std::priority_queue:在優先隊列中，優先級高的元素先出隊列，并非按照先進先出的要求，類似一個堆(heap)。其模板聲明帶有三個參數，priority_queue<Type, Container, Functional>, 其中Type為數據類型，Container為保存數據的容器，Functional為元素比較方式。Container必須是用數組實現的容器，比如 vector, deque. STL里面默認用的是vector. 比較方式默認用operator< , 所以如果把后面兩個參數缺省的話，優先隊列就是大頂堆，隊頭元素最大。

std::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 criterion. This context is similar to a heap, where elements can be inserted at any moment, and only the max heap element can be retrieved (the one at the top in the priority queue). 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.

A priority queue is a queue data structure that has the particular property of being sorted by priority. You can decide what priority to give items depending on the data type.

下面是從其他文章中copy的測試代碼，詳細內容介紹可以參考對應的reference：

#include "priority_queue.hpp"
#include <iostream>
#include <queue> // std::priority_queue
#include <string>
#include <vector>
#include <functional> // std::greater
#include <deque>
#include <list>//template < class T,
//	   class Container = vector<T>,
//	   class Compare = less<typename Container::value_type> >
//class priority_queue;///
// reference: http://www.cplusplus.com/reference/queue/priority_queue/
int test_priority_queue_1()
{
{ // std::priority_queue::push: Inserts a new element in the priority_queue.// The content of this new element is initialized to val. inserts element and sorts the underlying container// std::priority_queue::pop: Removes the element on top of the priority_queue, removes the top element,// effectively reducing its size by one. The element removed is the one with the highest value.// std::priority_queue::empty: Test whether container is emptystd::priority_queue<int> mypq;mypq.push(30);mypq.push(100);mypq.push(25);mypq.push(40);std::cout << "Popping out elements...";while (!mypq.empty()) {std::cout << ' ' << mypq.top();mypq.pop();}std::cout << '\n';
}{ // std::priority_queue::emplace: C++11, Adds a new element to the priority_queue.// This new element is constructed in place passing args as the arguments for its constructor.std::priority_queue<std::string> mypq;mypq.emplace("orange");mypq.emplace("strawberry");mypq.emplace("apple");mypq.emplace("pear");std::cout << "mypq contains:";while (!mypq.empty()) {std::cout << ' ' << mypq.top();mypq.pop();}std::cout << '\n';
}{// std::priority_queue::size: Returns the number of elements in the priority_queuestd::priority_queue<int> myints;std::cout << "0. size: " << myints.size() << '\n';for (int i = 0; i < 5; i++) myints.push(i);std::cout << "1. size: " << myints.size() << '\n';myints.pop();std::cout << "2. size: " << myints.size() << '\n';
}{ // std::priority_queue::top: Returns a constant reference to the top element in the priority_queue.// The top element is the element that compares higher in the priority_queue,// and the next that is removed from the container when priority_queue::pop is called.std::priority_queue<int> mypq;mypq.push(10);mypq.push(20);mypq.push(15);std::cout << "mypq.top() is now " << mypq.top() << '\n';
}{ // std::priority_queue::swap: C++11, Exchanges the contents of the container adaptor by those of x,// swapping both the underlying container value and their comparison function using// the corresponding swap non-member functions (unqualified)// std::swap (priority_queue): Exchange contents of priority queuesstd::priority_queue<int> foo, bar;foo.push(15); foo.push(30); foo.push(10);bar.push(101); bar.push(202);foo.swap(bar);//std::swap(foo, bar);std::cout << "size of foo: " << foo.size() << '\n';std::cout << "size of bar: " << bar.size() << '\n';
}return 0;
}//
// reference: http://en.cppreference.com/w/cpp/container/priority_queue
template<typename T>
static void print_queue(T& q) {while (!q.empty()) {std::cout << q.top() << " ";q.pop();}std::cout << '\n';
}int test_priority_queue_2()
{std::priority_queue<int> q;for (int n : {1, 8, 5, 6, 3, 4, 0, 9, 7, 2})q.push(n);print_queue(q);std::priority_queue<int, std::vector<int>, std::greater<int> > q2;for (int n : {1, 8, 5, 6, 3, 4, 0, 9, 7, 2})q2.push(n);print_queue(q2);// Using lambda to compare elements.auto cmp = [](int left, int right) { return (left ^ 1) < (right ^ 1); };std::priority_queue<int, std::vector<int>, decltype(cmp)> q3(cmp);for (int n : {1, 8, 5, 6, 3, 4, 0, 9, 7, 2})q3.push(n);print_queue(q3);return 0;
}/
// reference: http://www.technical-recipes.com/2011/priority-queues-and-min-priority-queues-in-c/
struct compare {bool operator()(const int& l, const int& r) {return l > r;}
};int test_priority_queue_3()
{using namespace std;priority_queue<int, vector<int>, compare > pq;pq.push(3);pq.push(5);pq.push(1);pq.push(8);std::cout << "pq contains:";while (!pq.empty()) {std::cout << ' ' << pq.top();pq.pop();}std::cout << '\n';return 0;
}/
// reference: https://msdn.microsoft.com/en-us/library/4ef4dae9.aspx
int test_priority_queue_4()
{
{ // priority_queue::container_type: A type that provides the base container to be adapted.// priority_queue::empty: Tests if a priority_queue is empty.// true if the priority_queue is empty; false if the priority_queue is nonempty.using namespace std;// Declares priority_queues with default deque base containerpriority_queue <int> q1, s2;q1.push(1);if (q1.empty()) cout << "The priority_queue q1 is empty." << endl;else cout << "The priority_queue q1 is not empty." << endl;if (s2.empty()) cout << "The priority_queue s2 is empty." << endl;else cout << "The priority_queue s2 is not empty." << endl;
