copy-on-write (was: globals considered harmful)

Urs Hoelzle urs at
Thu Mar 24 21:34:05 UTC 1994

> [...]
> I think there are definite advantages to unifying this behaviour by
> providing a mechanism for copy-on-write on a per-slot basis.  For
> example, in a word-processor I might want all paragraphs to appear in
> a default typeface unless specifically overridden by the user; if the
> default is changed all paragraphs which have not been so overridden
> should reflect that change; and a paragraph-object with an overridden
> "typeface" ("style", whatever) slot should be revertable to the default
> behaviour by deleting the slot.  I think that is what Alexis was getting at.

Interestingly enough, NewtonScript (the programming language used on
the Apple Newton) has "copy on write" (COW) slots.  One reason for
including COW (as I understand it, I'm not a Newton programmer) was to
save space: clones only need to hold changed slots, not all slots.  In
the Newton, the prototype's slots can even be in ROM (no prototype
corruption problem here :-), further saving precious RAM.

The problem with COW is that there's no straightforward efficient
implementation.  For example, if an assignment grows an object, the
implementation has to make up a new object and redirect all pointers
to the old object.  That's only fast if you use indirect pointers
(i.e., an object table), which is acceptable for an interpreter but
slows down a compiled system quite a bit.

However, there may be some clever trick to implement COW efficiently.
Sounds like a Ph.D. thesis to me... :-)


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