mxArray dynamic memory allocation

I want to read a struct (called ROAD) from a .mat file and create a map<string,double> from it so that the map is a reflection of the ROAD. The fields in the Matlab struct are arrays of different lengths, and this is the current state of the code.

I have a string vector called ROAD_fields that are just a list of the field names of the matlab struct. I also have a map<string,double> called ROAD that I want to fill.

      std::vector<std::string>  *ROAD_fields;
      std::map<std::string, double>  ROAD;

Here is the implementation so far:

      void Road::getRoadFromFile() {

  MATFile *file = matOpen(this->filename.c_str(),"r");
  mxArray *roadstruct = matGetVariable(file, "ROAD");

  int i;
  mxArray* field;
  for(i=0; i<this->ROAD_fields->size(); i++) {
    field = mxGetFieldByNumber(roadstruct,0,i);
    this->ROAD[this->ROAD_fields->at(i).c_str()] = this->extractField(field);
  }

      };

      double Road::extractField(mxArray *ptr) {
    return *(double*) mxGetPr(ptr);
      };

I get an access violation in the extractField call and I guess it's due to the fact that the arrays have different lengths? I think I need to reallocate memory somehow, but I haven't been able to figure out how.

Please help!