pop_heap 源码剖析

一：用法示例

一共两个重载：

default (1)   
template <class RandomAccessIterator>
  void pop_heap ( RandomAccessIterator first ,  RandomAccessIterator last ) ;
custom (2)   
template <class RandomAccessIterator, class Compare>
  void pop_heap (RandomAccessIterator first ,  RandomAccessIterator last  , Compare comp ) ;

作用范围是[ first , last )，使用pop_heap前必须保证在该作用范围的数满足（大顶或小顶）堆。

该函数的作用就是把堆顶的元素（也就是数组的第一个元素）放在末尾，原来末尾的元素移到前面，在移动过程，同样的，也要保证[ first , last - 1 )满足（大顶或小顶）堆。

例子：

#include<iostream>
#include<functional>
#include<algorithm>
#include<vector>

using namespace std;

int main()
{
  
  int a[10] = { 2,1,6,4,9,10,3,5,8,7 };
  vector<int> v(a, a + 10);
  sort(v.begin(), v.end());//现在是一个小顶堆

  pop_heap(v.begin(), v.end(), greater<>());//现在1已经到了数组末尾

  for (auto it = v.begin(); it != v.end(); it++)
    cout << *it << " ";//2 4 3 8 5 6 7 10 9 1
  cout << endl;

  return 0;
}

二：源码及剖析

记住源码阅读顺序是从最后一个往前读，该函数用到了push_heap源代码的知识，链接为： ​​push_heap​​   ，建议读者对照着看。

// TEMPLATE FUNCTION pop_heap WITH PRED
template<class _RanIt,
  class _Diff,
  class _Ty,
  class _Pr> inline
  void _Adjust_heap(_RanIt _First, _Diff _Hole, _Diff _Bottom,
    _Ty&& _Val, _Pr _Pred)
  { // percolate _Hole to _Bottom, then push _Val, using _Pred
    /*
          _Hole为0，表示起点位置
          _Bottom为begin()-end()-1，也就是数组长度减一
          _Val保存着最后位置的值，也就是*(end()-1)
    */
  _Diff _Top = _Hole;//_Top等于0
  _Diff _Idx = 2 * _Hole + 2;//转到_Hole的右孩子处

  for (; _Idx < _Bottom; _Idx = 2 * _Idx + 2)
    { // move _Hole down to larger child
    if (_DEBUG_LT_PRED(_Pred, *(_First + _Idx), *(_First + (_Idx - 1))))
      --_Idx;//_Idx指向左右孩子值较大(小）的位置
    *(_First + _Hole) = _STD move(*(_First + _Idx));
    _Hole = _Idx;
    }

  if (_Idx == _Bottom)
    { // only child at bottom, move _Hole down to it
    *(_First + _Hole) = _STD move(*(_First + (_Bottom - 1)));
    _Hole = _Bottom - 1;
    }
  _Push_heap(_First, _Hole, _Top, _STD move(_Val), _Pred);
  }

template<class _RanIt,
  class _Diff,
  class _Ty,
  class _Pr> inline
  void _Pop_heap(_RanIt _First, _RanIt _Last, _RanIt _Dest,
    _Ty&& _Val, _Pr _Pred, _Diff *)
  { // pop *_First to *_Dest and reheap, using _Pred
  *_Dest = _STD move(*_First);//把第一个元素赋给最后一个，最后一个位置的值早已经保存在_Val
  _Adjust_heap(_First, _Diff(0), _Diff(_Last - _First),
    _STD move(_Val), _Pred);
  }

template<class _RanIt,
  class _Ty,
  class _Pr> inline
  void _Pop_heap_0(_RanIt _First, _RanIt _Last, _Pr _Pred, _Ty *)
  { // pop *_First to *(_Last - 1) and reheap, using _Pred
  _Ty _Val = _STD move(*(_Last - 1));
  _Pop_heap(_First, _Last - 1, _Last - 1,
    _STD move(_Val), _Pred, _Dist_type(_First));
  }

template<class _RanIt,
  class _Pr> inline
  void _Pop_heap(_RanIt _First, _RanIt _Last, _Pr _Pred)
  { // pop *_First to *(_Last - 1) and reheap, using _Pred
  _Pop_heap_0(_Unchecked(_First), _Unchecked(_Last), _Pred,
    _Val_type(_First));
  }

template<class _RanIt,
  class _Pr> inline
  void pop_heap(_RanIt _First, _RanIt _Last, _Pr _Pred)
  { // pop *_First to *(_Last - 1) and reheap, using _Pred
  _DEBUG_RANGE_PTR(_First, _Last, _Pred);
  _DEBUG_HEAP_PRED(_First, _Last, _Pred);
  if (2 <= _Last - _First)
    _Pop_heap(_Unchecked(_First), _Unchecked(_Last), _Pred);
  }

    // TEMPLATE FUNCTION pop_heap
template<class _RanIt> inline
  void pop_heap(_RanIt _First, _RanIt _Last)
  { // pop *_First to *(_Last - 1) and reheap, using operator<
  _STD pop_heap(_First, _Last, less<>());
  }

源码摘抄自Visual Studio 2015安装目录algorithm文件中。

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