Is sizeof() is an operator or function

Answer 1

sizeof() is an operator. Check this out for more information: https://stackoverflow.com/questions/1393582/why-is-sizeof-considered-an-operator

Answer 2

Yes, this doubt is quite a common one.
The sizeof() operator, though it looks and works like a function is NOT a function but rather an operator, an operator which exists as a combination of characters.
The good thing is, you don't have to include any header file in order to use it...
The bad thing is, one can't modify it...