reimplementing an interface in a derived class

Steven Schveighoffer schveiguy at
Mon Jan 7 15:48:35 UTC 2019

On 1/4/19 7:16 PM, kdevel wrote:
> On Friday, 4 January 2019 at 20:21:56 UTC, Steven Schveighoffer wrote:
>>> missing in the source. But why is d a null reference in the first place?
>> Because when you dynamically cast one object or interface to another 
>> object or interface, and that result is not possible (if you remove 
>> ",D" from the example you quoted, then neither A nor B implement D), 
>> then the result is null.
> I overlooked that Alex wrote
>     class A
> and not
>     class A : D
> which I came across in the examples in #10 and #11 of
> and to which I referred to in my OP.
>> See parts 2 and 3.
>     part 2
>     "Any casting of a class reference to a derived class reference
>     is done with a runtime check to make sure it really is a downcast.
>     null is the result if it isn't."
> Part 2 is about downcasting. Does that apply here? 

Yes. If the requested interface or class isn't a base class, or a base 
interface, then a downcast must be involved. Essentially the downcast is 
a runtime check to see if this class instance actually is a derived one, 
or a derived class implements the specified interface. D does not have 
multiple inheritance.

> Due to the omission
> of ": D" in Alex' example the cast is not a cast "to a derived class
> reference" and hence this part does not apply.

Of course it applies :) The fact that the class is not an instance of D 
in a derived class means that it returns null. The spec statement could 
be worded better.

> OTOH upcasting is also
> not covered by part 2 and seems to be legal:

Upcasting is trivial, and the compiler will not even spend time doing a 
runtime check, it knows how to do this at compile time. It doesn't even 
require a cast.

Just like casting a short to an int doesn't require a cast, but you are 
free to put one in if you like.


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