What's the issue in custom comparator?

Question

Hi

Tried having set of pairs. Can anyone help me understand issue? I think it is due to custom comparator I am using for set of pair, but not able to figure out what issue.
https://code.sololearn.com/c26W5z2ex8fu

Thanks a lot....!

Answer 1

You simply forgot to pass the comparator to the constructor of the set. Since you are using a function pointer as the template type, the currently used default value is a null pointer, and the code fails at its attempted dereferenciation during a comparison.

Answer 2

The Compare (2nd) argument in set template is a class, whose requirement is that expression comp(a, b), where comp is an object whose type is the class, should be valid (for both non-const and const).

So, what you should do is make compare a class, and define the actual comparison in the overloaded (). You should also make it that the function works for both const and non-const object.

Define compare as the following will work

struct compare
{
bool operator()(const pair<int,int>& obj1,const pair<int,int>& obj2) const
{
if (obj1.second != obj2.second)
return obj1.second < obj2.second;

return obj1.first < obj2.first;
}
};

Answer 3

Good point..

I am using the set as

mySet;//(index , count)

If I use set as count and index, it works without any requirement of custom comparator.

Just wanted to try with custom comparator and it crashed so curious to know issue

Answer 4

Ipang yes, it's is called functor.

Answer 5

I thought if function with decltype is not allowed , it should not compile at all... In this case , it gets compiled and results into runtime error.... Are you sure that functor is the only thing we can use even though it compile with function as well

And I tried with functor also and it is not working

Answer 6

Also, reply to your code doesn't work with functor. Your operator() is not const.

Answer 7

Yeah functor works ...thanks

I am still interested to know issue in function with decltype

Answer 8

CarrieForle,
Is this how we imitate a function by overloading () operator in a class/struct?

Answer 9

Yeah CarrieForle , I got your point...

We can use functor like you said...also lambda should also work..

My concern is why custom comparator as function is not working like I have coded.... What is missing or it is not at all possible ?

Answer 10

I don't think it is. Remind that what set want is a "data type", which is made with keyword class, struct, or enum. Function is not a data type, and the return type of compare (bool) doesn't make a valid expression of bool(a, b).

Answer 11

decltype(&function) return the pointer of the return type of the function. decltype doesn't run the function.

EDIT: after testing. Yes, decltype(function) does work. My bad. However, declaring a variable of decltype(function) seems doesn't do anything.