使用`std::vector< std::unique_ptr< T > >`时出现错误。

Question

使用`std::vector< std::unique_ptr< T > >`时出现错误。

c++c++11stdvectormove-semanticsunique-ptr

9

我发现使用 std::move 传递 std::vector< std::unique_ptr< T > > 时出现了一些错误。导致问题的代码如下：

#include <memory>    // for std::unique_ptr
#include <utility>   // for std::move
#include <vector>    // for std::vector

struct bar {};
using vtype = std::vector<std::unique_ptr<bar>>;

struct foo
{
    foo(vtype v) : _v(std::move(v)) { }
private:
    vtype _v;
};

vtype getVector()
{
    return { std::move( std::unique_ptr<bar>(new bar()) ) };
};    

int main()
{
foo f(std::move(getVector()));
};

使用clang 3.4编译时，此代码会产生以下错误：

$ clang++ -std=c++11 test.cpp -o xtest
In file included from test.cpp:1:
In file included from /usr/bin/../lib/gcc/x86_64-linux-gnu/4.8/../../../../include/c++/4.8/memory:64:
/usr/bin/../lib/gcc/x86_64-linux-gnu/4.8/../../../../include/c++/4.8/bits/stl_construct.h:75:38: error: call to deleted constructor of
      'std::unique_ptr<bar, std::default_delete<bar> >'
    { ::new(static_cast<void*>(__p)) _T1(std::forward<_Args>(__args)...); }
                                     ^   ~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/bin/../lib/gcc/x86_64-linux-gnu/4.8/../../../../include/c++/4.8/bits/stl_uninitialized.h:75:8: note: in instantiation of function template specialization
      'std::_Construct<std::unique_ptr<bar, std::default_delete<bar> >, const std::unique_ptr<bar, std::default_delete<bar> > &>' requested here
                std::_Construct(std::__addressof(*__cur), *__first);
                     ^
/usr/bin/../lib/gcc/x86_64-linux-gnu/4.8/../../../../include/c++/4.8/bits/stl_uninitialized.h:117:2: note: in instantiation of function template specialization
      'std::__uninitialized_copy<false>::__uninit_copy<const std::unique_ptr<bar, std::default_delete<bar> > *, std::unique_ptr<bar, std::default_delete<bar> > *>'
      requested here
        __uninit_copy(__first, __last, __result);
        ^
/usr/bin/../lib/gcc/x86_64-linux-gnu/4.8/../../../../include/c++/4.8/bits/stl_uninitialized.h:258:19: note: in instantiation of function template specialization
      'std::uninitialized_copy<const std::unique_ptr<bar, std::default_delete<bar> > *, std::unique_ptr<bar, std::default_delete<bar> > *>' requested here
    { return std::uninitialized_copy(__first, __last, __result); }
                  ^
/usr/bin/../lib/gcc/x86_64-linux-gnu/4.8/../../../../include/c++/4.8/bits/stl_vector.h:1204:11: note: in instantiation of function template specialization
      'std::__uninitialized_copy_a<const std::unique_ptr<bar, std::default_delete<bar> > *, std::unique_ptr<bar, std::default_delete<bar> > *, std::unique_ptr<bar,
      std::default_delete<bar> > >' requested here
            std::__uninitialized_copy_a(__first, __last,
                 ^
/usr/bin/../lib/gcc/x86_64-linux-gnu/4.8/../../../../include/c++/4.8/bits/stl_vector.h:368:2: note: in instantiation of function template specialization
      'std::vector<std::unique_ptr<bar, std::default_delete<bar> >, std::allocator<std::unique_ptr<bar, std::default_delete<bar> > > >::_M_range_initialize<const
      std::unique_ptr<bar, std::default_delete<bar> > *>' requested here
        _M_range_initialize(__l.begin(), __l.end(),
        ^
test.cpp:17:12: note: in instantiation of member function 'std::vector<std::unique_ptr<bar, std::default_delete<bar> >, std::allocator<std::unique_ptr<bar,
      std::default_delete<bar> > > >::vector' requested here
    return { std::move( std::unique_ptr<bar>(new bar()) ) };
           ^
/usr/bin/../lib/gcc/x86_64-linux-gnu/4.8/../../../../include/c++/4.8/bits/unique_ptr.h:273:7: note: function has been explicitly marked deleted here
      unique_ptr(const unique_ptr&) = delete;
      ^
1 error generated.

