operator overloading for (), [], ->

[jinendrashankar]

why operators (), [], -> and = can be overloaded only by class members.

or
why operators (), [], -> and = can not be overloaded by friend function.

[Ganon11]

Sounds like a good question from a book or homework assignment. You can either (1) think about these operators, and try to understand why they cannot be friend functions, or (2) try a search engine such as Google.