如何修复具有 unique_ptr 的对象的向量

Question

我有以下代码，其中包含 class 的向量，其中某些成员声明为 unique_ptr。

struct Container
{
    struct Nested{
        std::unique_ptr<Container> node;

        Nested(std::unique_ptr<Container> t) : node(std::move(t)) {}
        Nested(const Nested& t) { node = std::move(t.node); };
    };

    std::vector<Nested> edges;
};

typedef std::unique_ptr<Container> UCont;
typedef Container::Nested Nested;


int main()
{
    std::unique_ptr<Container> object = UCont(new Container{{
                                    Nested(UCont(new Container{{}})),
                                    Nested(UCont(new Container{{}})),
                                    Nested(UCont(new Container{{}}))
                                }});
}

现在编译它会出现以下错误：

..[=12=].UniquePtrVector.cpp: In copy constructor 'Container::Nested::Nested(const Container::Nested&)':
..[=12=].UniquePtrVector.cpp:20:35: error: use of deleted function 'std::unique_ptr<_Tp, _Dp>& std::unique_ptr<_Tp, _Dp>::operator=(const std::unique_ptr<_Tp, _Dp>&) [with _Tp = Container; _Dp = std::default_delete<Container>]'
    Nested(const Nested& t) { node = std::move(t.node); };
                                   ^
In file included from c:\mingw\lib\gcc\mingw32.3.0\include\c++\bits\locale_conv.h:41:0,
                 from c:\mingw\lib\gcc\mingw32.3.0\include\c++\locale:43,
                 from c:\mingw\lib\gcc\mingw32.3.0\include\c++\iomanip:43,
                 from c:\mingw\lib\gcc\mingw32.3.0\include\c++\mingw32\bits\stdc++.h:71,
                 from ..[=12=].UniquePtrVector.cpp:10:
c:\mingw\lib\gcc\mingw32.3.0\include\c++\bits\unique_ptr.h:357:19: note: declared here
       unique_ptr& operator=(const unique_ptr&) = delete;

我不确定如何修复此错误。我想删除复制构造函数也不是一个选项。有帮助吗？

编辑：更改为

Nested(std::unique_ptr<Container>&& t) : node(std::move(t)) {}
Nested(Nested&& t) : node(std::move(t.node)) {}
Nested(const Nested& t) =delete;

也报错：

In file included from c:\mingw\lib\gcc\mingw32.3.0\include\c++\bits\stl_tempbuf.h:60:0,
                 from c:\mingw\lib\gcc\mingw32.3.0\include\c++\bits\stl_algo.h:62,
                 from c:\mingw\lib\gcc\mingw32.3.0\include\c++\algorithm:62,
                 from c:\mingw\lib\gcc\mingw32.3.0\include\c++\mingw32\bits\stdc++.h:64,
                 from ..[=14=].UniquePtrVector.cpp:10:
c:\mingw\lib\gcc\mingw32.3.0\include\c++\bits\stl_construct.h: In instantiation of 'void std::_Construct(_T1*, _Args&& ...) [with _T1 = Container::Nested; _Args = {const Container::Nested&}]':
c:\mingw\lib\gcc\mingw32.3.0\include\c++\bits\stl_uninitialized.h:75:18:   required from 'static _ForwardIterator std::__uninitialized_copy<_TrivialValueTypes>::__uninit_copy(_InputIterator, _InputIterator, _ForwardIterator) [with _InputIterator = const Container::Nested*; _ForwardIterator = Container::Nested*; bool _TrivialValueTypes = false]'
c:\mingw\lib\gcc\mingw32.3.0\include\c++\bits\stl_uninitialized.h:126:15:   required from '_ForwardIterator std::uninitialized_copy(_InputIterator, _InputIterator, _ForwardIterator) [with _InputIterator = const Container::Nested*; _ForwardIterator = Container::Nested*]'
c:\mingw\lib\gcc\mingw32.3.0\include\c++\bits\stl_uninitialized.h:281:37:   required from '_ForwardIterator std::__uninitialized_copy_a(_InputIterator, _InputIterator, _ForwardIterator, std::allocator<_Tp>&) [with _InputIterator = const Container::Nested*; _ForwardIterator = Container::Nested*; _Tp = Container::Nested]'
c:\mingw\lib\gcc\mingw32.3.0\include\c++\bits\stl_vector.h:1290:33:   required from 'void std::vector<_Tp, _Alloc>::_M_range_initialize(_ForwardIterator, _ForwardIterator, std::forward_iterator_tag) [with _ForwardIterator = const Container::Nested*; _Tp = Container::Nested; _Alloc = std::allocator<Container::Nested>]'
c:\mingw\lib\gcc\mingw32.3.0\include\c++\bits\stl_vector.h:377:21:   required from 'std::vector<_Tp, _Alloc>::vector(std::initializer_list<_Tp>, const allocator_type&) [with _Tp = Container::Nested; _Alloc = std::allocator<Container::Nested>; std::vector<_Tp, _Alloc>::allocator_type = std::allocator<Container::Nested>]'
..[=14=].UniquePtrVector.cpp:36:11:   required from here
c:\mingw\lib\gcc\mingw32.3.0\include\c++\bits\stl_construct.h:75:7: error: use of deleted function 'Container::Nested::Nested(const Container::Nested&)'
     { ::new(static_cast<void*>(__p)) _T1(std::forward<_Args>(__args)...); }
       ^
..[=14=].UniquePtrVector.cpp:20:11: note: declared here
           Nested(const Nested& t) =delete;
           ^

