STL源码剖析——容器配接器之priority_queue

前言

        priority_queue是拥有优先级的queue，不过它容器内的元素并不是根据加入顺序排列，而是根据用户定义的优先级进行排列。priority_queue只能在队列尾部加入元素，在头部取出元素。不能遍历容器，因此不需要自己设置迭代器。在SGI STL的源码<stl_queue.h>的class priority_queue设计中，它是基于某种容器作为底部结构的，默认容器是vector容器，用户也可以自己指定容器的类型。

priority_queue容器配接器

    下面给出源码剖析：

template <class _Tp, 
          class _Sequence __STL_DEPENDENT_DEFAULT_TMPL(vector<_Tp>),
          class _Compare
          __STL_DEPENDENT_DEFAULT_TMPL(less<typename _Sequence::value_type>) >
class priority_queue {

  // requirements:

  __STL_CLASS_REQUIRES(_Tp, _Assignable);
  __STL_CLASS_REQUIRES(_Sequence, _Sequence);
  __STL_CLASS_REQUIRES(_Sequence, _RandomAccessContainer);
  typedef typename _Sequence::value_type _Sequence_value_type;
  __STL_CLASS_REQUIRES_SAME_TYPE(_Tp, _Sequence_value_type);
  __STL_CLASS_BINARY_FUNCTION_CHECK(_Compare, bool, _Tp, _Tp);

public:
	// priority_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vector，用户可自行指定容器类型
  _Compare comp;//优先级决策方式
public:
	//***********************************************************
	//*	构造函数
	//*	priority_queue() 
	//*	explicit priority_queue(const Compare& __x) 
	//*	explicit priority_queue (const Compare& comp = Compare(),
    //*                    const Container& ctnr = Container());
	//*	template <class InputIterator>
    //* priority_queue (InputIterator first, InputIterator last,
    //*                    const Compare& comp = Compare(),
    //*                    const Container& ctnr = Container());
	//***********************************************************
  priority_queue() : c() {}
  explicit priority_queue(const _Compare& __x) :  c(), comp(__x) {}
  priority_queue(const _Compare& __x, const _Sequence& __s) 
    : c(__s), comp(__x) 
    { make_heap(c.begin(), c.end(), comp); }

#ifdef __STL_MEMBER_TEMPLATES
  template <class _InputIterator>
  priority_queue(_InputIterator __first, _InputIterator __last) 
    : c(__first, __last) { make_heap(c.begin(), c.end(), comp); }

  template <class _InputIterator>
  priority_queue(_InputIterator __first, 
                 _InputIterator __last, const _Compare& __x)
    : c(__first, __last), comp(__x) 
    { make_heap(c.begin(), c.end(), comp); }

  template <class _InputIterator>
  priority_queue(_InputIterator __first, _InputIterator __last,
                 const _Compare& __x, const _Sequence& __s)
  : c(__s), comp(__x)
  { 
    c.insert(c.end(), __first, __last);
    make_heap(c.begin(), c.end(), comp);
  }

#else /* __STL_MEMBER_TEMPLATES */
  priority_queue(const value_type* __first, const value_type* __last) 
    : c(__first, __last) { make_heap(c.begin(), c.end(), comp); }

  priority_queue(const value_type* __first, const value_type* __last, 
                 const _Compare& __x) 
    : c(__first, __last), comp(__x)
    { make_heap(c.begin(), c.end(), comp); }

  priority_queue(const value_type* __first, const value_type* __last, 
                 const _Compare& __x, const _Sequence& __c)
    : c(__c), comp(__x) 
  { 
    c.insert(c.end(), __first, __last);
    make_heap(c.begin(), c.end(), comp);
  }
#endif /* __STL_MEMBER_TEMPLATES */

  //判断容器是否为空
  bool empty() const { return c.empty(); }
  //返回容器元素的个数
  size_type size() const { return c.size(); }
  //返回优先级最高元素的引用
  const_reference top() const { return c.front(); }
  //新增元素，并根据优先级调整堆
  void push(const value_type& __x) {
    __STL_TRY {
      c.push_back(__x); 
      push_heap(c.begin(), c.end(), comp);
    }
    __STL_UNWIND(c.clear());
  }
  //弹出优先级最高的元素
  void pop() {
    __STL_TRY {
      pop_heap(c.begin(), c.end(), comp);
      c.pop_back();
    }
    __STL_UNWIND(c.clear());
  }
};

// no equality is provided

__STL_END_NAMESPACE

#endif /* __SGI_STL_INTERNAL_QUEUE_H */

// Local Variables:
// mode:C++
// End:

举例说明该容器的使用：

// constructing priority queues
#include <iostream>       // std::cout
#include <queue>          // std::priority_queue
#include <vector>         // std::vector
#include <functional>     // std::greater

class mycomparison
{
  bool reverse;
public:
  mycomparison(const bool& revparam=false)
    {reverse=revparam;}
  bool operator() (const int& lhs, const int&rhs) const
  {
    if (reverse) return (lhs>rhs);
    else return (lhs<rhs);
  }
};

int main ()
{
  int myints[]= {10,60,50,20};

  std::priority_queue<int> first;
  std::priority_queue<int> second (myints,myints+4);
  std::priority_queue<int, std::vector<int>, std::greater<int> >
                            third (myints,myints+4);
   std::cout << "third  = ( ";
   while ( !third.empty( ) )
   {
      std::cout << third.top( ) << " ";
      third.pop( );
   }
   std::cout << ")" << std::endl;
  // using mycomparison:
  typedef std::priority_queue<int,std::vector<int>,mycomparison> mypq_type;

  mypq_type fourth (myints,myints+4);                       // less-than comparison
  mypq_type fifth (mycomparison(true));   // greater-than comparison
    std::cout << "fourth  = ( ";
   while ( !fourth.empty( ) )
   {
      std::cout << fourth.top( ) << " ";
      fourth.pop( );
   }
   std::cout << ")" << std::endl;
   std::cout << "fifth  = ( ";
   while ( !fifth.empty( ) )
   {
      std::cout << fifth.top( ) << " ";
      fifth.pop( );
   }
   std::cout << ")" << std::endl;
   
   for (int i = 0; i < 5; i++)
   {
	   fifth.push(i*10);
   }
	std::cout <<"after push the elements,fifth size is :"<<fifth.size()<<std::endl;
    std::cout << "after push the elements,fifth = ( ";
   while ( !fifth.empty( ) )
   {
      std::cout << fifth.top( ) << " ";
      fifth.pop( );
   }
   std::cout << ")" << std::endl;
   

  system("pause");
  return 0;
}
Output:
third  = ( 10 20 50 60 )
fourth  = ( 60 50 20 10 )
fifth  = ( )
after push the elements,fifth size is :5
after push the elements,fifth = ( 0 10 20 30 40 )

参考资料：

《STL源码剖析》侯捷