Type introspection

In computing, type introspection is a capability of some object-oriented programming languagess to determine the type of an object at runtime. This is a notable capability of the Objective C language.

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

- desex: (id) to_desex
{
   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.