delegate !is null

[Steven Schveighoffer]

On Tue, 08 Sep 2009 12:40:13 -0400, Saaa <empty at needmail.com> wrote:

>> Hm... I'm still confused.  Why not just set the delegate to null?  Why  
>> do
>> you need to have the delegate set to something?
> It is for the gui. I give it a list of things to display.
> And some of these things don't yet exist or can be deleted at any time.
> I'd like it to display the last valid value.

That still doesn't explain why you need to have delegates set to something  
valid, yet have a null pointer value.  Either a) you are trying to subvert  
the type system, or b) you can simply set the whole delegate (function and  
pointer) once you get the class instance.

It's hard for me to say what is the best way to do it, but generally,  
things like this are difficult, and rightfully so.  Having a delegate  
without a valid pointer makes little sense.

>
>>
>> There are ways to do it, without having a class instance, but it is  
>> messy.
> how messy? :D

class C
{
   int val = 0;
   int method() {return val;}
}

int delegate() dg;

dg.func = &C.method; // note the upper-case C

// dg is now a "instanceless" delegate to C.method.

dg.ptr = new C;

Now you can call dg() and it will call the C.method function on the newly  
created C object.

Note that this method of manipulating methods does NOT obey inheritance.   
For example:

class B : C
{
   int method() {return 555;}
}

dg.ptr = new B;

dg(); // will return 0, since it calls C's version of method.

I also don't know how well it will work on interfaces.

> Kind of related:
> If you delete an object and later create a new object, what are the  
> chances
> they are located on the
> same place (deleted.ptr is new.ptr) ?
> Does the garbage collector try to reuse locations or is it the opposite  
> (or
> random) ?

The chances are non-zero :)  I wouldn't depend on this behavior either way  
if I were you.

-Steve