I recently had to change the linkage specification of several classes and ran into a problem. Two of the classes contain a std::map
with a std::unique_ptr
as the value type. After the linkage was changed the compiler started complaining with "cannot access private member declared in class 'std::unique_ptr<_Ty>'" errors.
Anyone know why this only happens when an export specification is supplied or have a solution?
Sample Code:
#include <map>
struct SomeInterface
{
virtual ~SomeInterface() = 0;
};
// This class compiles with no problems
struct LocalClass
{
std::map<int, std::unique_ptr<SomeInterface>> mData;
};
// This class fails to compile
struct __declspec(dllexport) ExportedClass
{
std::map<int, std::unique_ptr<SomeInterface>> mData;
};
Compiler output:
c:\program files (x86)\microsoft visual studio 10.0\vc\include\utility(163): error C2248: 'std::unique_ptr<_Ty>::unique_ptr' : cannot access private member declared in class 'std::unique_ptr<_Ty>'
with
[
_Ty=SomeInterface
]
c:\program files (x86)\microsoft visual studio 10.0\vc\include\memory(2347) : see declaration of 'std::unique_ptr<_Ty>::unique_ptr'
with
[
_Ty=SomeInterface
]
c:\program files (x86)\microsoft visual studio 10.0\vc\include\utility(195) : see reference to function template instantiation 'std::_Pair_base<_Ty1,_Ty2>::_Pair_base<const int&,_Ty2&>(_Other1,_Other2)' being compiled
with
[
_Ty1=const int,
_Ty2=std::unique_ptr<SomeInterface>,
_Other1=const int &,
_Other2=std::unique_ptr<SomeInterface> &
]
c:\program files (x86)\microsoft visual studio 10.0\vc\include\xmemory(208) : see reference to function template instantiation 'std::pair<_Ty1,_Ty2>::pair<const _Kty,_Ty>(std::pair<_Ty1,_Ty2> &)' being compiled
with
[
_Ty1=const int,
_Ty2=std::unique_ptr<SomeInterface>,
_Kty=int,
_Ty=std::unique_ptr<SomeInterface>
]
c:\program files (x86)\microsoft visual studio 10.0\vc\include\xmemory(280) : see reference to function template instantiation 'void std::allocator<_Ty>::construct<std::pair<_Ty1,_Ty2>&>(std::pair<_Ty1,_Ty2> *,_Other)' being compiled
with
[
_Ty=std::pair<const int,std::unique_ptr<SomeInterface>>,
_Ty1=const int,
_Ty2=std::unique_ptr<SomeInterface>,
_Other=std::pair<const int,std::unique_ptr<SomeInterface>> &
]
c:\program files (x86)\microsoft visual studio 10.0\vc\include\xtree(592) : see reference to function template instantiation 'void std::_Cons_val<std::allocator<_Ty>,_Ty,std::pair<_Ty1,_Ty2>&>(_Alloc &,std::pair<_Ty1,_Ty2> *,std::pair<_Ty1,_Ty2>)' being compiled
with
[
_Ty=std::pair<const int,std::unique_ptr<SomeInterface>>,
_Ty1=const int,
_Ty2=std::unique_ptr<SomeInterface>,
_Alloc=std::allocator<std::pair<const int,std::unique_ptr<SomeInterface>>>
]
c:\program files (x86)\microsoft visual studio 10.0\vc\include\xtree(1521) : see reference to function template instantiation 'std::_Tree_nod<_Traits>::_Node *std::_Tree_val<_Traits>::_Buynode<std::pair<_Ty1,_Ty2>&>(_Valty)' being compiled
with
[
_Traits=std::_Tmap_traits<int,std::unique_ptr<SomeInterface>,std::less<int>,std::allocator<std::pair<const int,std::unique_ptr<SomeInterface>>>,false>,
_Ty1=const int,
_Ty2=std::unique_ptr<SomeInterface>,
_Valty=std::pair<const int,std::unique_ptr<SomeInterface>> &
]
c:\program files (x86)\microsoft visual studio 10.0\vc\include\xtree(1516) : while compiling class template member function 'std::_Tree_nod<_Traits>::_Node *std::_Tree<_Traits>::_Copy(std::_Tree_nod<_Traits>::_Node *,std::_Tree_nod<_Traits>::_Node *)'
with
[
_Traits=std::_Tmap_traits<int,std::unique_ptr<SomeInterface>,std::less<int>,std::allocator<std::pair<const int,std::unique_ptr<SomeInterface>>>,false>
]
c:\program files (x86)\microsoft visual studio 10.0\vc\include\map(81) : see reference to class template instantiation 'std::_Tree<_Traits>' being compiled
with
[
_Traits=std::_Tmap_traits<int,std::unique_ptr<SomeInterface>,std::less<int>,std::allocator<std::pair<const int,std::unique_ptr<SomeInterface>>>,false>
]
c:\projects\so\so\so.cpp(18) : see reference to class template instantiation 'std::map<_Kty,_Ty>' being compiled
with
[
_Kty=int,
_Ty=std::unique_ptr<SomeInterface>
]
The error is given because the compiler can't create the copy constructor and copy assignment operator for ExportedClass. That would require copying unique_ptr objects which don't have copy constructors (they are movable but not copyable).
For the normal class the error is not given because the copy constructor/assignment is not actually used anywhere. However when __declspec(dllexport) is present all the compiler generated functions are instantiated (not sure about the right terminology here but something like that :).
One way to fix the error is to define those two functions for ExportedClass and mark them as private:
struct __declspec(dllexport) ExportedClass
{
std::map<int, std::unique_ptr<SomeInterface>> mData;
private:
ExportedClass(const ExportedClass&) {}
ExportedClass& operator=(const ExportedClass&) { return *this; }
};
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