count_if 源码剖析

一：用法解析

函数原型：

template <class InputIterator, class UnaryPredicate>
  typename iterator_traits<InputIterator>::difference_type
    count_if (InputIterator first, InputIterator last, UnaryPredicate pred);

不理解typename的含义

功能：

在区间[ first , last )找到使pred为真的数，返回其个数。

例子：

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

bool IsOdd (int i) { return ((i%2)==1); }

int main () {
  std::vector<int> myvector;
  for (int i=1; i<10; i++) myvector.push_back(i); // myvector: 1 2 3 4 5 6 7 8 9

  int mycount = count_if (myvector.begin(), myvector.end(), IsOdd);
  std::cout << "myvector contains " << mycount  << " odd values.\n";

  return 0;
}

运行如下：

myvector contains 5 odd values.

二：源码剖析

// TEMPLATE FUNCTION count_if
template<class _InIt,
  class _Pr> inline
  typename iterator_traits<_InIt>::difference_type
    _Count_if(_InIt _First, _InIt _Last, _Pr _Pred)
  { // count elements satisfying _Pred
  typename iterator_traits<_InIt>::difference_type _Count = 0;

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

template<class _InIt,
  class _Pr> inline
  typename iterator_traits<_InIt>::difference_type
    count_if(_InIt _First, _InIt _Last, _Pr _Pred)
  { // count elements satisfying _Pred
  _DEBUG_RANGE_PTR(_First, _Last, _Pred);
  return (_Count_if(_Unchecked(_First), _Unchecked(_Last), _Pred));
  }

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

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