C++ 如何移动包含唯一_ptr作为成员的对象向量？

我有一个函数，它返回一个类的

std:：vector

，该类包含一个

std:：unique\u ptr

作为成员。我需要将这个向量对象存储在堆上，这样我就可以通过C风格的DLL接口传递它

请参阅以下代码示例：

#include <iostream>
#include <cstdlib>
#include <memory>
#include <vector>

// I have control over the following two classes
class SomeBigClassWithManyMembers { };

class MyClass
{
    std::unique_ptr<SomeBigClassWithManyMembers> up;

public:
    static const std::vector<MyClass> GetMany()
    {
        // imagine this does lots of work
        return std::vector<MyClass>(50);
    }

    // following code is suggested in https://stackoverflow.com/questions/31430533/move-assignable-class-containing-vectorunique-ptrt but doesn't help
    /*
    MyClass() { }
    MyClass(MyClass&& other): up{std::move(other.up)} { }
    MyClass& operator=(MyClass&& other)
    {
        up = std::move(other.up);
        return *this;
    }
    */
};

// Imagine that I can't change this C-style code - it's a fixed interface provided by someone else
struct NastyCStyleStruct
{
    void* v;
};
void NastyCStyleInterface(NastyCStyleStruct s) { printf("%u", (unsigned int)((std::vector<MyClass>*)s.v)->size()); }

int main()
{
    NastyCStyleStruct s;
    s.v = new std::vector<MyClass>(std::move(MyClass::GetMany()));
    NastyCStyleInterface(s);
    return 0;
}

不够。向量必须存储在堆上

这里的问题是代码似乎试图使用

MyClass

的复制构造函数（不存在）。我不明白为什么要调用复制构造函数，因为我明确地要求使用

std:：move

进行移动语义。甚至将

s.v

初始化为合适大小的新

new std:：vector

，并调用

std:：move

的三参数版本都不起作用

来自g++的错误：

In file included from /usr/include/c++/7/memory:64:0,
                 from stackq.cpp:4:
/usr/include/c++/7/bits/stl_construct.h: In instantiation of ‘void std::_Construct(_T1*, _Args&& ...) [with _T1 = MyClass; _Args = {const MyClass&}]’:
/usr/include/c++/7/bits/stl_uninitialized.h:83:18:   required from ‘static _ForwardIterator std::__uninitialized_copy<_TrivialValueTypes>::__uninit_copy(_InputIterator, _InputIterator, _ForwardIterator) [with _InputIterator = __gnu_cxx::__normal_iterator<const MyClass*, std::vector<MyClass> >; _ForwardIterator = MyClass*; bool _TrivialValueTypes = false]’
/usr/include/c++/7/bits/stl_uninitialized.h:134:15:   required from ‘_ForwardIterator std::uninitialized_copy(_InputIterator, _InputIterator, _ForwardIterator) [with _InputIterator = __gnu_cxx::__normal_iterator<const MyClass*, std::vector<MyClass> >; _ForwardIterator = MyClass*]’
/usr/include/c++/7/bits/stl_uninitialized.h:289:37:   required from ‘_ForwardIterator std::__uninitialized_copy_a(_InputIterator, _InputIterator, _ForwardIterator, std::allocator<_Tp>&) [with _InputIterator = __gnu_cxx::__normal_iterator<const MyClass*, std::vector<MyClass> >; _ForwardIterator = MyClass*; _Tp = MyClass]’
/usr/include/c++/7/bits/stl_vector.h:331:31:   required from ‘std::vector<_Tp, _Alloc>::vector(const std::vector<_Tp, _Alloc>&) [with _Tp = MyClass; _Alloc = std::allocator<MyClass>]’
stackq.cpp:43:65:   required from here
/usr/include/c++/7/bits/stl_construct.h:75:7: error: use of deleted function ‘MyClass::MyClass(const MyClass&)’
     { ::new(static_cast<void*>(__p)) _T1(std::forward<_Args>(__args)...); }
       ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
stackq.cpp:10:7: note: ‘MyClass::MyClass(const MyClass&)’ is implicitly deleted because the default definition would be ill-formed:
 class MyClass
       ^~~~~~~
stackq.cpp:10:7: error: use of deleted function ‘std::unique_ptr<_Tp, _Dp>::unique_ptr(const std::unique_ptr<_Tp, _Dp>&) [with _Tp = SomeBigClassWithManyMembers; _Dp = std::default_delete<SomeBigClassWithManyMembers>]’
In file included from /usr/include/c++/7/memory:80:0,
                 from stackq.cpp:4:
/usr/include/c++/7/bits/unique_ptr.h:388:7: note: declared here
       unique_ptr(const unique_ptr&) = delete;
       ^~~~~~~~~~

