Trouble with passing class objects by reference

[tmorlan]

Hi, I am working on a program to simulate a zoo, essentially there are passengers that are waiting for a ride in cars around the zoo. This is a multi-threaded program using the std::thread class, and essentially I pass an array of passengers and an array of cars into a function as a thread is initialized, then based on some the values of the objects, I place cars and passengers each into a std::queue. Then each passenger is paired with a car until one of the queues is empty. There is much more to the program than this, but I think this is enough to show the problem I am having.

The problem is that when I modify the member variables of the "front" member of each queue, the values don't stick when I access the values directly. You can see in the code below that I try to print the value

Here is the header of the function that the next code snippets are from.
void carsManage(int time, car cars[], gasStation& station, visitor visitors[], int numVisitors, int numPumps, int numCars, int rideTime)

here you can see the queues are initialized and filled with certain objects

queue<visitor> rideQueue;
	queue<car> carQueue;

for (int i = 0; i < numVisitors; i++)//checking for visitors that need rides
	{
		if (!visitors[i].getRidden())
			rideQueue.push(visitors[i]);
	}

	for (int i = 0; i < numCars; i++)//adding all ready cars to queue
	{
		if (cars[i].getReady())
			carQueue.push(cars[i]);
	}

In the code below there are two std::cout statements that should return the same values for each object, but they do not.

while (rideQueue.size() > 0 && carQueue.size() > 0)//putting drivers in cars until one runs out
	{
		rideQueue.front().setRidden(true);
		rideQueue.pop();
		carQueue.front().setRides(carQueue.front().getRides() - 1);
		carQueue.front().setReady(false);
		carQueue.front().setLeaveTime(time);
		cout << carQueue.front().getLeaveTime() << "\n";
		carQueue.pop();
	}
	int carsOut = 0;
	
	for (int i = 0; i < numCars; i++)//calculating cars out driving
	{
		cout << cars[i].getLeaveTime() << "\n";
		if (cars[i].getLeaveTime() <= time && cars[i].getLeaveTime() > 0)
			carsOut += 1;
	}

also, here is the thread initialization, if that helps clarify anything

thread carAgent(carsManage, time, cars, ref(station), visitors, numVisitors, numPumps, numCars, rideTime);

If anything needs to be clarified, let me know; Any help would be greatly appreciated.

[tmorlan]

Alright after doing a lot of debugging I narrowed it down to the queue class. Apparently, modifying the front element of the queue class isn't done by reference, so idk what to do about that.

[tmorlan]

Is there a way to push an object into a queue by reference?

[LB]

std::queue::front returns by reference.

I'm not really sure what your code does, but this looks highly suspicious to me:

		rideQueue.front().setRidden(true);
		rideQueue.pop();
		carQueue.front().setRides(carQueue.front().getRides() - 1);
		carQueue.front().setReady(false);
		carQueue.front().setLeaveTime(time);
		cout << carQueue.front().getLeaveTime() << "\n";
		carQueue.pop();

Why set all that information and then destroy it? Do you understand what pop() does?

[tmorlan]

Well this particular thread is generated many times in the loop, so the queue itself is populated and depopulated with every iteration. That's not the problem though. And yeah I know front returns the reference, but the problem I am having is that push does not actually pass by reference, rather by value. If you know a way to fix this, that would be awesome, but either way thank you for the response.

[LB]

tmorlan wrote:

Is there a way to push an object into a queue by reference?

Maybe you want to use emplace?

http://www.cplusplus.com/reference/queue/queue/emplace/

I'm not sure why you are asking for what you are asking for. If an object is not in a container, there's no way to ever make that object be in that container. You would work around this by using e.g. smart pointers wrapper classes such as std::unique_ptr so that the object is persistent but the wrapper class that holds it can be juggled around, but you may want to reconsider your design.

[cire]

I think the problem is that you want to store a reference to an existing object in your containers, and you can't do that directly. Consider storing a std::reference_wrapper or a pointer to the object instead.

http://en.cppreference.com/w/cpp/utility/functional/reference_wrapper

[tmorlan]

Thanks for the responses guys. cire, you were right, that was the issue and making the queue of objects a queue of object pointers solved it. I know my design is sh*t, and I wish I could redesign it, but I'm just a lowly college student, and this is a project that was due yesterday. Thanks again!