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::greaterclass 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 :5after push the elements,fifth = ( 0 10 20 30 40 )

参考资料：

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