How to provide custom comparator for `std::multiset` without overloading `operator()`, `std::less`, `std::greater`?

Question

I want a custom comparator for the following code. However, I am not allowed to overload operator(), std::less, std::greater.

I tried to achieve this using lambda but gcc won't allow me to use auto as a non-static member. Any other way to make this work?

#include <iostream>
#include <map>
#include <set>

class Test 
{
public:
    // bool operator () (const int lhs, const int rhs) { // not allowed
    //     return lhs > rhs;
    // };    
    using list = std::multiset<int  /*, Test*/>;
    std::map<const char*, list> scripts;
};

int main() 
{
    Test t;
    t.scripts["Linux"].insert(5);
    t.scripts["Linux"].insert(8);
    t.scripts["Linux"].insert(0);

    for (auto a : t.scripts["Linux"]) {
        std::cout << a << std::endl;
    }

    std::cout << "end";
}

Edit: With lambdas

class Test 
{
  public:
    auto compare = [] (const int a, const int b) { return a < b;}
    using list = std::multiset<int, compare>;    //here
    std::map<const char*, list> scripts;
};

Error:

'auto' not allowed in non-static class member
 auto compare = [] (const int a, const int b) { return a < b;}

Answer 1

I want a custom comparator for the following code. However, I cannot overload operator(), std::less, std::greater.

I assume that you are not allowed to overload operator() of the Test class, but could be that of other class. If so, create a internal private functor which overloads operator() and that could be part of the alias using list = std::multiset<int, Compare>;

class Test
{
private:
    struct Compare
    {
        bool operator()(const int lhs, const int rhs) const /* noexcept */ { return lhs > rhs; }
    };

public:
    using list = std::multiset<int, Compare>;
    std::map<std::string, list> scripts;
};

---

I tried to achieve these using lambdas but gcc won't allow me to use auto as a non-static member. Any other way to make this work?

Update: After researching a while, I found a way to go which does work^† with a lambda function.

The idea is to use the decltype of std::multiset with custom lambda compare as the key of the std::map scripts. In addition to that, provide a wrapper method for inserting the entries to the CustomMultiList.

Complete example code: (See live)

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

// provide a lambda compare
const auto compare = [](int lhs, int rhs) noexcept { return lhs > rhs; };

class Test
{
private:
    // make a std::multi set with custom compare function  
    std::multiset<int, decltype(compare)> dummy{ compare };
    using CustomMultiList = decltype(dummy); // use the type for values of the map 
public:
    std::map<std::string, CustomMultiList> scripts{};
    // warper method to insert the `std::multilist` entries to the corresponding keys
    void emplace(const std::string& key, const int listEntry)
    {
        scripts.try_emplace(key, compare).first->second.emplace(listEntry);
    }
    // getter function for custom `std::multilist`
    const CustomMultiList& getValueOf(const std::string& key) const noexcept
    {
        static CustomMultiList defaultEmptyList{ compare };
        const auto iter = scripts.find(key);
        return iter != scripts.cend() ? iter->second : defaultEmptyList;
    }
};


int main()
{
    Test t{};
    // 1: insert using using wrapper emplace method
    t.emplace(std::string{ "Linux" }, 5);
    t.emplace(std::string{ "Linux" }, 8);
    t.emplace(std::string{ "Linux" }, 0);


    for (const auto a : t.getValueOf(std::string{ "Linux" }))
    {
        std::cout << a << '\n';
    }
    // 2: insert the `CustomMultiList` directly using `std::map::emplace`
    std::multiset<int, decltype(compare)> valueSet{ compare };
    valueSet.insert(1);
    valueSet.insert(8);
    valueSet.insert(5);
    t.scripts.emplace(std::string{ "key2" }, valueSet);

    // 3: since C++20 : use with std::map::operator[]
    t.scripts["Linux"].insert(5);
    t.scripts["Linux"].insert(8);
    t.scripts["Linux"].insert(0);

    return 0;
}

---

^† Until c++20 lambda are not default constructable and copyable. But, the std::map::operator[] does requered the mapped_type to be copy constructible and default constructible. Hence the insertion to the value of the scripts map(i.e. to std::multiset<int, decltype(/*lambda compare*/)>) using subscription operator of std::map is only possible from from C++20.