Answer 1

当您在复制构造函数中移动 t.node 时，t.node 需要更改。但是 t 是常量，所以移动是无效的。 unique_ptr 不能复制构造，因此 struct Nested 也不能复制构造。

要使其正常工作，您需要提供移动构造函数并删除复制构造函数。像这样：

    struct Nested{
        std::unique_ptr<Container> node;
        Nested(std::unique_ptr<Container>&& t) : node(std::move(t)) {}
        Nested(Nested&& t) : node(std::move(t.node)) {}
        Nested(const Nested& t) =delete;
    };

Answer 2

tl;dr ：考虑使用 rule of zero

我会谨慎地在您的 class 中明确指定所有这些构造函数。 unique_ptr 无法复制这一事实会让您在按照您尝试的方式做事时遇到各种麻烦。相反，当您不指定 any 构造函数时会发生以下情况：

#include <vector>
#include <memory>

struct Container
{
    struct Nested{
        std::unique_ptr<Container> node;
//
//        Nested(std::unique_ptr<Container> t) : node(std::move(t)) {}
//        Nested(const Nested& t) { node = std::move(t.node); };
    };

    std::vector<Nested> edges;
};

typedef std::unique_ptr<Container> UCont;
typedef Container::Nested Nested;


int main()
{
    auto c1 = new Container{};
    auto c2 = new Container{};
    auto c3 = new Container{};
    std::unique_ptr<Container> u1 {c1};
    std::unique_ptr<Container> u2 {c2};
    std::unique_ptr<Container> u3 {c3};
    Nested n1 {std::move(u1)};
    Nested n2 {std::move(u2)};
    Nested n3 {std::move(u3)};
    auto v = std::vector<Nested>{3};
    v.push_back(std::move(n1));
    v.push_back(std::move(n2));
    v.push_back(std::move(n3));
    auto c5 = new Container { std::move(v) };
    std::unique_ptr<Container> object = UCont(std::move(c5));
}

为了清楚起见，我已将所有内容分解为较短的陈述（大部分）。

Answer 3

我收到错误的根本原因是在初始化列表中使用的对象中使用了 unique_ptr。类似于简单 unique_ptr 的向量（如此处 Initializing container of unique_ptrs from initializer list fails with GCC 4.7 ），在其数据模型中也具有 unique_ptr 的对象也不能在初始化列表中使用。

与另一个link相似，原因相同； initializer list 始终执行复制，并且 unique_ptr 无法复制。所以我们要用emplace_back/push_back.

因此，即使使用构造函数，以下解决方案也有效

struct Container
{
    struct Nested{
            std::unique_ptr<Container> node;

            Nested(): node(nullptr) {}
            Nested(std::unique_ptr<Container> t) : node(std::move(t)) {}
        };

    std::vector<Nested> edges;
};

typedef std::unique_ptr<Container> UCont;
typedef Container::Nested Nested;


int main()
{
        auto v = std::vector<Nested>{3};
        v.push_back(std::move(Nested(std::move(std::unique_ptr<Container>(new Container{})))));
        v.push_back(std::move(Nested(std::move(std::unique_ptr<Container>(new Container{})))));
        v.push_back(std::move(Nested(std::move(std::unique_ptr<Container>(new Container{})))));
        std::unique_ptr<Container> object = UCont(new Container { std::move(v) });

}

如何修复具有 unique_ptr 的对象的向量

How to fix vector of objects that have unique_ptr

c++

vector

stdvector

unique-ptr

c++11

tl;dr ：考虑使用 rule of zero