is_heap和is_heap_until 用法与源码剖析

一：用法示例

is_heap一共两个重载：

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

返回值bool型，功能是判断在[ first , last ）范围的数是否能构成一个堆。根据第三参数comp确定是大顶堆还是小顶堆，默认是大顶堆。

如不理解堆的概念，建议先学习“堆排序“这个排序算法，具体内容请读者自行百度。

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);

  if (!is_heap(v.begin(), v.end()))
    cout << "not heap!\n";//不是一个堆

  sort(v.begin(), v.end());

  if (!is_heap(v.begin(), v.end()))
    cout << "not max_heap!\n";//不是一个大顶堆

  if (is_heap(v.begin(), v.end(),greater<>()))
    cout << "is min_heap!\n";//是小顶堆

  return 0;
}

运行如图：

is_heap_until也是两个重载：

default (1)   

template <class RandomAccessIterator>
  RandomAccessIterator is_heap_until ( RandomAccessIterator first , RandomAccessIterator last ) ;
custom (2)   
template <class RandomAccessIterator ,  class Compare>
  RandomAccessIterator is_heap_until ( RandomAccessIterator first , RandomAccessIterator last , Compare comp ) ;

返回值是一个迭代器，指向第一个不满足堆元素（Returns an iterator to the first element in the range [first,last) which is not in a valid position if the range is considered a heap (as if constructed with make_heap).）。

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[3] = 20;//现在既不是大顶堆也不是小顶堆

  auto it1 = is_heap_until(v.begin(), v.end(), less<>());
  auto it2 = is_heap_until(v.begin(), v.end(), greater<>());

  cout << "前" << it1 - v.begin() << "个元素满足大顶堆！\n";
  cout << "前" << it2 - v.begin() << "个元素满足小顶堆！\n";

  return 0;
}

运行如图：

二：源码剖析

is_heap和is_heap_until的源码实现都是依赖is_heap_until，也就是下面源码的第一个模板函数。

// TEMPLATE FUNCTIONS is_heap AND is_heap_until WITH PRED
template<class _RanIt,
  class _Diff,
  class _Pr> inline
  _RanIt _Is_heap_until(_RanIt _First, _RanIt _Last, _Pr _Pred, _Diff *)
  { // find extent of range that is a heap ordered by _Pred
  _Diff _Size = _Last - _First;

  if (2 <= _Size)//如果只有一个元素的话肯定满足堆
    for (_Diff _Off = 0; ++_Off < _Size; )
      if (_DEBUG_LT_PRED(_Pred, *(_First + (_Off - 1) / 2),*(_First + _Off)))//相当于if(_Pred(*(_First + (_Off - 1) / 2),*(_First + _Off)))
        return (_First + _Off);
  return (_Last);
  }

template<class _RanIt,
  class _Pr>
  _RanIt is_heap_until(_RanIt _First, _RanIt _Last, _Pr _Pred)
  { // find extent of range that is a heap ordered by _Pred
  _DEBUG_RANGE(_First, _Last);
  _DEBUG_POINTER_IF(2 <= _Last - _First, _Pred);
  return (_Rechecked(_First,
    _Is_heap_until(_Unchecked(_First), _Unchecked(_Last), _Pred,
      _Dist_type(_First))));
  }

template<class _RanIt,
  class _Pr>
  bool is_heap(_RanIt _First, _RanIt _Last, _Pr _Pred)
  { // test if range is a heap ordered by _Pred
  return (_STD is_heap_until(_First, _Last, _Pred) == _Last);
  }

    // TEMPLATE FUNCTIONS is_heap AND is_heap_until
template<class _RanIt>
  _RanIt is_heap_until(_RanIt _First, _RanIt _Last)
  { // find extent of range that is a heap ordered by operator<
  return (_STD is_heap_until(_First, _Last, less<>()));
  }

template<class _RanIt>
  bool is_heap(_RanIt _First, _RanIt _Last)
  { // test if range is a heap ordered by operator<
  return (_STD is_heap(_First, _Last, less<>()));
  }

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

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