X *)' being compiled
    with
    [
        _Ty=X
    ,   _Iter=std::move_iterator<std::_Vector_iterator<std::_Vector_val<std::_Simple_types<X>>>>
    ]
    C:\Program Files (x86)\Microsoft Visual Studio 12.0\VC\include\vector(789) : see reference to function template instantiation 'X *std::vector<X,std::allocator<_Ty>>::_Ucopy<_Iter>(_Iter,_Iter,X *)' being compiled
    with
    [
        _Ty=X
    ,   _Iter=std::move_iterator<std::_Vector_iterator<std::_Vector_val<std::_Simple_types<X>>>>
    ]
    C:\Program Files (x86)\Microsoft Visual Studio 12.0\VC\include\vector(766) : see reference to function template instantiation 'void std::vector<X,std::allocator<_Ty>>::_Construct<_Iter>(_Iter,_Iter,std::forward_iterator_tag)' being compiled
    with
    [
        _Ty=X
    ,   _Iter=std::move_iterator<std::_Vector_iterator<std::_Vector_val<std::_Simple_types<X>>>>
    ]
    C:\Program Files (x86)\Microsoft Visual Studio 12.0\VC\include\vector(766) : see reference to function template instantiation 'void std::vector<X,std::allocator<_Ty>>::_Construct<_Iter>(_Iter,_Iter,std::forward_iterator_tag)' being compiled
    with
    [
        _Ty=X
    ,   _Iter=std::move_iterator<std::_Vector_iterator<std::_Vector_val<std::_Simple_types<X>>>>
    ]
    C:\Program Files (x86)\Microsoft Visual Studio 12.0\VC\include\vector(854) : see reference to function template instantiation 'void std::vector<X,std::allocator<_Ty>>::_Construct<std::move_iterator<std::_Vector_iterator<std::_Vector_val<std::_Simple_types<X>>>>>(_Iter,_Iter)' being compiled
    with
    [
        _Ty=X
    ,   _Iter=std::move_iterator<std::_Vector_iterator<std::_Vector_val<std::_Simple_types<X>>>>
    ]
    C:\Program Files (x86)\Microsoft Visual Studio 12.0\VC\include\vector(854) : see reference to function template instantiation 'void std::vector<X,std::allocator<_Ty>>::_Construct<std::move_iterator<std::_Vector_iterator<std::_Vector_val<std::_Simple_types<X>>>>>(_Iter,_Iter)' being compiled
    with
    [
        _Ty=X
    ,   _Iter=std::move_iterator<std::_Vector_iterator<std::_Vector_val<std::_Simple_types<X>>>>
    ]
    C:\Program Files (x86)\Microsoft Visual Studio 12.0\VC\include\vector(849) : while compiling class template member function 'void std::vector<X,std::allocator<_Ty>>::_Assign_rv(std::vector<_Ty,std::allocator<_Ty>> &&,std::false_type)'
    with
    [
        _Ty=X
    ]
    C:\Program Files (x86)\Microsoft Visual Studio 12.0\VC\include\vector(860) : see reference to function template instantiation 'void std::vector<X,std::allocator<_Ty>>::_Assign_rv(std::vector<_Ty,std::allocator<_Ty>> &&,std::false_type)' being compiled
    with
    [
        _Ty=X
    ]
    Test.cpp(16) : see reference to class template instantiation 'std::vector<X,std::allocator<_Ty>>' being compiled
    with
    [
        _Ty=X
    ]

Answer 1

正如 dyp 在评论中提到的，这是一个 reported bug in C++11 *。表达式

a = rv

_{(其中 a 是 X 类型的容器，元素类型为 T，而 rv 是 X 类型的非常量右值)}
在表 99“可识别分配器的容器要求”中具有以下要求:

If allocator_traits<allocator_type>::propagate_on_container_move_assignment
::value is false, T is MoveInsertable into X and MoveAssignable. All existing elements of a are either move assigned to or destroyed.

allocator_traits propagate_on_container_move_assignment 的定义如下:

typedef see below propagate_on_container_move_assignment;

Type: Alloc::propagate_on_container_move_assignment if such a type exists,
otherwise false_type.

问题是忘记将相应的 typedef 放入 std::allocator , 所以 propagate_on_container_move_assignment一直是 false .只需添加 typedef 即可在 C++14 中解决此问题。

* _{注意 [default.allocator] 和 [allocator.traits.types] 实际上在 N3337 的 §20.6 中，而不是 §20.7。}