Re: some casting questions
On Jun 16, 3:22 pm, Jess <w...@hotmail.com> wrote:
It seems both static_cast and dynamic_cast can cast a base class
pointer/reference to a derived class pointer/reference. If so, is
there any difference between them?
Several. The dynamic_cast does actual run-time type checking.
It will fail (return a null pointer or throw an exception) if
the actual object is not of the right type. In addition,
dynamic_cast can do many casts that aren't possible with
static_cast; roughly speaking, static_cast can only move up and
down in the inheritance lattice; dynamic_cast can move anywhere.
Also, static_cast won't work from base to derived if the
inheritance is virtual. Finally, dynamic_cast only works from
base to derived if the types are polymorphic; static_cast
doesn't have this restriction.
In general, when casting within an inheritance hierarchy, you'll
almost always want dynamic_cast. static_cast is generally only
used to or from void*, or with various scalar or class types
(although I tend to prefer either C styles casts or functional
style casts here).
In addition, if I have a derived class object and then upcast
it to its base class, which cast operator should I use? Is it
static_cast, or, can I simply cast it implicitly without any
operator?
It depends on the context. Most of the time, you won't use a
cast at all, since the conversion is implicit. If you want to
force it, for example (say when calling a template function, so
that the template instantiates for the base class, and not for
the derived), either static_cast or dynamic_cast are fine; they
both have exactly the same behavior in this case.
Does this upcasting remove the derived class portion of the
object?
If you actually cast the object. Most of the time, casting
will in fact involve pointers or references to the object.
If after the upcasting, I'd like to downcast it again, is it
still possible? I think if the derived class portion of the
object is removed by upcasting, then it's impossible to
downcast it again, but should be possible otherwise.
Think of a static_cast of a class type as calling a constructor;
that's what it does, always. So it will depend on what
constructors are available. For pointers, you can generally go
back and forth, with a few restrictions: static_cast allows
derived to base even with a virtual base class, but you can't
use static_cast in that case to go back.
--
James Kanze (Gabi Software) email: james.kanze@gmail.com
Conseils en informatique orient=E9e objet/
Beratung in objektorientierter Datenverarbeitung
9 place S=E9mard, 78210 St.-Cyr-l'=C9cole, France, +33 (0)1 30 23 00 34