How can I create my own comparator for a map?

c++ stl stdmap
Xitrum picture Xitrum · Apr 20, 2011 · Viewed 130.6k times · Source 
typedef map<string, string> myMap;

When inserting a new pair to myMap, it will use the key string to compare by its own string comparator. Is it possible to override that comparator? For example, I'd like to compare the key string by its length, not by the alphabet. Or is there any other way to sort the map?

Answer

Georg Fritzsche picture Georg Fritzsche · Apr 20, 2011

std::map takes up to four template type arguments, the third one being a comparator. E.g.:

struct cmpByStringLength {
    bool operator()(const std::string& a, const std::string& b) const {
        return a.length() < b.length();
    }
};

// ...
std::map<std::string, std::string, cmpByStringLength> myMap;

Alternatively you could also pass a comparator to maps constructor.

Note however that when comparing by length you can only have one string of each length in the map as a key.

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