The constructor is defined on the environment env. This constructor will be called from the main function. This code is provided for you:

RoutingModel::RoutingModel(IloEnv env)
  : _env(env),
    _depotPath("../../../examples/data/mdpdp/depots.csv"),
    _visitPath("../../../examples/data/mdpdp/vrp100.csv"),
    _nodePath ("../../../examples/data/mdpdp/node100.csv"),
    _nbOfDepots(0),
    _depots(0)
{
}