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In Objective C, for example, both the generic Object and NSObject (in Cocoa/ OpenStep) provide the method isMemberOfClass: which returns true if the argument to the method is an instance of the specified class. The method isKindOfClass: analogously returns true if the argument inherits from the specified class.
For example, say we have a Puppy and Kitten class inheriting from Animal, and a Vet class.
Now, in the desex method we can write
{
if([to_desex isKindOfClass:[Animal class]]) { //we're actually desexing an Animal, so continue if([to_desex isMemberOfClass:[Puppy class]]) desex_dog(to_desex); else if([to_desex isMemberOfClass:[Kitten class]]) desex_cat(to_desex); else error(); } else { error(); }}
Now, when desex is called with a generic object (an id), the function will behave correctly depending on the type of the generic object.
Type introspection can be used to implement polymorphism.