【STL】ptr_fun详解

ptr_fun是将一个普通的函数适配成一个仿函数(functor), 添加上argument_type和result type等类型，它的定义如下：

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

下面的例子就是说明了使用ptr_fun将普通函数(两个参数, 如果有多个参数, 要改用boost::bind)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)  
{  
	std::cout<< "arg1 = " << arg1 << std::endl;  
	std::cout<< "arg2 = " << arg2 << std::endl;  

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

	return sum;  
}

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

	return 0;
}