ptr_fun学习笔记

ptr_fun是将一个普通的函数适配成一个functor,添加上argument type和result type等类型， 其实现如下（例子里面是binary_function，unary_function同理)： 

template<class _Arg1,
    class _Arg2,
    class _Result> inline
    pointer_to_binary_function<_Arg1, _Arg2, _Result,
        _Result(__fastcall *)(_Arg1, _Arg2)>
            ptr_fun(_Result (__fastcall *_Left)(_Arg1, _Arg2))
    {   // return pointer_to_binary_function functor adapter
    return (std::pointer_to_binary_function<_Arg1, _Arg2, _Result,
        _Result (__fastcall *)(_Arg1, _Arg2)>(_Left));
    }

由上面的代码可见，ptr_fun只是将一个普通的函数（或者函数指针）适配成类pointer_to_binary_function，而该类实际上是binary_function的子类，这样出来的functor就有利于同STL的算法等适配。 
下面的例子就是说明了使用ptr_fun将普通的函数适配成bind1st或bind2nd能够使用的functor，否则对bind1st或bind2nd直接绑定普通函数，则编译出错。 

#include <algorithm>
#include <functional>
#include <iostream>

using namespace std;

int sum(int arg1, int arg2)
{
    cout<<"ARG 1: "<<arg1<<endl;
    cout<<"ARG 2: "<<arg2<<endl;

    int sum = arg1 + arg2;
    cout<<"SUM: "<<sum<<endl;

    return sum;
}

int main(int argc,char* argv[])
{
    bind1st(ptr_fun(sum),1)(2); // the same as sum(1,2)
    bind2nd(ptr_fun(sum),1)(2); //the same as sum(2,1)

    getchar();
    return 0;
}