使用g++ 4.8似乎情况并没有变得更好：

 $ g++-4.8 -std=c++11 test.cpp -o xtest
In file included from /usr/include/c++/4.8/memory:64:0,
                 from test.cpp:1:
/usr/include/c++/4.8/bits/stl_construct.h: In instantiation of ‘void std::_Construct(_T1*, _Args&& ...) [with _T1 = std::unique_ptr<bar>; _Args = {const std::unique_ptr<bar, std::default_delete<bar> >&}]’:
/usr/include/c++/4.8/bits/stl_uninitialized.h:75:53:   required from ‘static _ForwardIterator std::__uninitialized_copy<_TrivialValueTypes>::__uninit_copy(_InputIterator, _InputIterator, _ForwardIterator) [with _InputIterator = const std::unique_ptr<bar>*; _ForwardIterator = std::unique_ptr<bar>*; bool _TrivialValueTypes = false]’
/usr/include/c++/4.8/bits/stl_uninitialized.h:117:41:   required from ‘_ForwardIterator std::uninitialized_copy(_InputIterator, _InputIterator, _ForwardIterator) [with _InputIterator = const std::unique_ptr<bar>*; _ForwardIterator = std::unique_ptr<bar>*]’
/usr/include/c++/4.8/bits/stl_uninitialized.h:258:63:   required from ‘_ForwardIterator std::__uninitialized_copy_a(_InputIterator, _InputIterator, _ForwardIterator, std::allocator<_Tp>&) [with _InputIterator = const std::unique_ptr<bar>*; _ForwardIterator = std::unique_ptr<bar>*; _Tp = std::unique_ptr<bar>]’
/usr/include/c++/4.8/bits/stl_vector.h:1206:27:   required from ‘void std::vector<_Tp, _Alloc>::_M_range_initialize(_ForwardIterator, _ForwardIterator, std::forward_iterator_tag) [with _ForwardIterator = const std::unique_ptr<bar>*; _Tp = std::unique_ptr<bar>; _Alloc = std::allocator<std::unique_ptr<bar> >]’
/usr/include/c++/4.8/bits/stl_vector.h:369:36:   required from ‘std::vector<_Tp, _Alloc>::vector(std::initializer_list<_Tp>, const allocator_type&) [with _Tp = std::unique_ptr<bar>; _Alloc = std::allocator<std::unique_ptr<bar> >; std::vector<_Tp, _Alloc>::allocator_type = std::allocator<std::unique_ptr<bar> >]’
test.cpp:17:59:   required from here
/usr/include/c++/4.8/bits/stl_construct.h:75:7: error: use of deleted function ‘std::unique_ptr<_Tp, _Dp>::unique_ptr(const std::unique_ptr<_Tp, _Dp>&) [with _Tp = bar; _Dp = std::default_delete<bar>]’
     { ::new(static_cast<void*>(__p)) _T1(std::forward<_Args>(__args)...); }
       ^
In file included from /usr/include/c++/4.8/memory:81:0,
                 from test.cpp:1:
/usr/include/c++/4.8/bits/unique_ptr.h:273:7: error: declared here
       unique_ptr(const unique_ptr&) = delete;
       ^

根据这个答案和评论，这不应该在这些编译器上发生，但我并没有完全相同：我正在尝试使用初始化列表初始化向量。

有什么想法可以让这段代码正确构建？

- lurscher

1

可以使用不可复制的 bar 轻松地进行复制。http://coliru.stacked-crooked.com/a/f8344e87a2d4374e - David G

2个回答

-1

你的vtype是一个唯一指针的向量，而你正在将一个唯一指针返回给它。

由于你在构造函数foo中进行了移动，所以不需要返回移动操作，请看一下这段代码：

#include <memory>    // for std::unique_ptr
#include <utility>   // for std::move
#include <vector>    // for std::vector

struct bar {};
using vtype = /*std::vector<*/std::unique_ptr<bar>/*>*/;

struct foo
{
    foo(vtype v) : _v(std::move(v)) { }
private:
    vtype _v;
};

vtype getVector()
{
    return /*{ std::move(*/ std::unique_ptr<bar>(new bar()) /*) }*/;
};    

int main()
{
foo f(std::move(getVector()));
};

- Mido

网页内容由stack overflow 提供, 点击上面的

可以查看英文原文，
原文链接

- Praetorian · Accepted Answer

getVector函数中使用了大括号初始化列表作为返回语句。

return { std::move( std::unique_ptr<bar>(new bar()) ) };

这会导致调用接受initializer_list<T>的std::vector<T>构造函数。即使您移动了unique_ptr，initializer_list 只允许对其元素进行const访问，因此vector会尝试复制unique_ptr，导致出现您看到的错误。

您可以通过使用更详细的方式构造vector来修复错误。

vtype getVector()
{
    vtype v;
    v.push_back(std::unique_ptr<bar>(new bar()));
    return v;
}

在线演示

出于好奇，可以从移动只能（move-only）对象的数组构造一个vector，但需要通过std::move_iterator来移动元素。

vtype getVector()
{
    std::unique_ptr<bar> arr[] = {std::unique_ptr<bar>(new bar())};
    return {std::make_move_iterator(std::begin(arr)),
            std::make_move_iterator(std::end(arr))};
}