Object index

Explanation
The object_index variable is a property of every instance. It is the index of the object which the specified instance is an instance of. This allows one to determine the type of object that an instance is in code, and then treat the instance appropriately.

Examples
show_message("My object index is: " + string(object_index))

This code would show a message to the user with the text "My object index is: " plus the object index of the instance calling the function.

Remarks
This variable cannot be changed, for obvious reasons. It is accessible to all versions of GM, and is an attribute of all instances of any object.