priority_queue

<queue>

Priority queue

Priority queues are a type of container adaptors, specifically designed such that its first element is always the greatest of the elements it contains, according to some strict weak ordering condition.

This context is similar to a heap where only the max heap element can be retrieved (the one at the top in the priority queue) and elements can be inserted indefinitely.

Priority queues are implemented as container adaptors, which are classes that use an encapsulated object of a specific container class as its underlying container, providing a specific set of member functions to access its elements. Elements are popped from the "back" of the specific container, which is known as the top of the priority queue.

The underlying container may be any of the standard container class templates or some other specifically designed container class. The only requirement is that it must be accessible through random access iterators and it must support the following operations:

front()

push_back()

pop_back()

Therefore, the standard container class templates and can be used. By default, if no container class is specified for a particular priority_queue class, the standard container class template is used.

Support for random access iterators is required to keep a heap structure internally at all times. This is done automatically by the container adaptor by calling the algorithms , and when appropriate.

In their implementation in the C++ Standard Template Library, priority queues take three template parameters:

1 2	template < class T, class Container = vector , class Compare = less > class priority_queue;

Where the template parameters have the following meanings:

T: Type of the elements.

Container: Type of the underlying container object used to store and access the elements.

Compare: Comparison class: A class such that the expression comp(a,b), where comp is an object of this class anda and b are elements of the container, returns true if a is to be placed earlier than b in a strict weak ordering operation. This can either be a class implementing a function call operator or a pointer to a function. This defaults to less<T>, which returns the same as applying the less-than operator (a<b).
The priority_queue object uses this expression when an element is inserted or removed from it (using or, respectively) to grant that the element popped is always the greatest in the priority queue.

In the reference for the priority_queue member functions, these same names (T, Container and Compare) are assumed for the template parameters.

Member functions

Construct priority queue (public member function )

Test whether container is empty (public member function)

Return size (public member function)

Access top element (public member function)

Insert element (public member function)

Remove top element (public member function)

priority_queue是一种容器适配器。容器适配器的意思就是说其底层实现依赖于某一特定容器，使用该容器来存储数据。容器适配器只是对该容器的一层封装，以满足上下文应用的需要。容器适配器对其成员函数的调用最终是对容器的函数的调用。C++ STL中定义了三种Container Adapter：Stack(LIFO)、Queue(FIFO)和Priority_queue(max heap)。其中priority_queue底层封装的容器默认是vector。

priority_queue构造的好像是大根堆，而且当插入或删除元素时会自动调用“algorithm.h”（主要是STL中常用算法）文件中的 , and 函数来调整，使其保持大根堆的特性。由于是大根堆，每次调用top()函数将会返回priority_queue中最大值的引用。

注意：priority_queue的定义在头文件“queue.h”中，所以使用时要添加

#include <queue>

下面这个例子就很好地说明了priority_queue的内部存储结构：

// priority_queue::push/pop#include 
    
     #include 
     
      using namespace std;int main (){  priority_queue
      
        mypq;  mypq.push(30);  mypq.push(100);  mypq.push(25);  mypq.push(40);  cout << "Popping out elements...";  while (!mypq.empty())  {     cout << " " << mypq.top();     mypq.pop();  }  cout << endl;  return 0;}

以下转自：

在优先队列中，优先级高的元素先出队列。

标准库默认使用元素类型的<操作符来确定它们之间的优先级关系。

优先队列的第一种用法，也是最常用的用法：