Thanks for your explanations, Rob,
OK, also for the records of this e-list:
list:=TObjectList.create;
list.add(TSubObject.create);
boolVar:=supports(list[0],IBlah,IBlahRef);
if bool
then IBlahRef.hello;
So the sollution is:
* use a TObjectList or a TInterfaceList to make the elements of the list
assignable to
Interface variables. Both seem to work.
* For the actual assignment, the supports function is used.
Rinke
(by the way: testRef3 in my original post should of course be subObjRef)
On 16 Feb 2006 at 22:02, Rob Kennedy wrote:
> rinke hoekstra wrote:
> > I cannot assign elements from a list to an interface type reference.
>
> You might benefit from reading this article:
>
> http://www.cs.wisc.edu/~rkennedy/interface-object
>
> > Given the following (simplified) class and interface definitions:
> >
> > IBlah = Interface
> > procedure hello;
> > end;
> >
> > TTestObject = class(TInterfacedObject, IBlah)
> > procedure hello;
> > end;
> >
> > TSubObject = class(TTestObject)
> > procedure boeh;
> > end;
> >
> >
> > Assigning a variable of tyep TsubObject to an IBlah reference type works
> > perfectly:
> >
> > var
> > iBlahRef: IBlah;
> > subObjRef: TSubObject;
> >
> > begin
> > subObjRef:=TSubObject.create;
> > iBlahRef:=IBlah(testref3); //works fine
>
> Works fine? It doesn't even compile. What's testref3?
>
> > When I first put the refs in a list, and then try to get them from the
> > list, it does not work
> > anymore:
> >
> > var
> > iBlahRef: IBlah;
> > subObjRef: TSubObject;
> >
> > begin
> > l:=TList.create;
> > l.add(TSubObject.create);
> > iBlahRef:=IBlah(l[0]); //does not work, null pointer exception
>
> That's not a safe type cast. What you put into the list was a TSubObject
> reference. That's what you get out, too, except that its type is reduced
> to Pointer since that's what TList stores.
>
> Once you get the Pointer back, you type-cast it to IBlah. But the
> address you got doesn't represent an IBlah. It represents a TSubObject.
> The compiler doesn't know that, though, so it simply treats the Pointer
> as an IBlah when it does the assignment to iBlahRef.
>
> When you type-cast an actual object reference to an interface type, the
> compiler inserts extra code to fetch the object's interface reference.
> It's not just a simple re-interpretation of the pointer value as a
> different type. But type-casting a Pointer to an interface type *is*
> just a re-interpretation.
>
> > while, from the same list, this works:
> > testObjectRef:=TTestObject(l[0]); //works fine
> >
> > As both TTestObject variables and IBlah variables should be assignable to
> > the elements
> > of the list, both should work, isn't it?
>
> Just because it's assignable *to* the list doesn't mean it's assignable
> *from* the list.
>
> > Then why can't I assign the elements from the list to an IBlah, where I can
> > assign them
> > to an TTestObject? Am I missing something, or is there some very silly
> > error I am
> > overlooking?
> >
> > Is there any way in which I can assign list elements to interface typed
> > variables??
>
> Well, you could start by using list classes that know something more
> about the things they store. If you want to store objects, then use a
> TObjectList. If you want to store interfaces, then use a TInterfaceList.
>
> When you want to type-cast to an interface type, use the "as" keyword or
> the Supports function, found in the SysUtils unit.
>
> --
> Rob
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