在/usr/include/c++/7/memory:64:0中包含的文件中，
来自stackq.cpp:4：
/usr/include/c++/7/bits/stl_-construct.h:在“void std:：_-construct（_-T1*，_-Args&&&…[其中_-T1=MyClass；_-Args={const-MyClass&}]”的实例化中：
/usr/include/c++/7/bits/stl_uninitialized.h:83:18:从“静态”转发器std:：__uninitialized_copy:：_uninitialized_copy（_inputierator，_inputierator，_ForwardIterator，_ForwardIterator）[带_inputierator=u gnu cxx:_normal u迭代器；_ForwardIterator=MyClass*；bool _-valuetypes=false]”
/usr/include/c++/7/bits/stl\u uninitialized.h:134:15:从“\u ForwardIterator std:：uninitialized\u copy（\u InputIterator，\u InputIterator，\u ForwardIterator，\u ForwardIterator）[with\u InputIterator=\u gnu\u cxx:：\u normal\u ForwardIterator=MyClass*]中需要
/usr/include/c++/7/bits/stl_uninitialized.h:289:37:从“_forwarditeratorstd:：__uninitialized_copy_a（_inputierator，_inputierator，_ForwardIterator，std:：allocator&”）中需要[with_inputierator=_gnu_cxx:_normal_iterator；_ForwardIterator=MyClass*_Tp=MyClass]”
/usr/include/c++/7/bits/stl_vector.h:331:31:必须来自“std:：vector:：vector（const std:：vector&）[with _Tp=MyClass；_Alloc=std:：allocator]”
stackq.cpp:43:65:从这里开始需要
/usr/include/c++/7/bits/stl_construct.h:75:7:错误：使用了已删除的函数“MyClass:：MyClass（const MyClass&）”
{：：新建（静态_-cast（u-p））_-T1（标准：：转发（u-args））；}
^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
stackq.cpp:10:7:注意：“MyClass:：MyClass（const MyClass&）”被隐式删除，因为默认定义的格式可能不正确：
类MyClass
^~~~~~~
stackq.cpp:10:7:错误：使用删除的函数“std:：unique\u ptr:：unique\u ptr（const std:：unique\u ptr&）[使用_Tp=SomeBigClassWithManyMembers；_Dp=std:：default\u delete]”
在/usr/include/c++/7/memory:80:0中包含的文件中，
来自stackq.cpp:4：
/usr/include/c++/7/bits/unique_ptr.h:388:7：注意：此处声明
唯一\u ptr（const unique \u ptr&）=删除；
^~~~~~~~~~

