使用自定义比较器的std::set触发程序崩溃问题排查

错误原因 #

1. 自定义比较器空指针崩溃：原代码用函数指针decltype(cmp)*作为std::set的比较器类型，但通过bucketCuisines[cuisines[i]]创建新集合时，默认构造的set会使用空指针作为比较器。插入元素时调用空指针，直接触发崩溃（DEADLYSIGNAL）。
2. 评分更新逻辑错误：changeRating函数中，插入新条目时误用了旧评分oldRating，而非新评分newRating，导致结果不符合预期。

修复方案 #

1. 替换函数指针为仿结构体比较器 #

使用重载operator()的结构体作为比较器，std::set默认构造时会自动生成合法的比较器实例，彻底避免空指针调用问题。

2. 修正评分更新的插入参数 #

将插入操作的oldRating改为newRating，确保更新后的评分正确存入集合。

修复后的完整代码 #

class FoodRatings {
public:
    struct Cmp {
        bool operator()(const pair<int, string>& a, const pair<int, string>& b) const {
            if (a.first == b.first)
                return a.second < b.second;
            return a.first > b.first;
        }
    };
    
    unordered_map<string, int> bucketFoods;
    unordered_map<string, string> bucketFoodNCuisine;
    map<string, set<pair<int, string>, Cmp>> bucketCuisines;
    
    FoodRatings(vector<string>& foods, vector<string>& cuisines, vector<int>& ratings) {
        int n = foods.size();
        for (int i = 0; i < n; ++i) {
            bucketFoods[foods[i]] = ratings[i];
            bucketFoodNCuisine[foods[i]] = cuisines[i];
            bucketCuisines[cuisines[i]].insert({ratings[i], foods[i]});
        }
    }
    
    void changeRating(string food, int newRating) {
        int oldRating = bucketFoods[food];
        bucketFoods[food] = newRating;
        string cuisine = bucketFoodNCuisine[food];
        bucketCuisines[cuisine].erase({oldRating, food});
        bucketCuisines[cuisine].insert({newRating, food});
    }
    
    string highestRated(string cuisine) {
        return bucketCuisines[cuisine].begin()->second;
    }
};

验证结果 #

针对给定输入：

["FoodRatings","highestRated","highestRated","changeRating","highestRated","changeRating","highestRated"]
[[["kimchi","miso","sushi","moussaka","ramen","bulgogi"],["korean","japanese","japanese","greek","japanese","korean"],[9,12,8,15,14,7]],["korean"],["japanese"],["sushi",16],["japanese"],["ramen",16],["japanese"]]

修复后的代码输出与预期完全一致：

[null, "kimchi", "ramen", null, "sushi", null, "ramen"]

内容的提问来源于stack exchange，提问作者Eunno An