From lechner@cs.uml.edu  Wed Sep 20 22:09:30 2006
Subject: Fox alt to bde hasType: Check subtype by dynamic cast
To: lechner@cs.uml.edu (Bob Lechner)
RJLRef: $PH/COOL-BDE/bdegen13/hasTypeAlternative_C++DynamicCast.txt
> 
> This may be useful on a bde graphobject display (List) class item,
> to see if it is in the graph, node, caption or hlink subclass.
> 
> Right now bde/src uses macro hasType(id,XX) below.
> (267 calls or comments, including assert(hasType(...) checks.)  
> This is analogous to fox's isMemberOf(...)  below.
> 
> hasType is application-independent, like dprint:
> ------
> mercury(16)> grep '#define hasType' ../pr_util_nolog/9*.h
> 792:#define hasType(key, ttabbr) \
>  ((&key)?(strstr(decode_retstr(&key),#ttabbr)?1:0):0)
> --------
> decode_retstr looks up a ttabbr at an index and does a string compare.  
> But (&key)==encode(ttabbr) is slower since encode serialy
> searches for ttabbr in a list of string values. 
> 
> 
> The polymorphic list at topobject->subobjects[->next...] 
> was never dis-aggregated into subtype lists. 
> The pr_loaded VMNetDB segregates types in bde's component hierarchy.
> This is more faithful to bde's [relational] data model.
> 
> R Lechner
> 
> Forwarded message:
>
>> > From foxgui-users-bounces@lists.sourceforge.net  Tue Sep 19 22:16:47 2006
>> > To: foxgui-users@lists.sourceforge.net
>> > Cc: The Devils Jester <thedevilsjester@gmail.com>
>> > Subject: Re: [Foxgui-users] Check what type a widget is?
>> > 
>> > On Tuesday 19 September 2006 19:57, The Devils Jester wrote:
>> > > What is the proper method of checking to see what class a specific widget=
>> >  is?
>> > > (I.E. I need to determine if its a button or a toggle button, etc...)
>> > 
>> > 
>> > Even so, for general reference:
>> > 
>> > 	if(window->isMemberOf(FXMETACLASS(ClassName))){
>> > 		...
>> > 	}
>> > 
>> > This would work; however, the C++ way is better:
>> > 
>> > 	ClassName *someclass=dynamic_cast<ClassName*>(window);
>> > 	if(someclass){
>> > 		...
>> > 	}
>> > 
>> > 
>> > This is language-supported, and since you usually want to know
>> > what class something is in order to cast the pointer, it kills
>> > two birds with one stone, so to speak.
>> > 
>> > Apart from that, a C++ style cast takes care of potential multiple-
>> > inheritance issues [not that this is relevant inside FOX itself,
>> > which is strictly single-inheritance; however it may be an issue
>> > in your own classes].
>> > 
>> > 	- Jeroen
>> 
>

> Message 3/1369  From Jeroen van der Zijp  Sep 20, 06 08:17:46 PM -0500
> Cc: The Devils Jester <thedevilsjester@gmail.com>
> Subject: Re: [Foxgui-users] Check what type a widget is?

> On Wednesday 20 September 2006 17:23, The Devils Jester wrote:
> > So in the C++ example you gave, ClassName would be say FXLabel,  and
> > if the passed object 'window' has FXLabel in its inheritance history,
> > the new pointer 'someclass' will be a non null value?  Sorry I am a
> > bit green as far as OOP goes.
> 
> C++ has added dynamic_cast<> as a checked downcast specifically for these
> situations.  It only requires that the object have a vtable [which FXObject
> and its derivatives do].
> 
> The other one is static_cast<>, which you can deploy if you KNOW that
> the cast will be successful [it doesn't check, like dynamic_cast does,
> thus its faster].
> 
> A C++ object which inherits from more than one base class comprises
> all of its base classes in its struct.  When you "cut" it down to
> one of its bases, the C++ compiler knows the type and can add the
> necessary struct-offsets.
> 
> However, when you down-cast, with the C-style cast you'd just be
> reinterpreting the pointer to this as a pointer to that, and thus
> in case of multiple inheritance this will be total garbage.
> 
> So its *highly* recommended to use the new C++ style casts when navigating
> a class hierarchy, and when the class-hierarchy uses M.I. it is *essential*.
> 
> Regards,
> 
>         - Jeroen