protected data structure elements passed by value across functions - c++

Question

I have a network application where multiple users have their userContext saved in the server. Users continuously send different requests to the server and the server processes them asynchronously using a thread pool and an event loop (like epoll for example).
All the users have their own ID. This id is unique with respect to the server. The server stores the user context in a map<int,userContext>. When the server receives a message from one user, it decodes the message. It also finds what is the request and userID etc. Then the server processes the request and it also updates (as required, state machine updates in the userContext current state) the stored userContext in the map.
In my application, there is a lot of such procedure calls (few are nested also) and I am passing the user context by value. I can not pass the reference of the map values. (then protection no longer exists).
Below is a sample implementation of userContextStore and two procedure.
class userContext {
public:
int id;
int value1;
int value2;
userContext():id(-1),value1(-1),value2(-1){}
userContext(const userContext &context) {
this->id = context.id;
this->value1 = context.value1;
this->value2 = context.value2;
}
};
class contextStore {
public:
map<int,userContext> Map;
std::mutex m;
void update(userContext context,int id) {
lock_guard<std::mutex> lock(m);
if(Map.find(id) != Map.end()) {
Map[id] = context;
return;
}
Map[id] = context;
}
userContext getUserContext(int id) {
lock_guard<std::mutex> lock(m);
userContext context(Map[id]);
return context;
}
void printContext(int id) {
lock_guard<std::mutex> lock(m);
if(Map.find(id) != Map.end()) {
userContext temp(Map[id]);
cout << temp.value1 << endl;
cout << temp.value2 << endl;
}
}
};
void procedureA(contextStore &store,userContext context) {
// do some long processing using the provided context
// change context copy in between
// based on the above processing
context.value1 += 20; // example of a change
int id = context.id;
store.update(context,id);
}
void procedureB(contextStore &store,int id) {
userContext context(store.getUserContext(id));
// do some other long processing
// change context copy in between
context.value1 -= 10; // example of a change
store.update(context,id);
}
Is there any better (possibly way to avoid object copy multiple times) to pass the userContext objects for the required modification inside a particular procedure call?
The second issue is that I am using a fat lock protecting the entire map. In this case, if one user request is under process in the server, then other user request's will not be processed. (because of the lock protects the entire map). Is there any way to have to fine grain lock for such situation?
Thanks!