How to access member variable from nested class

I need to make a Solution class that holds several 1D points and it can be given a center and a number k to calculated the k nearest points to this center.

My code,

class Solution {
private:
    int center_;
    struct Point {
        int x;
        bool operator<(const Point &other) const {
            return (x - center_) * (x - center_) < (other.x - center_) * (other.x - center_);
        }
    };
public:
    vector<int> findNearestKPoints(vector<int> &nums, int k, int center) {
        center_ = center;

        // vetor<int> to vector<Point>
        vector<Point> points;
        for (int num : nums) {
            points.push_back({num});
        }

        // partition vector<Point>
        nth_element(points.begin(), points.begin() + k - 1, points.end(), less<int>());

        // vector<Point> to vector<int>
        vector<int> res;
        for (int i = 0; i < k; ++i) {
            const Point &point = points[i];
            res.push_back(point.val);
        }
        return res;
    }
}

but it cannot compile.

The compile error is

use of non-static data member 'center_' of 'Solution' from nested type 'Point'

So how to fix it? Maybe there are other ways to calculate nearest points.