You define a function that takes the model for the whole problem _mdl and a solution, and synchronizes the solution with the model. As discussed, the objective stored in the solution may be inconsistent with the model you wish to work on. You correct this with the following function which extracts the model passed, then restores and stores the solution. A subgoal is also executed. After storing, the solution's objective is consistent with the model. This function is used in the improvement iteration phase.

Add the following code after the comment //Synchronize the solution

void RoutingSolver::syncSolution(IloModel mdl, IloSolution s, IloGoal g) {
  _solver.extract(mdl);
  if ( !_solver.solve( IloRestoreSolution( _solver.getEnv(), s) && g))
    _solver.out() << "Synchronization failed" << endl;
  else
    s.store(_solver);
}

The following member function is called after the iterative improvements have ended to do a final solution synchronization. This code is provided for you:

void RoutingSolver::syncSolution() {
  syncSolution(_mdl, _solution, _instantiateCost);
}