What is need of custom comparator post object name of map

Question

Hi

Refer code below:

https://code.sololearn.com/c6WexWpRFRa3

refer line #35 from function void testComparator() as below:
map<test,int,decltype(comparator)> m(comparator);

I understand that map template has three argument and hence map<test,int,decltype(comparator)> is ok for me to understand.
What is meaning of m(comparator)? I mean why comparator is passed to object?

Answer 1

When you not pass the lambda as a parameter, it will try to create one ( using default constructor of decltype(comparator) ) which is not possible as lambdas doesn't have any default constructors.

Answer 2

Thanks Arsenic