expression having type 'const CompareVPtrs' would lose some const-volatile qualifiers in order to call

Question

I am implementing fifteen puzzle console game in C++, raises the following error

Error 4  error C3848: expression having type 'const CompareVPtrs' would lose some
   const-volatile qualifiers in order to call 
   'bool CompareVPtrs::operator ()(Vertex *,Vertex *)'    
   c:\program files\microsoft visual studio 11.0\vc\include\xfunctional   
  324 1  puzzle15

This is my structure

struct CompareVPtrs: public binary_function<Vertex*, Vertex*, bool>
{
    bool operator()( Vertex *lhs, Vertex *rhs)
    {
        return equal((int *)lhs->state, (int *)lhs->state+16,
            (int *)rhs->state);
    }
}
CompareVP;

The full game source code https://gist.github.com/sunloverz/7338003

Answer 1

It means your comparison operator needs to be const:

bool operator()( Vertex *lhs, Vertex *rhs) const
{ //                                       ^^^^^
  ....
}