Priority Queue Sorting Objects Incorrectly (User-Defined Compare)

Question

I have a class Customer which has a member variable arrivalTime. I have defined a getter function getArrivalTime(). I am storing Customer in a priority queue, and have defined a custom predicate as follows:

class ArrivalQueueCompare
{
public:
    bool operator()(const Customer &a, const Customer &b)
    {
        return a.getArrivalTime() > b.getArrivalTime();
    }
};

The priority queue is declared as:

std::priority_queue<Customer, std::vector<Customer>, ArrivalQueueCompare> arrivalQueue;

When I push four Customer objects, a0, a1, a2, and a3, into the priority queue, with arrival times 20, 0, 50, and 30 respectively, the priority queue seems to have stored them in the order a1-a0-a2-a3.

According to my predicate the order should be a1-a0-a3-a2, but the priority queue is storing them otherwise. Why might this be?

I have attached screenshots from Xcode's debugger screen as a proof: Screenshot 1 Screenshot 2 Screenshot 3 Screenshot 4

Update:

I am reading in lines from a file and creating Customer objects with:

 while (std::getline(file, line))
    {
        Customer newCustomer = createCustomerObject(line);

        arrivalQueue.push(newCustomer);
    }

The createCustomerObject() function just creates, and returns, a Customer object by initialising the member variables of Customer using line.

Answer 1

A priority_queue doesn't store the object in a sorted fashion. It only guarantees that the first element is the largest one (depending on your compare).

So, observing the objects stored in a non-ordered fashion in the storage vector is expected.

When you start popping the objects with pop(), you'll see the remaining objects get reordered.