如何使用Visual Studio在dll导出的类中使用唯一指针的向量

Question

一个简单的例子

class __declspec(dllexport) A
{
public:
    vector<unique_ptr<int>> v;
};

VS2013编译中已删除的复制构造函数的错误unique_ptr.如果我删除__declspec(dllexport),它很好.如果我只使用unique_ptr<int> v,它也没关系.这是编译器错误吗？有办法解决吗？谢谢.

您可以在http://webcompiler.cloudapp.net/上使用以下完整代码进行尝试

#include <iostream>
#include <vector>
#include <memory>
using namespace std;

class __declspec(dllexport) A
{
public:
    vector<unique_ptr<int>> v;
};

int main()
{
   cout << "Hello World" << endl; 
}

产生编译错误:

Compiled with /EHsc /nologo /W4 /c
main.cpp
main.cpp(9): warning C4251: 'A::v': class 'std::vector<std::unique_ptr<int,std::default_delete<_Ty>>,std::allocator<std::unique_ptr<_Ty,std::default_delete<_Ty>>>>' needs to have dll-interface to be used by clients of class 'A'
        with
        [
            _Ty=int
        ]
c:\tools_root\cl\inc\xutility(2144): error C2280: 'std::unique_ptr<int,std::default_delete<_Ty>> &std::unique_ptr<_Ty,std::default_delete<_Ty>>::operator =(const std::unique_ptr<_Ty,std::default_delete<_Ty>> &)': attempting to reference a deleted function
        with
        [
            _Ty=int
        ]
c:\tools_root\cl\inc\memory(1430): note: see declaration of 'std::unique_ptr<int,std::default_delete<_Ty>>::operator ='
        with
        [
            _Ty=int
        ]
c:\tools_root\cl\inc\xutility(2165): note: see reference to function template instantiation '_OutIt std::_Copy_impl<_InIt,_OutIt>(_InIt,_InIt,_OutIt,std::_Nonscalar_ptr_iterator_tag)' being compiled
        with
        [
            _OutIt=std::unique_ptr<int,std::default_delete<int>> *,
            _InIt=std::unique_ptr<int,std::default_delete<int>> *
        ]
c:\tools_root\cl\inc\vector(973): note: see reference to function template instantiation '_OutIt std::_Copy_impl<std::unique_ptr<int,std::default_delete<_Ty>>,std::unique_ptr<_Ty,std::default_delete<_Ty>>*>(_InIt,_InIt,_OutIt)' being compiled
        with
        [
            _OutIt=std::unique_ptr<int,std::default_delete<int>> *,
            _Ty=int,
            _InIt=std::unique_ptr<int,std::default_delete<int>> *
        ]
c:\tools_root\cl\inc\vector(956): note: while compiling class template member function 'std::vector<std::unique_ptr<int,std::default_delete<_Ty>>,std::allocator<std::unique_ptr<_Ty,std::default_delete<_Ty>>>> &std::vector<std::unique_ptr<_Ty,std::default_delete<_Ty>>,std::allocator<std::unique_ptr<_Ty,std::default_delete<_Ty>>>>::operator =(const std::vector<std::unique_ptr<_Ty,std::default_delete<_Ty>>,std::allocator<std::unique_ptr<_Ty,std::default_delete<_Ty>>>> &)'
        with
        [
            _Ty=int
        ]
main.cpp(10): note: see reference to function template instantiation 'std::vector<std::unique_ptr<int,std::default_delete<_Ty>>,std::allocator<std::unique_ptr<_Ty,std::default_delete<_Ty>>>> &std::vector<std::unique_ptr<_Ty,std::default_delete<_Ty>>,std::allocator<std::unique_ptr<_Ty,std::default_delete<_Ty>>>>::operator =(const std::vector<std::unique_ptr<_Ty,std::default_delete<_Ty>>,std::allocator<std::unique_ptr<_Ty,std::default_delete<_Ty>>>> &)' being compiled
        with
        [
            _Ty=int
        ]
main.cpp(9): note: see reference to class template instantiation 'std::vector<std::unique_ptr<int,std::default_delete<_Ty>>,std::allocator<std::unique_ptr<_Ty,std::default_delete<_Ty>>>>' being compiled
        with
        [
            _Ty=int
        ]

Answer 1

它似乎添加__declspec(dllexport)强制编译器定义隐式声明的复制构造函数和复制赋值运算符(通常,只有在使用它们时才会发生这种情况).这些反过来调用复制构造函数/赋值运算符v.但是复制操作std::vector<T>是不可复制的T,例如std::unique_ptr.因此错误.

当成员只是std::unique_ptr,问题不会发生,因为它具有显式删除的复制操作,因此默认的复制操作A也会被删除.

因此,如果您明确删除了复制操作,问题就解决了:

class __declspec(dllexport) A
{
public:
    A(const A&) = delete;
    A& operator=(const A&) = delete;
    vector<unique_ptr<int>> v;
};

当然,如果你想要复制功能,自己定义它们也会有所帮助.