4.6.1 queue概述

       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。

4.6.2 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>  
bool  
operator<(const queue<_Tp, _Sequence>& __x, const queue<_Tp, _Sequence>& __y)  
{  
  return __x.c < __y.c;  
}  
  
#ifdef __STL_FUNCTION_TMPL_PARTIAL_ORDER  
  
template <class _Tp, class _Sequence>  
bool  
operator!=(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::queue  
  
int 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;  
}  

输出：

size of first: 0  
size of second: 3  
size of third: 0  
size of fourth: 2  
The element at the front of queue second is: 100  
The element at the back of queue second is: 10