template<typename DataValue>
class Cache {
private:

    class Data {
    public:
        DataValue dataValue;
        std::list<std::string>::iterator it;

        Data(const DataValue& dataValue, std::list<std::string>::iterator newIterator)
            : dataValue(dataValue)
            , it(newIterator)
        {}

        Data() {}
    };

    std::unordered_map<std::string, Data> map;
    std::list<std::string> listOfKeys;
    std::function<DataValue(std::string)> GetFromServer;
    int MAX_SIZE;

    void RemoveMostNotRelevant() {
        const std::string& mostNotRelevantKey = listOfKeys.front();
        map.erase(mostNotRelevantKey);
        listOfKeys.pop_front();
    }

public:
    Cache(int maxSize = 1, std::function<DataValue(std::string)> getFromServer)
        : MAX_SIZE(maxSize)
        , GetFromServer(getFromServer)
    {}

    Data operator () (const std::string& key) {
        if (std::size(map) == MAX_SIZE) {
            if (map.count(key)) {
                Data& data = map[key];

                listOfKeys.erase(data.it);
                listOfKeys.push(key);
                data.it = std::prev(listOfKeys.end());

                return data.dataValue;
            } else {
                RemoveMostNotRelevant();

                DataValue dataValue = GetFromServer(key);
                listOfKeys.push(key);

                Data data(dataValue, std::prev(listOfKeys.end()));
                map[key] = data;

                return dataValue;
            }
        } else {
            auto [it, inserted] = map.insert({key, Data()});
            Data& data = it->second;

            if (inserted) {
                data.dataValue = GetFromServer(key);
                listOfKeys.push(key);
                data.it = std::prev(listOfKeys.end());
            } else {
                listOfKeys.erase(data.it);
                listOfKeys.push(key);
                data.it = std::prev(listOfKeys.end());
            }

            return data.dataValue;
        }
    }

};


class MyClass
{

};

int myFunc()
{
    Cache<double> myCache(10, [&] (std::string key) { return DataValue(key); });
    Cache<MyClass> myCache(10);
}