Covariant callback functions, or assigning base class members through a subclass reference

[Rene Zwanenburg via Digitalmars-d-learn]

On Tuesday, 14 July 2015 at 17:26:32 UTC, Steven Schveighoffer 
wrote:
> On 7/14/15 11:28 AM, Rene Zwanenburg wrote:
>> Given the following code:
>>
>> class Base
>> {
>>      alias CallbackType = void delegate(Base);
>>
>>      CallbackType callback;
>>
>>      void foo()
>>      {
>>          callback(this);
>>      }
>> }
>>
>> class Derived : Base
>> {
>>
>> }
>>
>> void main()
>> {
>>      auto d = new Derived();
>>      d.callback = (Derived d) { /* Do something */ }
>> }
>>
>> Obviously this won't compile, since the callback function 
>> needs to have
>> Base as parameter, not Derived. But the given code is 
>> perfectly safe
>> because in main d typed as Derived, not Base. Does anyone know 
>> a clean
>> way to support the code given in main(), preferably by 
>> defining some
>> smart CallbackType so Derived doesn't need to be modified?
>
> No, this isn't possible. Only thing you can do is:
>
> d.callback = (Base b) { if(auto d = cast(Derived)b) { /* Do 
> something */ } else assert(0); };
>
> -Steve

Thanks, I'll have to think of something else then.

At my day job I mostly work with C#, and I strongly dislike how 
event handler parameters there have to be broader-typed (is that 
even a valid term?) than they logically need to be, which is why 
I'm trying to avoid doing something similar here.