---

如何修复此代码以使向量对象本身存储在堆上？

这里的问题是

GetMany

被定义为返回

const std:：vector

。即使它是按值返回的，因此如果您改变结果，也不会产生不必要的副作用，编译器仍然强制执行该类型（

auto

毕竟只是复制了函数的确切返回类型），因此无法从中移动。因此，与其廉价地复制几个指针大小的值（到

向量

内容、大小和容量），它必须进行完整的复制构造，包括复制构造原始

向量

中存储的所有值。由于存储实例中的

唯一\u ptr

成员，该操作失败

---

只需从返回类型中删除

const

就可以让

std:：move

/

vector

的move构造函数完成它们的工作，从而可以超便宜地提取

GetMany

返回的

vector

的内容，而无需复制（甚至移动）对于

MyClass

的任何实例，都会调用copy构造函数，因为

GetMany

被定义为返回

const std:：vector

，而不是

std:：vector

，因此

std:：move

实际上无法将其转换为r值；离开它将违反

常量

-ness，因此复制是唯一的选择。很可能你不需要堆上的

向量

（虽然它可能会使代码更容易），你只需要停止禁止复制省略/移动语义。啊，是的，我真傻，没有注意到这一点！删除

GetMany

上的

const

现在可以编译代码了。@MarekR:OP的代码似乎是为了使用一个奇怪的C类接口，所以它们可能没有太多选择。@ShadowRanger我知道这一点，但将指向

std:：vector

的指针传递给C API显然是错误的（我不认为这种情况可能是正确的）。很可能他需要

std:：vector:：data

返回的值。还要注意的是，他是个新手，所以这一定是置换编程的结果。事实上，我怀疑。

In file included from /usr/include/c++/7/memory:64:0,
                 from stackq.cpp:4:
/usr/include/c++/7/bits/stl_construct.h: In instantiation of ‘void std::_Construct(_T1*, _Args&& ...) [with _T1 = MyClass; _Args = {const MyClass&}]’:
/usr/include/c++/7/bits/stl_uninitialized.h:83:18:   required from ‘static _ForwardIterator std::__uninitialized_copy<_TrivialValueTypes>::__uninit_copy(_InputIterator, _InputIterator, _ForwardIterator) [with _InputIterator = __gnu_cxx::__normal_iterator<const MyClass*, std::vector<MyClass> >; _ForwardIterator = MyClass*; bool _TrivialValueTypes = false]’
/usr/include/c++/7/bits/stl_uninitialized.h:134:15:   required from ‘_ForwardIterator std::uninitialized_copy(_InputIterator, _InputIterator, _ForwardIterator) [with _InputIterator = __gnu_cxx::__normal_iterator<const MyClass*, std::vector<MyClass> >; _ForwardIterator = MyClass*]’
/usr/include/c++/7/bits/stl_uninitialized.h:289:37:   required from ‘_ForwardIterator std::__uninitialized_copy_a(_InputIterator, _InputIterator, _ForwardIterator, std::allocator<_Tp>&) [with _InputIterator = __gnu_cxx::__normal_iterator<const MyClass*, std::vector<MyClass> >; _ForwardIterator = MyClass*; _Tp = MyClass]’
/usr/include/c++/7/bits/stl_vector.h:331:31:   required from ‘std::vector<_Tp, _Alloc>::vector(const std::vector<_Tp, _Alloc>&) [with _Tp = MyClass; _Alloc = std::allocator<MyClass>]’
stackq.cpp:43:65:   required from here
/usr/include/c++/7/bits/stl_construct.h:75:7: error: use of deleted function ‘MyClass::MyClass(const MyClass&)’
     { ::new(static_cast<void*>(__p)) _T1(std::forward<_Args>(__args)...); }
       ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
stackq.cpp:10:7: note: ‘MyClass::MyClass(const MyClass&)’ is implicitly deleted because the default definition would be ill-formed:
 class MyClass
       ^~~~~~~
stackq.cpp:10:7: error: use of deleted function ‘std::unique_ptr<_Tp, _Dp>::unique_ptr(const std::unique_ptr<_Tp, _Dp>&) [with _Tp = SomeBigClassWithManyMembers; _Dp = std::default_delete<SomeBigClassWithManyMembers>]’
In file included from /usr/include/c++/7/memory:80:0,
                 from stackq.cpp:4:
/usr/include/c++/7/bits/unique_ptr.h:388:7: note: declared here
       unique_ptr(const unique_ptr&) = delete;
       ^~~~~~~~~~