The following code defines a selector which selects the earliest activity to which no resource is assigned. This selector needs a visitor to visit all the activities of the environment, a predicate to filter the ones that are still not allocated, and an evaluator to select the activity with the smallest minimal start time.

// selector for non-assigned activity
 
ILOVISITOR0(ForAllActivitiesInEnv,
	    IloActivity, 
	    IloEnv, env) {
  for(IloIterator<IloActivity> ite(env); ite.ok(); ++ite) {
    visit(*ite);
  }
}
 
ILOPREDICATE0(IsAllocatedPredicate,
	      IloActivity, act) {
  if (act.getObject() == 0)
    return IloFalse;
  else
    return IloTrue;
}
 
ILOEVALUATOR0(StartMinEvaluator,
	      IloActivity, act) {
  return act.getStartMin(); 
}
 
  IloPredicate<IloActivity> notAllocated = !IsAllocatedPredicate(env);
  IloSelector<IloActivity,IloEnv> selector =
    IloBestSelector<IloActivity,IloEnv>
    (ForAllActivitiesInEnv(env), 
     notAllocated , 
     StartMinEvaluator(env).makeLessThanComparator());
 

Solving the problem consequently consists of iterating the following loop.

The following function implements the algorithm. It returns IloTrue if the problem has a solution. Otherwise, it returns IloFalse. The function can return IloFalse only if there exists a time point t such that the number of activities executing at t exceeds the number of unary resources that are available. In other words, the function returns IloTrue if the global resource capacity constraint is satisfied by the solution; it returns IloFalse if the global resource capacity constraint is not satisfied by the solution.

IloBool
SolveProblem(IloSolver solver, IloModel model) {
  IloEnv env = model.getEnv();
  IlcScheduler scheduler = IlcScheduler(solver);
  IloActivity act;
 
  IloPredicate<IloActivity> notAllocated = !IsAllocatedPredicate(env);
  IloSelector<IloActivity,IloEnv> selector =
    IloBestSelector<IloActivity,IloEnv>
    (ForAllActivitiesInEnv(env), 
     notAllocated , 
     StartMinEvaluator(env).makeLessThanComparator());
 
  while ( selector.select(act, env) ) {
    for (IloIterator<IloUnaryResource> iterator(model.getEnv());
         notAllocated(act);
         ++iterator) {
      if (iterator.ok()) {
        IloUnaryResource res = *iterator;
        IloConstraint ct = act.requires(res);
        model.add(ct);
 
        if(solver.solve()) 
        {
          // store the assigned resource for the given activity
          act.setObject( (void*)(*iterator).getImpl() );
        }
        else {
          model.remove(ct);
        }
      }
      else
        return IloFalse;
    }
  }
  return IloTrue;
}