How to specify a lambda function as comparator to a class std::map member variable

Question

How to use a custom comparator for a class std::map variable using lambda function. I'm getting error as error: 'comp' is not a type, std::map<std::pair<std::string, int>, int, decltype(comp)> mapp(comp);. This is my code

#include <iostream>
#include <string>
#include <map>

auto comp = [] (const std::pair<std::string, int>& p1, const std::pair<std::string, int>& p2) {
    if (p1.second > p2.second)
        return true;
    else if (p1.second == p2.second)
        return p1.first < p2.first;

    return false;
};

class A {
    // below also doesn't work
    /*static constexpr auto comp = [] (const std::pair<std::string, int>& p1, const std::pair<std::string, int>& p2) {
        if (p1.second > p2.second)
            return true;
        else if (p1.second == p2.second)
            return p1.first < p2.first;

        return false;
    };*/
    std::map<std::pair<std::string, int>, int, decltype(comp)> mapp(comp);

public:
    A() {

    }

    void add(const std::string& name, int score) {
        
    }

    std::string get() {
        return "";
    }
};

int main() {
}

In your map, key is a pair and value is an int. Your comparator should also have a pair of pair and int, right? — kiner_shah, Dec 12 '21 at 07:26
"*How to use a custom comparator for a class std::map variable using lambda function.*" Can you not just use a named struct? — Nicol Bolas, Dec 12 '21 at 07:27
@kiner_shah the lambda function takes two pairs of type `std::pair` as parameters — Harry, Dec 12 '21 at 07:28
Try initializing `mapp` in the constructor `A() : mapp(comp) {}` — kiner_shah, Dec 12 '21 at 07:33
@kiner_shah can you post that as an answer with some explanation, so that I can close this post. — Harry, Dec 12 '21 at 07:41
@Evg, is `()` not preferred for some reason? It should work right? — kiner_shah, Dec 12 '21 at 07:42
@kiner_shah It shouldn't. Please see linked dupes for an explanation. — Evg, Dec 12 '21 at 07:43

How to specify a lambda function as comparator to a class std::map member variable

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