}{ // priority_queue::pop: Removes the largest element of the priority_queue from the top position.using namespace std;priority_queue <int> q1, s2;q1.push(10);q1.push(5);q1.push(30);priority_queue <int>::size_type i, iii;i = q1.size();cout << "The priority_queue length is " << i << "." << endl;const int& ii = q1.top();cout << "The element at the top of the priority_queue is " << ii << "." << endl;q1.pop();iii = q1.size();cout << "After a pop, the priority_queue length is " << iii << "." << endl;const int& iv = q1.top();cout << "After a pop, the element at the top of the " << "priority_queue is " << iv << "." << endl;
}{ // priority_queue::priority_queue: Constructs a priority_queue that is empty or// that is a copy of a range of a base container object or of another priority_queueusing namespace std;// The first member function declares priority_queue// with a default vector base containerpriority_queue <int> q1;cout << "q1 = ( ";while (!q1.empty()) {cout << q1.top() << " ";q1.pop();}cout << ")" << endl;// Explicitly declares a priority_queue with nondefault// deque base containerpriority_queue <int, deque <int> > q2;q2.push(5);q2.push(15);q2.push(10);cout << "q2 = ( ";while (!q2.empty()) {cout << q2.top() << " ";q2.pop();}cout << ")" << endl;// This method of printing out the elements of a priority_queue// removes the elements from the priority queue, leaving it emptycout << "After printing, q2 has " << q2.size() << " elements." << endl;// The third member function declares a priority_queue// with a vector base container and specifies that the comparison// function greater is to be used for ordering elementspriority_queue <int, vector<int>, greater<int> > q3;q3.push(2);q3.push(1);q3.push(3);cout << "q3 = ( ";while (!q3.empty()) {cout << q3.top() << " ";q3.pop();}cout << ")" << endl;// The fourth member function declares a priority_queue and// initializes it with elements copied from another container:// first, inserting elements into q1, then copying q1 elements into q4q1.push(100);q1.push(200);q1.push(5);priority_queue <int> q4(q1);cout << "q4 = ( ";while (!q4.empty()) {cout << q4.top() << " ";q4.pop();}cout << ")" << endl;// Creates an auxiliary vector object v5 to be used to initialize q5vector <int> v5;vector <int>::iterator v5_Iter;v5.push_back(10);v5.push_back(30);v5.push_back(20);cout << "v5 = ( ";for (v5_Iter = v5.begin(); v5_Iter != v5.end(); v5_Iter++)cout << *v5_Iter << " ";cout << ")" << endl;// The fifth member function declares and// initializes a priority_queue q5 by copying the// range v5[ first,  last) from vector v5  priority_queue <int> q5(v5.begin(), v5.begin() + 2);cout << "q5 = ( ";while (!q5.empty()) {cout << q5.top() << " ";q5.pop();}cout << ")" << endl;// The sixth member function declares a priority_queue q6// with a comparison function greater and initializes q6// by copying the range v5[ first,  last) from vector v5priority_queue <int, vector<int>, greater<int> >q6(v5.begin(), v5.begin() + 2);cout << "q6 = ( ";while (!q6.empty()) {cout << q6.top() << " ";q6.pop();}cout << ")" << endl;
}{ // priority_queue::push: Adds an element to the priority queue based on the priority of the element from operator<.using namespace std;priority_queue<int> q1;q1.push(10);q1.push(30);q1.push(20);priority_queue<int>::size_type i;i = q1.size();cout << "The priority_queue length is " << i << "." << endl;const int& ii = q1.top();cout << "The element at the top of the priority_queue is " << ii << "." << endl;
}{ // priority_queue::size: Returns the number of elements in the priority_queue.// priority_queue::size_type: An unsigned integer type that can represent the number of elements in a priority_queue.using namespace std;priority_queue <int> q1, q2;priority_queue <int>::size_type i;q1.push(1);i = q1.size();cout << "The priority_queue length is " << i << "." << endl;q1.push(2);i = q1.size();cout << "The priority_queue length is now " << i << "." << endl;
}{ // priority_queue::top: Returns a const reference to the largest element at the top of the priority_queue.using namespace std;priority_queue<int> q1;q1.push(10);q1.push(30);q1.push(20);priority_queue<int>::size_type i;i = q1.size();cout << "The priority_queue length is " << i << "." << endl;const int& ii = q1.top();cout << "The element at the top of the priority_queue is " << ii << "." << endl;
}{ // priority_queue::value_type: A type that represents the type of object stored as an element in a priority_queue.using namespace std;// Declares priority_queues with default deque base containerpriority_queue<int>::value_type AnInt;AnInt = 69;cout << "The value_type is AnInt = " << AnInt << endl;priority_queue<int> q1;q1.push(AnInt);cout << "The element at the top of the priority_queue is " << q1.top() << "." << endl;
}return 0;
}

GitHub： https://github.com/fengbingchun/Messy_Test

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