A simple example
class __declspec(dllexport) A
{
public:
vector<unique_ptr<int>> v;
};
Error in VS2013 compilation for deleted copy constructor of unique_ptr
. If I remove __declspec(dllexport)
, it is fine. If I use only unique_ptr<int> v
, it is fine too. Is this a compiler bug? Any way to work around it? Thanks.
You may try it on http://webcompiler.cloudapp.net/ with the following complete code
#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;
}
yielding the compiler error:
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
]
It appears adding __declspec(dllexport)
forces the compiler to define the implicitly-declared copy constructor and copy assignment operator (normally, this only happens if they're used). These in turn call the copy constructor/assignment operator of v
. But the copy operations of std::vector<T>
are ill-formed for a non-copyable T
, such as std::unique_ptr
. Hence the error.
When the member is just std::unique_ptr
, the problem does not occur, because that has copy operations explicitly deleted and so the default copy operations of A
become deleted as well.
So the issue is solved if you explicitly delete the copy operations:
class __declspec(dllexport) A
{
public:
A(const A&) = delete;
A& operator=(const A&) = delete;
vector<unique_ptr<int>> v;
};
Of course, if you want copy functionality, defining them yourself would help as well.
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