Editing Immutable object (section)

=== Referencing vs copying objects ===
If an object is known to be immutable, it is preferred to create a [[reference (computer science)|reference]] of it instead of copying the entire object. This is done to conserve memory by preventing data duplication and avoid calls to constructors and destructors; it also results in a potential boost in execution speed.

The reference copying technique is much more difficult to use for mutable objects, because if any user of a mutable object reference changes it, all other users of that reference see the change. If this is not the intended effect, it can be difficult to notify the other users to have them respond correctly. In these situations, [[defensive copy]]ing of the entire object rather than the reference is usually an easy but costly solution. The [[observer pattern]] is an alternative technique for handling changes to mutable objects.