All queries about std::map having key as custom class object

Question

Hi

Once we use to have map for custom class , we need to have < operator defined... It is because third default argument for the map is less function object and hence we need to have this defined. If we have custom comparator , < is not must...

What is priority by compiler if both are defined and have different implementation?

Another question is need of < operator... I belive that it is required when we insert element... Does it require for deletion or find operation as well ?

Find means comparision to check equal to or not... Then how it works without asking us to have overloaded == operator for custom class ?

Answer 1

Last part ans...

std::map does not care about literal unicity of the keys. It cares about keys equivalence. The keys a and b are equivalent by definition when neither a < b nor b < a is true.

Simply operator < is enough. Equality can be checked by testing a < b and b < a both returning false.

Original ans is https://stackoverflow.com/questions/20168173/when-using-stdmap-should-i-overload-operator-for-the-key-type#:~:text=You%20should%20overload%20operator%3C%20.,as%20a%20test%20for%20uniqueness.

Answer 2

Thanks for your help ROOKIE

Answer 3

Regarding my first question about choice between custom comparator and <, it is never the case.

Refer code below:
https://code.sololearn.com/c6WexWpRFRa3

If we provide third argument into map declaration as comparator function, compiler choses the same . Other wise < is chosen and it is must to be defined.