ld.lld: error: undefined symbol: Solution::m

今天在写 leetcode 的时候, 出现了这么一个报错:

源代码

class Solution {

public:

    static unordered_map<int, int> m;

    static bool compare(int a, int b) {
        if (m[a] < m[b]) {
            return true;
        } else if (m[a] == m[b]) {
            return a > b;
        }
        return false;
    }

    vector<int> frequencySort(vector<int> &nums) {

        m.clear();

        for (auto i : nums) {
            m.find(i) == m.end() ? m[i] = 1 : ++m[i];
        }

        sort(nums.begin(), nums.end(), compare);
        return nums;
    }
};

报错

执行结果:
编译出错
ld.lld: error: undefined symbol: Solution::m
>>> referenced by prog_joined.cpp
>>>               /tmp/prog_joined-c167f6.o:(Solution::frequencySort(std::vector<int, std::allocator<int> >&))
>>> referenced by prog_joined.cpp
>>>               /tmp/prog_joined-c167f6.o:(Solution::frequencySort(std::vector<int, std::allocator<int> >&))
>>> referenced by prog_joined.cpp
>>>               /tmp/prog_joined-c167f6.o:(Solution::frequencySort(std::vector<int, std::allocator<int> >&))
>>> referenced 7 more times
clang-11: error: linker command failed with exit code 1 (use -v to see invocation)

原因分析

我感觉写的没毛病啊, 后来发现, 类的静态成员变量需要在类外面初始化:

unordered_map<int, int> Solution::m;

这样就行了