ptr_fun

頭文件：<functional>

ptr_fun是將一個普通的函數適配成一個仿函數(functor), 添加上argument_type和result type等類型，它的定義如下：

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));
    }

下面的例子就是說明了使用ptr_fun將普通函數(兩個參數, 如果有多個參數, 要改用boost::bind)適配成bind1st或bind2nd能夠使用的functor，否則對bind1st或bind2nd直接綁定普通函數，則編譯出錯。

#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;
}