Can multimaps be implemented with structures as key? If yes then how?

Question

For example two structures like point and square as given below if it is possible to do so how may i insert new elements into it?

typedef struct _point{
int x;
int y;
}Point;
typedef struct _square{
float belowLeftX;
float belowLeftY;
}Square;
multimap <Square, Point > dictionary;

You'll find a detaile description of the requirements for _`Key`_ types [here](https://en.cppreference.com/w/cpp/container/multimap). — πάντα ῥεῖ, Dec 12 '20 at 13:28
A map is an ordered container, the ordering is based on the keys, so naturally they need to be compared to one another (using `operator<`). Also, don't prepend your type names with an underscore. — Mansoor, Dec 12 '20 at 13:32
Thanks for your response, So i think it is better to look for some other solution then ? @M.A — Ali Akber Faiz, Dec 12 '20 at 13:39

score 2 · Accepted Answer · answered Dec 12 '20 at 13:55

Yes, a structure as key is not different compared to a class as key. You have two options to get your code working.

Option 1: Supply ordering to the Square type.

typedef struct _square {
    float belowLeftX;
    float belowLeftY;
    bool operator<(struct _square const& rhs) const
    {
        if (belowLeftX < rhs.belowLeftX) return true;
        if (rhs.belowLeftX < belowLeftX) return false;
        return belowLeftY < rhs.belowLeftY;
    }

Option 2: Supply ordering of the Square type to the dictionary.

auto comparator = [](Square const& lhs, Square const& rhs)
{
    if (lhs.belowLeftX < rhs.belowLeftX) return true;
    if (rhs.belowLeftX < lhs.belowLeftX) return false;
    return lhs.belowLeftY < rhs.belowLeftY;
};

std::multimap <Square, Point, decltype(comparator)> dictionary(comparator);

Can multimaps be implemented with structures as key? If yes then how?

1 Answers1