C++ STL priority_queue

与普通的queue的不同是，程序员可以自定义其中数据的优先级, 让优先级高的排在队列前面,优先出队。优先队列具有队列的所有特性，并在这基础上添加了内部的一个排序，它本质是一个堆实现的，堆的插入和删除操作的时间复杂度都是log。

A priority queue is a container adaptor that provides constant time lookup of the largest (by default) element, at the expense of logarithmic insertion and extraction. （在stl container上面又封装了一次）。

A user-provided Compare can be supplied to change the ordering, e.g. using std::greater would cause the smallest element to appear as the top().

Working with a priority_queue is similar to managing a  heap in some random access container, with the benefit of not being able to accidentally invalidate the heap.

Defined in header

三个模板参数， container默认为vector, compare默认为std::less仿函数：

T	-	The type of the stored elements. The behavior is undefined if T is not the same type as Container::value_type. (since C++17)
Container	-	The type of the underlying container to use to store the elements. The container must satisfy the requirements of SequenceContainer, and its iterators must satisfy the requirements of LegacyRandomAccessIterator. Additionally, it must provide the following functions with the usual semantics: front() push_back() pop_back() The standard containers std::vector and std::deque satisfy these requirements.
Compare	-	A Compare type providing a strict weak ordering. Note that the Compare parameter is defined such that it returns true if its first argument comes before its second argument in a weak ordering. But because the priority queue outputs largest elements first, the elements that "come before" are actually output last. That is, the front of the queue contains the "last" element according to the weak ordering imposed by Compare.

template<

    class T,
    class Container = std::vector,
    class Compare = std::less

> class priority_queue;

 

Example:

#include 
#include 
#include 
#include 
 
template
void print_queue(T q) { // NB: pass by value so the print uses a copy
    while(!q.empty()) {
        std::cout << q.top() << ' ';
        q.pop();
    }
    std::cout << '\n';
}
 
int main() {
    std::priority_queue q;
 
    const auto data = {1,8,5,6,3,4,0,9,7,2};
 
    for(int n : data)
        q.push(n);
 
    print_queue(q);
 
    std::priority_queue, std::greater>
        q2(data.begin(), data.end());
 
    print_queue(q2);
 
    // Using lambda to compare elements.
    auto cmp = [](int left, int right) { return (left ^ 1) < (right ^ 1); };
    std::priority_queue, decltype(cmp)> q3(cmp);
 
    for(int n : data)
        q3.push(n);
 
    print_queue(q3);
}

9 8 7 6 5 4 3 2 1 0 
0 1 2 3 4 5 6 7 8 9 
8 9 6 7 4 5 2 3 0 1

关于legacy random access iterator:

C++ named requirements: LegacyRandomAccessIterator - cppreference.com

Ref:

std::priority_queue - cppreference.com