C++ STL bind1st bind2nd bind 的使

说明

bind1st() 和 bind2nd()，在 C++11 里已经 deprecated 了，建议使用新标准的 bind()。
下面先说明bind1st() 和 bind2nd()的用法，然后在说明bind()的用法。

头文件

#include <functional>

作用

bind1st()和bind2nd()都是把二元函数转化为一元函数，方法是绑定其中一个参数。
bind1st()是绑定第一个参数。
bind2nd()是绑定第二个参数。

例子

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

using namespace std;

int main()
{
    int numbers[] = { 10,20,30,40,50,10 };
    int cx;
    cx = count_if(numbers, numbers + 6, bind2nd(less<int>(), 40));
    cout << "There are " << cx << " elements that are less than 40.\n";

    cx = count_if(numbers, numbers + 6, bind1st(less<int>(), 40));
    cout << "There are " << cx << " elements that are not less than 40.\n";

    system("pause");
    return 0;
}

结果:

There are 4 elements that are less than 40.
There are 1 elements that are not less than 40.

分析
less()是一个二元函数，less(a, b)表示判断a<b是否成立。

所以bind2nd(less<int>(), 40)相当于x<40是否成立,用于判定那些小于40的元素。

bind1st(less<int>(), 40)相当于40<x是否成立,用于判定那些大于40的元素。

bind()

bind1st() 和 bind2nd()，在 C++11 里已经 deprecated 了.bind()可以替代他们，且用法更灵活更方便。

上面的例子可以写成下面的形式：

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

using namespace std;

int main()
{
    int numbers[] = { 10,20,30,40,50,10 };
    int cx;
    cx = count_if(numbers, numbers + 6, bind(less<int>(), std::placeholders::_1, 40));
    cout << "There are " << cx << " elements that are less than 40.\n";

    cx = count_if(numbers, numbers + 6, bind(less<int>(), 40, std::placeholders::_1));
    cout << "There are " << cx << " elements that are not less than 40.\n";

    system("pause");
    return 0;
}

结果:

There are 4 elements that are less than 40.
There are 1 elements that are not less than 40.

std::placeholders::_1 是占位符，标定这个是要传入的参数。
所以bind()不仅可以用于二元函数，还可以用于多元函数，可以绑定多元函数中的多个参数，不想绑定的参数使用占位符表示。
此用法更灵活，更直观，更便捷。

参考

http://www.cplusplus.com/reference/functional/bind1st/
http://www.cplusplus.com/reference/functional/bind2nd/
http://www.cplusplus.com/reference/functional/bind/