Question

How does [array containsObject:id] method works? whether is it compare with addresses or is it compare the given objects properties with the array elements properties ?

Answer 1

Did you first try looking at the documentation for this method?

This method determines whether anObject is present in the array by sending an isEqual: message to each of the array’s objects (and passing anObject as the parameter to each isEqual: message).

And from the isEqual: method:

If two objects are equal, they must have the same hash value. This last point is particularly important if you define isEqual: in a subclass and intend to put instances of that subclass into a collection. Make sure you also define hash in your subclass.

What we can take from this is, unless you’re comparing two objects of predefined types (e.g. NSArray, NSString) where there is already a system-described way of hashing the object’s contents and comparing the hashes, you need to provide a - (NSUInteger)hash method which returns an unsigned integer whose value, for objects you consider equal, will be equal.