我们需要unary_function和binary_function做什么？

#include<iostream>
#include<functional>

class Square : public std::unary_function<int, int> {
public:
    int operator()(int x) const {
        return x * x;
    }
};

int main() {
    Square square;
    std::cout<< square(5)<< std::endl; // 输出 25
    return 0;
}

#include<iostream>
#include<functional>

class Add : public std::binary_function<int, int, int> {
public:
    int operator()(int x, int y) const {
        return x + y;
    }
};

int main() {
    Add add;
    std::cout<< add(3, 4)<< std::endl; // 输出 7
    return 0;
}