Question

The problem I’m having is that I want to use STL’s sort with a custom compare function inside a templated class.

The idea from using typedef came from another Stackoverflow Post

Anyway, here is the code:

template <typename R,typename S>
class mesh{
  /* some stuff */

  void sortData(){
    typedef bool (*comparer_t)(const S,const S);
    comparer_t cmp = &mesh::compareEdgesFromIndex;
    sort(indx,indx+sides*eSize,cmp);
  }

  /* more stuff */

  // eData and cIndx are member variables
  bool compareEdgesFromIndex(const S a,const S b){
    return compareEdges(eData[cIndx[2*a]],eData[cIndx[2*a+1]],eData[cIndx[2*b]],eData[cIndx[2*b+1]]); 
  }
};

The error I’m getting is

mesh.h:130:29: error: cannot convert ‘bool (mesh<float, unsigned int>::*)(unsigned int, unsigned int)’ to ‘comparer_t {aka\
bool (*)(unsigned int, unsigned int)}’ in initialization

Thank you in advance!

Answer 1

You are trying to mix a member-function-pointer where a function-pointer is required. You can either refactor the predicate to be a static function, or use bindings to associate your member-function-pointer with an instance of your class mesh.

In order to bind an instance to your member-function-pointer you would do

std::bind( mesh_instance, &mesh::compareEdgesFromIndex, _1, _2 )

if working with C++11. If you don’t have the luxury, then you can use equivalent functionality from Boost (replacing std::bind by boost::bind). C++03 offers some binding functionality but it’s limited, and I believe obsoleted now that generic binding functionality is available.