Priority queues are a type of container adapters, specifically designed such that the first element of the queue is either the greatest or the smallest of all elements in the queue and elements are in nonincreasing order (hence we can see that each element of the queue has a priority {fixed order}).
However in C++ STL, by default, the top element is always the greatest element. We can also change it to the smallest element at the top. Priority queues are built on the top to the max heap and uses array or vector as an internal structure.