non virtual interfaces

[Alexandr Druzhinin]

20.09.2013 12:45, Ali Çehreli пишет:
> On 09/19/2013 10:31 PM, Alexandr Druzhinin wrote:
>
>  > if I use protected instead of private in interface like:
>
> private member functions are non-virtual.
But I just use code example from TDPL russian edition. And TDPL says 
clearly that (un)transmogrify() are private and CardboardBox _maynot_ 
make (un)transmogrify() non-private - this is highlighted as compiler 
support of NVI idiom. Is it wrong example, I guess?
>
>  > interface Transmogrifier
>  > {
>  >      final void thereAndBack()
>  >      {
>  >          transmogrify();
>  >          untransmogrify();
>  >      }
>  >
>  >      protected:
>  >          void transmogrify();
>  >          void untransmogrify();
>  > }
>
> If they were non-virtual (i.e. private), the calls to transmogrify() and
> untransmogrify() from thereAndBack() would be bound to
> Transmogrifier.transmogrify and Transmogrifier.untransmogrify at compile
> time. That happens and the linker cannot find their definitions.
I see. Thanks, I understand it now better
>
>  > class CardboardBox: Transmogrifier
>  > {
>  >      override protected void transmogrify() { }
>  >      override void untransmogrify() {}
>  > }
>
>  > it compiles, but why does compiler permit making untransmogrify() be
>  > public?
>
> It is up to CardboardBox to decide whether untransmogrify() is public or
> not. Note that untransmogrify() is still protected when objects are used
> through the Transmogrifier interface. However, when an object is known
> to be a CardboardBox so that it is being used through the CardboardBox
> interface, it is not bound to be a Transmogrifier at that point. Yes,
> CardboardBox inherits from Transmogrifier but it is CardboardBox's
> interface that is being used at that point so it decides.
Thanks again. So there is no compiler support for NVI idiom? Because if 
CardboardBox may define its own (un)transmogrify() - TDPL says it 
possible only if (un)transmogrify() have other signatures.
>
>  > How can I prohibit this? May be it just unrealized yet?
>
> You cannot prohibit from Transmogrifier.
>
> Ali
>
Unfortunately I tried to use NVI for it namely.