Question

I have a map as follows:

std::map<A, long> myMap

The ordering of this map is important so I am trying to figure out how the map will be ordered. I have found that on the documentation of this class it says Internally, the elements in the map are sorted from lower to higher key value following a specific strict weak ordering criterion set on construction but I don’t understand what this means. Will it call the ‘<‘ operator on the two objects to figure out the ordering?

I am also aware that I could just pass the map a struct as follows on initialisation and it will do as I want. I am just curious as to what it does by default.

struct classcomp {
  bool operator() (const A& lhs, const A& rhs) const
   {return lhs<rhs;}
};

Answer 1

Will it call the ‘<‘ operator on the two objects to figure out the ordering?

Yes, it will:

struct A {
    bool operator<(const A& other) const
    {
        return ...; // return true if *this is less than other; false otherwise
    }
};

Make sure that the “less than” relation defined by your operator< is transitive, i.e. if A < B and B < C, then A must be less than C as well.