count 源码剖析

一：用法解析

函数原型：

template <class InputIterator, class T>
  typename iterator_traits<InputIterator>::difference_type
    count (InputIterator first, InputIterator last, const T& val);

功能：

在区间[ first , last )找到与val相等的数，返回其个数。

例子：

// count algorithm example
#include <iostream>     // std::cout
#include <algorithm>    // std::count
#include <vector>       // std::vector

int main () {
  // counting elements in array:
  int myints[] = {10,20,30,30,20,10,10,20};   // 8 elements
  int mycount = std::count (myints, myints+8, 10);
  std::cout << "10 appears " << mycount << " times.\n";

  // counting elements in container:
  std::vector<int> myvector (myints, myints+8);
  mycount = std::count (myvector.begin(), myvector.end(), 20);
  std::cout << "20 appears " << mycount  << " times.\n";

  return 0;
}

运行如下：

10 appears 3 times.
20 appears 3 times.

二：源码剖析

// TEMPLATE FUNCTION count
template<class _InIt,
  class _Ty> inline
  typename iterator_traits<_InIt>::difference_type
    _Count_np(_InIt _First, _InIt _Last, const _Ty& _Val)
  { // count elements that match _Val
  typename iterator_traits<_InIt>::difference_type _Count = 0;

  for (; _First != _Last; ++_First)
    if (*_First == _Val)
      ++_Count;
  return (_Count);
  }

template<class _InIt,
  class _Ty> inline
  typename iterator_traits<_InIt>::difference_type
    count(_InIt _First, _InIt _Last, const _Ty& _Val)
  { // count elements that match _Val
  _DEBUG_RANGE(_First, _Last);
  return (_Count_np(_Unchecked(_First), _Unchecked(_Last), _Val));
  }

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

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