std::queue: 模板類queue定義在<queue>頭文件中。隊列(Queue)是一個容器適配器(Container adaptor)類型，被特別設計用來運行于FIFO(First-in first-out)場景，在該場景中，只能從容器一端添加(Insert)元素，而在另一端提取(Extract)元素。只能從容器”后面”壓進(Push)元素,從容器”前面”提取(Pop)元素。用來實現隊列的底層容器必須滿足順序容器的所有必要條件。

queue模板類需要兩個模板參數，一個是元素類型，一個容器類型，元素類型是必要的，容器類型是可選的，默認為deque類型。

std::queue: FIFO queue, queues are a type of container adaptor, specifically designed to operate in a FIFO context (first-in first-out), where elements are inserted into one end of the container and extracted from the other. queues are implemented as containers 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 pushed into the "back"of the specific container and popped from its "front".

The std::queue class is a container adapter that gives the programmer the functionality of a queue - specifically, a FIFO (first-in, first-out) data structure. The class template acts as a wrapper to the underlying container - only a specific set of functions is provided. The queue pushes the elements on the back of the underlying container and pops them from the front. ?Queue does not allow iteration through its elements.

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 "queue.hpp"
#include <iostream>
#include <queue> // std::queue
#include <vector>
#include <string>
#include <list>//
// reference: http://www.cplusplus.com/reference/queue/queue/
int test_queue_1()
{
{ // std::queue::back: Access last element, Returns a reference to the last element in the queue.// queue::front: access the first element, queue::back: access the last elementstd::queue<int> myqueue;myqueue.push(12);myqueue.push(20);myqueue.push(75);   // this is now the backfprintf(stderr, "myqueue.back: %d, myqueue.front: %d\n", myqueue.back(), myqueue.front());myqueue.back() -= myqueue.front(); // 75 -= 12std::cout << "myqueue.back() is now " << myqueue.back() << '\n'; // myqueue.back() is now 63
}{ // std::queue::front: Access next element, Returns a reference to the next element in the queue.// The next element is the "oldest" element in the queue and// the same element that is popped out from the queue when queue::pop is called.std::queue<int> myqueue;myqueue.push(77);myqueue.push(20);myqueue.push(16);fprintf(stderr, "myqueue.back: %d, myqueue.front: %d\n", myqueue.back(), myqueue.front());myqueue.front() -= myqueue.back();    // 77-16=61std::cout << "myqueue.front() is now " << myqueue.front() << '\n';
}{ // std::queue::push: Inserts a new element at the end of the queue, after its current last element// std::queue::pop: Remove next element, Removes the next element in the queue, effectively reducing its size by one.// std::queue::empty: Test whether container is empty, Returns whether the queue is empty: i.e. whether its size is zero.std::queue<int> myqueue;int myint;std::cout << "Please enter some integers (enter 0 to end):\n";std::vector<int> vec{ 1, 2, 3, 4, 5 };for (const auto& value : vec) {myqueue.push(value);}fprintf(stderr, "myqueue.back: %d, myqueue.front: %d\n", myqueue.back(), myqueue.front());std::cout << "myqueue contains: ";while (!myqueue.empty()) {std::cout << ' ' << myqueue.front();myqueue.pop();}std::cout << '\n';
}{ // std::queue::emplace: C++11, Construct and insert element.// Adds a new element at the end of the queue, after its current last element.// The element is constructed in-place, i.e. no copy or move operations are performed.// queue::push: inserts element at the end , queue::emplace: constructs element in-place at the end// queue::pop: removes the first element std::queue<std::string> myqueue;myqueue.emplace("First sentence");myqueue.emplace("Second sentence");std::cout << "myqueue contains:\n";while (!myqueue.empty()) {std::cout << myqueue.front() << '\n';myqueue.pop();}
}{ // std::queue::size: Returns the number of elements in the queuestd::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::queue::swap: C++11, Exchanges the contents of the container adaptor// std::swap (queue): Exchanges the contents of x and y.std::queue<int> foo, bar;foo.push(10); foo.push(20); foo.push(30);bar.push(111); bar.push(222);foo.swap(bar);// std::swap(foo, bar);std::cout << "size of foo: " << foo.size() << '\n';std::cout << "size of bar: " << bar.size() << '\n';std::cout << "foo contains: ";while (!foo.empty()) {std::cout << ' ' << foo.front();foo.pop();}std::cout << '\n';std::cout << "bar contains: ";while (!bar.empty()) {std::cout << ' ' << bar.front();bar.pop();}std::cout << '\n';
}return 0;
