c++ - 如何在另一个类数据成员上定义 `std::set` 排序？

Question

我试过但不起作用的代码:

class A {
public:
    struct cmpr_t {
        bool operator() (int k1, int k2) {
            return mp[k1] < mp[k2];  // doesn't compile
        }
    };

    map<int, int> mp;  // storing key->value
    set<int, cmpr_t> ss;  // just keys, ordered by corresponding value in mp
};

我只想要一个map和一个set，map存储数据(键，值)，而set 只包含键，我希望 set 按键的对应值排序。

那么如何定义集合呢？

更新

编译器错误:

In member function ‘bool SSet::cmpr_t::operator()(int, int)’:
error: invalid use of non-static data member ‘SSet::mp’
     unordered_map<int, int> mp;  // k -> v
                             ^
error: from this location
             return mp[l] < mp[r];
                    ^
error: invalid use of non-static data member ‘SSet::mp’
     unordered_map<int, int> mp;  // k -> v
                             ^
error: from this location
             return mp[l] < mp[r];
                            ^

Answer 1

class A 
{
    struct cmpr_t
    {
        A* a;
        explicit cmpr_t(A* a) : a(a) {}
        //                      ~~~^
        bool operator()(int k1, int k2) const
        {
            return a->mp[k1] < a->mp[k2];
            //     ~~^         ~~^
        }
    };
    std::map<int, int> mp;
    std::set<int, cmpr_t> ss;

public:         
    A() : ss(cmpr_t(this)) {}
    //       ~~~~~~~~~~~^ 
};