C++: multimap custom comparator for object keys

Question

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.

Answer 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()) {}