static constexpr遇到的undefined 问题

class Solution {
    static constexpr int check[10] = {0, 0, 1, -1, -1, 1, 1, -1, 0, 1};
public:
    int rotatedDigits(int n) {
        int ans = 0;
        for (int i = 1; i <= n; ++i) {
            std::string num = std::to_string(i);
            bool valid = true, diff = false;
            for (char ch: num) {
                if (check[ch - '0'] == -1) {
                    valid = false;
                }
                else if (check[ch - '0'] == 1) {
                    diff = true;
                }
            }
            if (valid && diff) {
                ++ans;
            }
        }
        return ans;
    }


};

编译遇到undefined Solution::check问题,这是因为函数里的check[xx]用到了check的地址

而static声明的东西没有地址，需要外部声明给定一个地址

所以加上类外部声明，cxx17以后不用了，constexpr被隐式inline了

int constexpr Solution::check[];