它是一个模板类：
template <class T, class Container = vector<T>, class Compare = less<typename Container::value_type> > class priority_queue;
默认情况下是 max heap, 默认的比较函数是 std::less<T>.
如果需要 min heap,或者是处理自定义的数据类型，需要提供定制的 比较函数。
priority_queue 的实现使用了 std::make_heap, std::push_heap, std::pop_heap.
在实现自定义的比较函数的时候，可以参考下面的提示：
std::make_heap
| default (1) | template <class RandomAccessIterator> void make_heap (RandomAccessIterator first, RandomAccessIterator last); |
|---|---|
| custom (2) | template <class RandomAccessIterator, class Compare> void make_heap (RandomAccessIterator first, RandomAccessIterator last, Compare comp ); |
Make heap from range
Rearranges the elements in the range [first,last) in such a way that they form a heap.
A heap is a way to organize the elements of a range that allows for fast retrieval of the element with the highest value at any moment (with pop_heap), even repeatedly, while allowing for fast insertion of new elements (with push_heap).
The element with the highest value is always pointed by first. The order of the other elements depends on the particular implementation, but it is consistent throughout all heap-related functions of this header.
The elements are compared using operator< (for the first version), or comp (for the second): The element with the highest value is an element for which this would return false when compared to every other element in the range.
The standard container adaptor priority_queue calls make_heap, push_heap and pop_heap automatically to maintain heap properties for a container.