count_if 源码剖析

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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，可以参考http://blog.csdn.net/laojiu_/article/details/51737114

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::vectorbool 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_iftemplate<class _InIt,class _Pr> inlinetypename iterator_traits<_InIt>::difference_type_Count_if(_InIt _First, _InIt _Last, _Pr _Pred){// count elements satisfying _Predtypename iterator_traits<_InIt>::difference_type _Count = 0;for (; _First != _Last; ++_First)if (_Pred(*_First))++_Count;return (_Count);}template<class _InIt,class _Pr> inlinetypename iterator_traits<_InIt>::difference_typecount_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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