CODEWITHSUNDEEP
c++
exco
exco

Reputation: 15

How to initialize a map of set with a comparison function?

I have a map where the key is a string and the value is a set of object Msg.

class Log{
std::map<std::string, std::set<Msg>> messages;
}

class Msg{
friend class Log;
std::string message;
std::chrono::system_clock::time_point timestamp;
}

The key is a phone number and the set contains objects with a text message from that number as well as the message's timestamp. I decided to use a set so that when I insert the messages, it will be sorted by its timestamp which is type time_point. The source is a text file of unsorted messages.

I wrote a comparison function for the set:

bool compareTimestamp(const Msg &lhs, const Msg &rhs){
return lhs.timestamp < rhs.timestamp;
}

From my research, if this were just a set and not a map of a set, to use the comparison function I would have to define the set like this

std::set<Msg, decltype(compareTimestamp)*> set;

and initialize the set in the constructor like this:

Log() : set(compareTimestamp){}

That worked when I tested it by inserting unsorted time_points.

However, I'm having trouble figuring out how to initialize the set with the comparison function inside of the map.

The map is defined in class Log like this:

std::map<std::string, std::set<Msg, decltype(compareTimepoint)*>> messages;

I tried to initialize this way but it's obviously wrong since it's initializing the map and not the set inside (I tested it anyway and it didn't work):

Log() : messages(compareTimepoint){}

Does anyone know how this can be done?

Upvotes: 0

Views: 1353

Answers (1)

Potatoswatter
Potatoswatter

Reputation: 137780

Each set has its own comparison function. The map doesn't know about the functions of the sets under it, or that those functions are all the same.

Since you don't plan to assign any other functions to the function pointer, you can use a comparison class instead. Such a class doesn't need initialization as a pointer does.

struct compareTimestamp {
    bool operator () (const Msg &lhs, const Msg &rhs) const {
        return lhs.timestamp < rhs.timestamp;
    }
};

std::map<std::string, std::set<Msg, compareTimestamp>> messages;

Upvotes: 3

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