-xbt_dict_t created_categories = NULL;
-xbt_dict_t declared_marks = NULL;
-xbt_dict_t user_host_variables = NULL;
-xbt_dict_t user_vm_variables = NULL;
-xbt_dict_t user_link_variables = NULL;
+xbt_dict_t created_categories = nullptr;
+xbt_dict_t declared_marks = nullptr;
+xbt_dict_t user_host_variables = nullptr;
+xbt_dict_t user_vm_variables = nullptr;
+xbt_dict_t user_link_variables = nullptr;