Editing Immutable object (section)

=== Copy-on-write ===
A technique that blends the advantages of '''mutable''' and '''immutable''' objects, and is supported directly in almost all modern hardware, is [[copy-on-write]] (COW). Using this technique, when a user asks the system to copy an object, it instead merely creates a new reference that still points to the same object. As soon as a user attempts to modify the object through a particular reference, the system makes a real copy, applies the modification to that, and sets the reference to refer to the new copy. The other users are unaffected, because they still refer to the original object. Therefore, under COW, all users appear to have a mutable version of their objects, although in the case that users do not modify their objects, the space-saving and speed advantages of immutable objects are preserved. Copy-on-write is popular in [[virtual memory]] systems because it allows them to save memory space while still correctly handling anything an application program might do.