我如何移动包含unique_ptr作为成员的对象的向量？

Question

我有一个函数，该函数返回包含std::vector作为成员的类的std::unique_ptr。我需要将此向量对象存储在堆上，以便可以通过C风格的DLL接口传递它。

请参见下面的代码示例：

#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;
}

请注意，在我的实际代码中，向量需要使创建它的函数失效（因为这是在DLL中完成的，因此编写）>

auto vec = MyClass::GetMany();
s.v = &vec;
将不是

就足够了。该矢量必须存储在堆中。

这里的麻烦是代码似乎试图使用MyClass的（不存在）复制构造函数。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。甚至没有将s.v初始化为适当大小的新new std::vector并调用std::move的三参数版本也有效。

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;
       ^~~~~~~~~~
如何修复此代码，以便将矢量对象本身存储在堆中？

我有一个函数返回一个包含std :: unique_ptr作为成员的类的std :: vector。我需要将此向量对象存储在堆上，以便可以通过C风格的DLL接口传递它。 ...

Answer 1

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，所以如果您对结果进行变异，也不会有不良副作用的风险，编译器仍然会强制执行该类型（auto只是复制了该函数的确切返回类型），因此无法移动从中。因此，它不必廉价地复制几个指针/ size_t大小的值（复制到vector的内容，大小和容量），而是必须进行完整的复制构造，包括复制构造存储在原始vector中的所有值。 。由于存储实例中的unique_ptr成员，该操作失败。