"David Abrahams" <dave@boost-consulting.com> wrote in message news:u8y342xnj.fsf@boost-consulting.com... | "Thorsten Ottosen" <nesotto@cs.auc.dk> writes: | | > "Peter Dimov" <pdimov@mmltd.net> wrote in message | > news:099701c54ab2$8f8fb080$6401a8c0@pdimov2... | > | Thorsten Ottosen wrote: | > | > | > I think you have to explain why they are not different then. | > | The burden of proof lies usually with the person making the | > | statement. ;-) | > | > no, the burden of proof lies on the person making the controversial | > statement; in this case you. | | Controversialness is in the eye of the beholder, and regardless, | Peter's right: by convention, the burden of proof lies with the | claimant. I disagree. | That's very practical, because if you're saying something | so obvious and "uncontroversial" it should be very _very_ simple to | demonstrate it. Pick up you're favourite book on OO and on STL and see if the programming techniques look alike. | > stuff like "value-based programming is not different from | > OO-programming" | | Nobody made that statement. Peter asked the question. | > | The reference semantics vs value semantics axis is orthogonal to OO, | > | in my opinion. The fact that in C++ you can't (efficiently) have | > | polymorphism with value semantics doesn't mean that copying a value | > | and copying a polymorphic object are conceptually or fundamentally | > | different. A cloning pointer is a very good approximation of a | > | polymorphic value. I don't see why you consider it conceptually | > | different from a value. | > | > because you had to put "polymorphic" in front of "value" to describe | > what it means; | | The user can't tell a Pimpl with a polymorphic impl from one without. | They both just look like values, and all those behavioral variations | that are implemented with virtual functions could just as well be | implemented with "if" statements. Nobody (except maybe you) has a | problem with allowing pimpls to have copy ctors -- impls often have | clone() functions just to serve those copy ctors. A ptr_container is | almost exactly like a multi-pimpl or a pimpl aggregate. I'm not prohibiting you from copying something with clone(), it just not the default behavior. | > polymorphic objects don't have value-based copy-semantics; you can't | > provide meaningful copy-constructor and copy-assignment operators. | | Sure they do. You only can't provide a meaningful copy ctor and copy | assignment for an abstract base class. and then you get the nice slicing behavior. -Thorsten