"cannot access private member'" error only when class has export linkage

Question

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>
      ]

Answer 1

Often a 'std::move(iUniquePtr)' is missing somewhere.

Answer 2

I ran into this a long time ago, so the details to this are a bit blurry.

In essence when you export a class you have to export all contained classes as well - public or not. In your case that would be std::map and std::unique_ptr. I'm not sure how classes in standard libraries behave here, that's the blurry part, but I remember having had problems with that.

The solution is to either export those classes or use a PIMPL implementation (which is a good idea for exported classes anyways).

Answer 3

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; }
};