2 Questions: Do I need an interface & C++ -> D code part, how?

[Daniel Keep]

Benjamin Schulte wrote:
> Hi,
> 
> I've got two questions about D - all containing those class structure:
> 
> ...
> 
> now my first question:
> I've got another class, called MyApplication - I want to have class Application as base class and MainLoopEvent as 2nd base class. So I wrote:
> 
> class MyApplication : Application, MainLoopEvent { }
> 
> But I got an error, that MainLoopEvent has to be an interface. Is there now another way than saying:

D only has single inheritance; you cannot inherit from multiple base
classes.  Instead, the normal practice is to inherit from one base class
and many interfaces.

> ---------------
> 2nd question:
> 
> MainLoopEvent has the method onMainLoop( );
> Say we create a second class with Event as Base class
> 
> class AnotherEvent : Event
> {
>   abstract void onBeingHappy( int a, int b );
> }
> 
> Now, now I need a new function. In C++ I could write a macro: (I mixed C++ with D to show you what I mean:)
> 
> #define callEvent(classType,event) foreach(Event e; event.getEventList() ) ((classType*)e)

I imagine the macro would not have been quite so easy to read had it
been written for C++ :P

> I could now just call
> callEvent(MainLoopEvent, myApplication)->onMainLoop( );
> callEvent(AnotherEvent, myEvent)->onBeingHappy( 12, 31 );
> 
> At the moment my solution for MainLoopEvent looks like this:
> 	static void callMainLoop( )
> 	{
> 		// Call main loop event
> 		foreach( MainLoopEvent e; events ) e.onMainLoop( );
> 	}
> 
> But that's not my favorite way to do this, because I would have to rewrite this for every abstract method.
> 
> Might have some bugs in here, but I hope you understand what I mean. A template that calls methods I don't really know from the structure.

You could try something like this:

void callEvent(EventType, DgType)(DgType dg)
{
    foreach( e ; events )
        if( auto mle = cast(EventType)e )
            dg(mle);
}

callEvent!(MainLoopEvent)((MainLoopEvent e){e.onMainLoop();});

It's hard to suggest what to do since I'm not 100% sure what you're
trying to accomplish.  Looking at D from a C++ perspective is tricky
since D is *not* C++, nor is it descended from it.

If you're doing GUI code, you might want to look at DFL
(http://www.dprogramming.com/dfl.php) which might give you some ideas on
how to do event callbacks.

For instance, I wouldn't bother with classes myself, I'd just use
delegates, or the Signal and Slot stuff in std.signals.  Like I said,
have a poke around, and keep in mind that D is not a superset of C++, so
there will be things you can't directly translate (like multiple
inheritance).  Anything in C you can generally assume behaves in roughly
the same way, but outside of that, be careful.

	-- Daniel

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