如何在地图中找到最小值

我想在地图上找出最小值。我有一个函数来自 Finding minimum value in a map

#import tool.mm
std::map<std::string, float> direction;
std::pair<std::string, float> min;

direction["up"] = 50.0;
direction["down"] = 20.0;
direction["right"] = 100.0;
direction["left"] = 200.0;

min = *min_element(direction.begin(), direction.end(), &Tool::compare);

这是什么 Tool 类的外观如下:

//tool.mm
class Tool {
public:
    bool compare(std::pair<std::string, float> i, std::pair<std::string, float> j) {
        return (i.second < j.second);
    }
};

当我运行此函数时,显示一个错误

Called object type 'bool (Tool::*)(std::__1::pair<std::__1::basic_string<char>, float>, std::__1::pair<std::__1::basic_string<char>, float>)' is not a function or function pointer