delegate cannot handle polymorphism?

[Nub Public]

On 6/28/2011 7:13 PM, Steven Schveighoffer wrote:
> On Mon, 27 Jun 2011 22:23:32 -0400, Nub Public <nubpublic at gmail.com> wrote:
>
>
>> I have to disagree on this.
>> Member function pointers are useful in many event handling systems.
>> Imagine a signal and slot system. To register a member function as a
>> slot, the most direct and intuitive way is to take a member function
>> pointer. In c++0x and boost, <function>, <functional> and related
>> libraries all work with member function pointers.
>>
>> My premise is simple: a virtual function should always be resolved by
>> the dynamic identity of the object, regardless of whether it is called
>> directly or through a function pointer.
>
> This can be had using delegates, but it's not straightforward, you need
> to do a little bit of extra work.
>
> For example, if you did this:
>
> class A
> {
> final void callFoo() { foo(); }
> void foo() {writeln("A");}
> }
>
> class B : A
> {
> void foo() {writeln("B");}
> }
>
> A delegate to callFoo would do polymorphism, even when you change the
> context pointer (to a compatible instance).
>
> Note that it is highly unusual to change anything about a delegate, as
> type checking the context pointer is out the window. In fact, it is the
> property of delegates which makes them so useful -- because you don't
> have to care what the type of the context pointer is, the delegate's
> type does not depend on it.
>
>> Perhaps the contextptr of D delegate cannot do this because it can
>> refer to the enclosing context besides an object. But I'd love it for
>> D to have the functionality of a truly polymorhic member function
>> pointer. Maybe the restrictions on the keyword "function" can be
>> relaxed to work with such a pointer?
>
> The easiest thing to do is the suggestion from Michel Fortin -- create a
> function/delegate that accepts the base type. This is much safer as
> well. I would be highly suspect of code that alters any delegate
> components.
>
> -Steve


I see. Thank you.