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 _______________________________________________ Delphi mailing list -> [email protected] http://www.elists.org/mailman/listinfo/delphi
