Re: Virtual Ctor Idiom and auto_ptr
Terry G wrote:
In the FAQ, the virtual constructor idioms, create() and clone(), use raw
pointer return values.
Wouldn't it be better if these returned auto_ptr's instead?
But then covariant return types can't be exploited.
Here's some sample code.
Note that the use of clone() in main() is difficult to upcast.
I suppose that if one is using clone() then polymorphic behavior is probably
desired, so the upcast typically isn't necessary.
I forgot to mention this in my first answer, but clone() and
create() typically have a very strict post condition, which must
be met by all derived classes, and so are almost never virtual,
but rather call a private virtual function to do the work, and
then verify the post-condition, i.e.:
Base*
Base::clone() const
{
Base* result = doClone() ;
assert ( typeid( *result ) == typeid( *this ) ) ;
return result ;
}
So covariant return types don't apply anyway.
And of course, if a derived class did want to provide a clone()
which returned a Derived*, it is free to do so. Most of the
time, of course, there's no point in it, but I suppose there are
exceptions, e.g. if the derived class defines some sort of
extended contract.
--
James Kanze (Gabi Software) email: james.kanze@gmail.com
Conseils en informatique orient?e objet/
Beratung in objektorientierter Datenverarbeitung
9 place S?mard, 78210 St.-Cyr-l'?cole, France, +33 (0)1 30 23 00 34
--
[ See http://www.gotw.ca/resources/clcm.htm for info about ]
[ comp.lang.c++.moderated. First time posters: Do this! ]