push_heap 源码剖析

一：用法示例

一共两个重载：

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

在说明这个函数的功能之前，我们先来看看对它的英文解释：

Push element into heap range

Given a heap in the range

​​[first,last-1)​​, this function extends the range considered a heap to

​​[first,last)​​ by placing the value in

​​(last-1)​​ into its corresponding location within it.

大概意思是，push_heap的作用范围是[ first , last），在使用push_heap必须保证[ first , last-1）满足堆，而*（last-1）是新增在末尾的一个值，那么此时对于整个[ first , last ) , 因为*（last-1）的新增可能已经不满足堆。push_back的作用就是重新调整*（last-1）的位置，使其整个[ first , last )满足堆。

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

  v.push_back(-1);//现在在末尾插入一个-1，是整个数组的最小值

  push_heap(v.begin(), v.end(), greater<>());//现在-1已经在头部位置

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

  return 0;
}

二：源码及剖析

源码方面的话，其实一点都不难，一共4个模板函数，重点是第一个，其余三个都是基于第一个模板函数的。

// TEMPLATE FUNCTION push_heap WITH PRED
template<class _RanIt,
  class _Diff,
  class _Ty,
  class _Pr> inline
  void _Push_heap(_RanIt _First, _Diff _Hole,
    _Diff _Top, _Ty&& _Val, _Pr _Pred)
  { // percolate _Hole to _Top or where _Val belongs, using _Pred
  for (_Diff _Idx = (_Hole - 1) / 2;
    _Top < _Hole && _DEBUG_LT_PRED(_Pred, *(_First + _Idx), _Val);
    _Idx = (_Hole - 1) / 2)
    { // move _Hole up to parent
    *(_First + _Hole) = _STD move(*(_First + _Idx));
    _Hole = _Idx;
    }
/*
_Top < _Hole是对边界的判断；
_DEBUG_LT_PRED(_Pred, *(_First + _Idx), _Val)相当于_Pred(*(_First + _Idx), _Val)
*/

  *(_First + _Hole) = _STD move(_Val);  // drop _Val into final hole
  }

template<class _RanIt,
  class _Diff,
  class _Ty,
  class _Pr> inline
  void _Push_heap_0(_RanIt _First, _RanIt _Last, _Pr _Pred, _Diff *, _Ty *)
  { // push *_Last onto heap at [_First, _Last), using _Pred
  _Diff _Count = _Last - _First;
  if (0 < _Count)
    { // worth doing, percolate *_Last
    _Ty _Val = _STD move(*_Last);
    _Push_heap(_First, _Count, _Diff(0), _STD move(_Val), _Pred);
    }
  }

template<class _RanIt,
  class _Pr> inline
  void push_heap(_RanIt _First, _RanIt _Last, _Pr _Pred)
  { // push *(_Last - 1) onto heap at [_First, _Last - 1), using _Pred
  _DEBUG_RANGE_PTR(_First, _Last, _Pred);
  if (_First != _Last)
    { // check and push to nontrivial heap
    --_Last;
    _DEBUG_HEAP_PRED(_First, _Last, _Pred);
    _Push_heap_0(_Unchecked(_First), _Unchecked(_Last), _Pred,
      _Dist_type(_First), _Val_type(_First));
    }
  }

    // TEMPLATE FUNCTION push_heap
template<class _RanIt> inline
  void push_heap(_RanIt _First, _RanIt _Last)
  { // push *(_Last - 1) onto heap at [_First, _Last - 1), using operator<
  _STD push_heap(_First, _Last, less<>());
  }

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

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