学步园

#include <iostream>
#include <vector>
#include <algorithm>
using namespace std;

class A {
public:
    A(): x_(0) {
    }

    void operator()(int x) {
        cout << ++x_ << endl;
    }

    int X() const {
        return x_;
    }

private:
    int x_;

};

/*
 *
 */
int main(int argc, char** argv) {
    vector<int> v;
    v.push_back(1);
    v.push_back(2);
    
    A a1;
    A a2 = for_each(v.begin(), v.end(), a1);
    cout <<a1.X() << endl;
    cout <<a2.X() << endl;
    return 0;
}

输出结果是：

for_each的实现

通过查看for_each的源代码，可以观察到stl中使用function object的基本特征。下面是gcc4.6.1的实现代码：

  /**
   *  @brief Apply a function to every element of a sequence.
   *  @ingroup non_mutating_algorithms
   *  @param  first  An input iterator.
   *  @param  last   An input iterator.
   *  @param  f      A unary function object.
   *  @return   @p f (std::move(@p f) in C++0x).
   *
   *  Applies the function object @p f to each element in the range
   *  @p [first,last).  @p f must not modify the order of the sequence.
   *  If @p f has a return value it is ignored.
  */
  template<typename _InputIterator, typename _Function>
    _Function
    for_each(_InputIterator __first, _InputIterator __last, _Function __f)
    {
      // concept requirements
      __glibcxx_function_requires(_InputIteratorConcept<_InputIterator>)
      __glibcxx_requires_valid_range(__first, __last);
      for (; __first != __last; ++__first)
        __f(*__first);
      return _GLIBCXX_MOVE(__f);
    }

_Function __f 这个参数表明for_each会对function object做一次复制。