}// reference: http://stackoverflow.com/questions/709146/how-do-i-clear-the-stdqueue-efficiently
static void clear(std::queue<int> &q)
{std::queue<int> empty;std::swap(q, empty);
}int test_queue_2()
{// clear queue, avoid to pop in a loop// A common idiom for clearing standard containers is swapping with an empty version of the containerstd::queue<int> foo;foo.push(10); foo.push(20); foo.push(30);fprintf(stderr, "foo size: %d\n", foo.size());clear(foo);fprintf(stderr, "foo size: %d\n", foo.size());return 0;
}//
// reference: https://msdn.microsoft.com/en-us/library/s23s3de6.aspx
int test_queue_3()
{
{ // queue::back: Returns a reference to the last and most recently added element at the back of the queue.// The last element of the queue. If the queue is empty, the return value is undefined.using namespace std;queue <int> q1;q1.push(10);q1.push(11);int& i = q1.back();const int& ii = q1.front();cout << "The integer at the back of queue q1 is " << i << "." << endl;cout << "The integer at the front of queue q1 is " << ii << "." << endl;
}{ // queue::empty: true if the queue is empty; false if the queue is nonempty.using namespace std;// Declares queues with default deque base containerqueue <int> q1, q2;q1.push(1);if (q1.empty()) cout << "The queue q1 is empty." << endl;else cout << "The queue q1 is not empty." << endl;if (q2.empty()) cout << "The queue q2 is empty." << endl;else cout << "The queue q2 is not empty." << endl;
}{ // queue::front: Returns a reference to the first element at the front of the queue// queue::size_type: An unsigned integer type that can represent the number of elements in a queue.using namespace std;queue <int> q1;q1.push(10);q1.push(20);q1.push(30);queue <int>::size_type i;i = q1.size();cout << "The queue length is " << i << "." << endl;int& ii = q1.back();int& iii = q1.front();cout << "The integer at the back of queue q1 is " << ii << "." << endl;cout << "The integer at the front of queue q1 is " << iii << "." << endl;
}{ // queue::pop: Removes an element from the front of the queueusing namespace std;queue <int> q1, s2;q1.push(10);q1.push(20);q1.push(30);queue <int>::size_type i;i = q1.size();cout << "The queue length is " << i << "." << endl;i = q1.front();cout << "The element at the front of the queue is " << i << "." << endl;q1.pop();i = q1.size();cout << "After a pop the queue length is " << i << "." << endl;i = q1.front();cout << "After a pop, the element at the front of the queue is " << i << "." << endl;
}{ // queue::push: Adds an element to the back of the queue.using namespace std;queue <int> q1;q1.push(10);q1.push(20);q1.push(30);queue <int>::size_type i;i = q1.size();cout << "The queue length is " << i << "." << endl;i = q1.front();cout << "The element at the front of the queue is " << i << "." << endl;
}{ // queue::queue: Constructs a queue that is empty or that is a copy of a base container object.using namespace std;// Declares queue with default deque base containerqueue <char> q1;// Explicitly declares a queue with deque base containerqueue <char, deque<char> > q2;// These lines don't cause an error, even though they// declares a queue with a vector base containerqueue <int, vector<int> > q3;q3.push(10);// but the following would cause an error because vector has// no pop_front member function// q3.pop( );// Declares a queue with list base containerqueue <int, list<int> > q4;// The second member function copies elements from a containerlist<int> li1;li1.push_back(1);li1.push_back(2);queue <int, list<int> > q5(li1);cout << "The element at the front of queue q5 is " << q5.front() << "." << endl;cout << "The element at the back of queue q5 is " << q5.back() << "." << endl;
}{ // queue::size: Returns the number of elements in the queueusing namespace std;queue <int> q1, q2;queue <int>::size_type i;q1.push(1);i = q1.size();cout << "The queue length is " << i << "." << endl;q1.push(2);i = q1.size();cout << "The queue length is now " << i << "." << endl;
}{ // queue::value_type: A type that represents the type of object stored as an element in a queue.using namespace std;// Declares queues with default deque base containerqueue<int>::value_type AnInt;AnInt = 69;cout << "The value_type is AnInt = " << AnInt << endl;queue<int> q1;q1.push(AnInt);cout << "The element at the front of the queue is " << q1.front() << "." << endl;
}return 0;
}

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

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