CODEWITHSUNDEEP
c++comparatormultimap
John
John

Reputation: 59

C++: multimap custom comparator for object keys

Can I somehow use my own function for ordering the pairs in multimap? I have three classes CTimeStamp, CMail and CMailLog. And the thing is in the CMailLog I have multimap<CTimeStamp, CMail> which I use because for this task I need solution which will be very fast for huge amounts of data and therefor I would need to somehow use method Compare from CTimeStamp when inserting into this multimap. The classes look something like this.

class CTimeStamp {
   public:
     int compare (const CTimeStamp &x) const;
     ...
}
class CMail {
   ...
}
class CMailLog {
   public:
     ...
   private:
     multimap<CTimeStamp, CMail> logs;
}

I'm not sure how to do this or if it's even possible.

Upvotes: 1

Views: 734

Answers (1)

πάντα ῥεῖ
πάντα ῥεῖ

Reputation: 1

I would need to somehow use method Compare from CTimeStamp when inserting into this multimap

As from the std::multimap documentation, all you need is to either

  • provide a specialisation for std::less<CTimeStamp>

    namespace std {
    bool less<CTimeStamp>(const CTimeStamp& a, const CTimeStamp& b) {
        return a.compare(b) < 0;
    }
}

or

  • provide a custom comparator at the constructor:

    CMailLog() : 
    logs([](const CTimeStamp& a, const CTimeStamp& b) { return a.compare(b) < 0; }) 
{}

I used a lambda expression in my last example for the constructor as I consider that's the shortest and most comprehensible form.
In fact any callable with the signature bool (const CTimeStamp&,const CTimeStamp&) would fit well.

You might also write a simple global function

bool foo(const CTimeStamp& a,const CTimeStamp& b) {
    return a.compare(b) < 0;
}

or appropriate callable type

struct foo {
    bool operator()(const CTimeStamp& a,const CTimeStamp& b) {
        return a.compare(b) < 0;
    }
};

and pass that one at the 

multimap<CTimeStamp, CMail> logs;

in the constructor initializer list:

CMailLog() : logs(foo) {}

Callable struct version

CMailLog() : logs(foo()) {}

Upvotes: 3

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