How to save nav_msgs/GetMap service and pass it into a function?

edit

So, what I'm going for is to rather having a callback function I want to make my node a client to that service and then pass it to the another function.

        ros::init(argc, argv, "dijkstra");
        ros::NodeHandle n("~");
        ros::ServiceClient client = n.serviceClient<nav_msgs::GetMap>("map");
        nav_msgs::GetMap srv;
        client.call(srv);
        nav_msgs::OccupancyGrid = srv.response;

Running the above code gives me the error-

 error: expected unqualified-id before ‘=’ token
   nav_msgs::OccupancyGrid = srv.response;

I need to pass the map to a function void dijkstra(???)