c++ - 非静态函数作为 C++ 中另一个函数的参数

Question

我创建了一个类student，它的子类是comparator。 comparator 的构造函数采用一个名为 cmp_mode 的参数，该参数指定我们应该如何比较学生。

class student
{
public:
    std::string name;
    int course, group;

    student(std::string name,
            int course,
            int group): name(name)
    {
        this->course = course;
        this->group = group;
    }

    enum cmp_mode
    {
        NAME,
        COURSE,
        GROUP
    };

    class comparator
    {
        cmp_mode mode;

    public:
        comparator(cmp_mode mode)
        {
            this->mode = mode;
        }

        bool compare(student s1, student s2)
        {
            if (mode == NAME)
                return s1.name < s2.name;
            else if (mode == COURSE)
                return s1.course < s2.course;
            else if (mode == GROUP)
                return s1.group < s2.group;
            else
                throw "Oh god! We can't compare variables using these keys!";
        }
    };

};

此外，我还创建了一个列表 的students，现在我想使用比较器子类对该列表进行排序。

std::list<student> l;

student st1("Anya", 2, 5);
student st2("Misha", 4, 2);
student st3("Yasha", 1, 3);

l.push_back(st1);
l.push_back(st2);
l.push_back(st3); 

student::comparator by_group(student::GROUP);

l.sort(by_group.compare);

但我收到以下错误。

ERROR: Reference to non-static member function must be called.

那我该怎么办呢？如何以更好的方式调整排序？

Answer 1

我从你的评论开始:

I thought that I can write compare function for each case, but it seems even worse.

为什么更糟？我个人认为它运行速度更快(至少编译速度更快)并且更易于维护(功能更短)。

这样写是不是简单多了:

l.sort(student::compare_by_group);

而且这个实现更容易维护:

class student
{
...    
    static bool compare_by_name(const student& s1, const student& s2)
    {
        return s1.name < s2.name;
    }
    static bool compare_by_course(const student& s1, const student& s2)
    {
        return s1.course < s2.course;
    }
    static bool compare_by_group(const student& s1, const student& s2)
    {
        return s1.group < s2.group;
    }

    // Add the followings only if you really need this stuff 
    // e.g. to get comparator type from elsewhere
    enum cmp_mode
    {
        NAME,
        COURSE,
        GROUP
    };

    static bool compare(cmp_mode, const student& s1, const student& s2)
    {
        switch(cmp_mode) {
          case NAME: return compare_by_name(s1, s2);
          ...
        }
    } 

};