Question

I’m experiencing an constness issue when attempting to insert a pair into a map. The compiler error is:

              c:\program files (x86)\microsoft visual studio 11.0\vc\include\xutility(2089) : see reference to function template instantiation 'std::pair<_Ty1,_Ty2> &std::pair<_Ty1,_Ty2>::operator =(const std::pair<_Ty1,_Ty2> &)' being compiled
1>          with
1>          [
1>              _Ty1=const Assets::AssetId,
1>              _Ty2=std::shared_ptr<Assets::Material>
1>          ]
1>          c:\fusionengine\meshgl.cpp(85) : see reference to class template instantiation 'std::pair<_Ty1,_Ty2>' being compiled
1>          with
1>          [
1>              _Ty1=const Assets::AssetId,
1>              _Ty2=std::shared_ptr<Assets::Material>
1>          ]

The line causing the error is:

m_materials.insert( MaterialsMap::value_type(pMaterial->AssetId(), pMaterial) );

The m_materials map is declared as follows:

typedef std::map< Assets::AssetId, std::shared_ptr<Material> > MaterialsMap;    
typedef std::pair< Assets::AssetId, std::shared_ptr<Material> > MtlPair;

MaterialsMap  m_materials;

Error 1 error C2166: l-value specifies const object c:\program files (x86)\microsoft visual studio 11.0\vc\include\utility 114

Can anyone explain how I resolve this issue?

Answer 1

This code compiles fine with GCC:

#include <map>
#include <memory>
using namespace std;

typedef int AssetId;
struct Material {
    int _id;
    Material(int id) : _id(id) {}
    int AssetId() const { return _id; }
};
typedef std::map< AssetId, std::shared_ptr<Material> > MaterialsMap;    

MaterialsMap  m_materials;

int main() {
    std::shared_ptr<Material> pMaterial(new Material(42));
    m_materials.insert( MaterialsMap::value_type(pMaterial->AssetId(), pMaterial) );
}

Either your example is incomplete or wrong, or this is a bug in the implementation of the standard library of MSVC2012. It shall not call operator= in the above code.