STL源码剖析——容器配接器之queue

前言

       queue是一种“先进先出”的数据结构，可以对两端进行操作，但是只能在队列头部进行移除元素，只能在队列尾部新增元素，可以访问队列尾部和头部的元素，但是不能遍历容器，所以queue不需要设计自己的容器。在SGI STL的源码<stl_queue.h>的class queue设计中，它是基于某种容器作为底部结构的，默认容器是deque容器，用户也可以自己指定容器的类型。

queue容器配接器

     源码剖析如下：

#ifndef __SGI_STL_INTERNAL_QUEUE_H#define __SGI_STL_INTERNAL_QUEUE_H#include <sequence_concepts.h>__STL_BEGIN_NAMESPACE// Forward declarations of operators < and ==, needed for friend declaration.//默认底层容器为deque容器template <class _Tp,           class _Sequence __STL_DEPENDENT_DEFAULT_TMPL(deque<_Tp>) >class queue;template <class _Tp, class _Seq>inline bool operator==(const queue<_Tp, _Seq>&, const queue<_Tp, _Seq>&);template <class _Tp, class _Seq>inline bool operator<(const queue<_Tp, _Seq>&, const queue<_Tp, _Seq>&);template <class _Tp, class _Sequence>class queue {  // requirements:  __STL_CLASS_REQUIRES(_Tp, _Assignable);  __STL_CLASS_REQUIRES(_Sequence, _FrontInsertionSequence);  __STL_CLASS_REQUIRES(_Sequence, _BackInsertionSequence);  typedef typename _Sequence::value_type _Sequence_value_type;  __STL_CLASS_REQUIRES_SAME_TYPE(_Tp, _Sequence_value_type);#ifdef __STL_MEMBER_TEMPLATES   template <class _Tp1, class _Seq1>  friend bool operator== (const queue<_Tp1, _Seq1>&,                          const queue<_Tp1, _Seq1>&);  template <class _Tp1, class _Seq1>  friend bool operator< (const queue<_Tp1, _Seq1>&,                         const queue<_Tp1, _Seq1>&);#else /* __STL_MEMBER_TEMPLATES */  friend bool __STD_QUALIFIER  operator== __STL_NULL_TMPL_ARGS (const queue&, const queue&);  friend bool __STD_QUALIFIER  operator<  __STL_NULL_TMPL_ARGS (const queue&, const queue&);#endif /* __STL_MEMBER_TEMPLATES */public:// queue仅支持对头部和尾部的操作, 所以不定义STL要求的    // pointer, iterator, difference_type   typedef typename _Sequence::value_type      value_type;  typedef typename _Sequence::size_type       size_type;  typedef          _Sequence                  container_type;  typedef typename _Sequence::reference       reference;  typedef typename _Sequence::const_reference const_reference;protected:  _Sequence c;//底层容器，默认为deque容器，用户可自行指定容器类型public://下面对queue的维护完全依赖于底层容器的操作   queue() : c() {}  explicit queue(const _Sequence& __c) : c(__c) {}  //判断容器是否为空  bool empty() const { return c.empty(); }  //返回容器中元素的个数  size_type size() const { return c.size(); }  //返回队头元素的引用  reference front() { return c.front(); }  const_reference front() const { return c.front(); }  //返回队尾元素的引用  reference back() { return c.back(); }  const_reference back() const { return c.back(); }  //只能在队尾新增元素  void push(const value_type& __x) { c.push_back(__x); }  //只能在队头移除元素  void pop() { c.pop_front(); }};//下面是依赖于底层容器的操作运算符  template <class _Tp, class _Sequence>bool operator==(const queue<_Tp, _Sequence>& __x, const queue<_Tp, _Sequence>& __y){  return __x.c == __y.c;}template <class _Tp, class _Sequence>booloperator<(const queue<_Tp, _Sequence>& __x, const queue<_Tp, _Sequence>& __y){  return __x.c < __y.c;}#ifdef __STL_FUNCTION_TMPL_PARTIAL_ORDERtemplate <class _Tp, class _Sequence>booloperator!=(const queue<_Tp, _Sequence>& __x, const queue<_Tp, _Sequence>& __y){  return !(__x == __y);}template <class _Tp, class _Sequence>bool operator>(const queue<_Tp, _Sequence>& __x, const queue<_Tp, _Sequence>& __y){  return __y < __x;}template <class _Tp, class _Sequence>bool operator<=(const queue<_Tp, _Sequence>& __x, const queue<_Tp, _Sequence>& __y){  return !(__y < __x);}template <class _Tp, class _Sequence>bool operator>=(const queue<_Tp, _Sequence>& __x, const queue<_Tp, _Sequence>& __y){  return !(__x < __y);}#endif /* __STL_FUNCTION_TMPL_PARTIAL_ORDER */

下面举例说明：

// constructing queues#include <iostream>       // std::cout#include <deque>          // std::deque#include <list>           // std::list#include <queue>          // std::queueint main (){  std::deque<int> mydeck (3,100);        // deque with 3 elements  std::list<int> mylist (2,200);         // list with 2 elements  std::queue<int> first;                 // empty queue  std::queue<int> second (mydeck);       // queue initialized to copy of deque  std::queue<int,std::list<int> > third; // empty queue with list as underlying container  std::queue<int,std::list<int> > fourth (mylist);  std::cout << "size of first: " << first.size() << '\n';  std::cout << "size of second: " << second.size() << '\n';  std::cout << "size of third: " << third.size() << '\n';  std::cout << "size of fourth: " << fourth.size() << '\n';  std::cout << "The element at the front of queue second is: "        << second.front( )  << std::endl;  second.push(10);  std::cout << "The element at the back of queue second is: "        << second.back( ) << std::endl;  return 0;}Output:size of first: 0size of second: 3size of third: 0size of fourth: 2The element at the front of queue second is: 100The element at the back of queue second is: 10

参考资料：

《STL源码剖析》侯捷