on Fri Jun 04 2010, "Bartlett, Roscoe A" <rabartl-AT-sandia.gov> wrote:
David,
From: David Abrahams [mailto:dave@boostpro.com]
The whole notion of deep-vs-shallow copy is a fallacy. If you nail down what it means to make a “copy”---which will force you to nail down what constitutes an object's value---you'll see that.
[Bartlett, Roscoe A]
It is not a fallacy. The only obvious behavior for value semantics is deep copy such that if you do:
A a(...); B b = a;
then any change to 'b' will have *no* impact on the behavior of 'a' at all, period.
And that, right there, is the definition of “copy” (assuming A and B are the same type). There's no such thing as a “shallow copy” of a that's different from what you just described.
Anything else is not value semantics and will confuse people. Make it simple; most types should have either value semantics or reference semantics as described in Section 4.1 in the Teuchos MM report. Anything in between is just confusing and counter-intuitive.
Your “reference types” as defined there are not copyable, so there's no “shallow copy” going on. Period. Viewing it that way makes it *really* simple. :-) Cheers, -- Dave Abrahams BoostPro Computing http://boostpro.com