Abstract functions in child classes

Jonathan M Davis jmdavisProg at gmx.com
Fri Dec 2 14:28:33 PST 2011

On Friday, December 02, 2011 21:13:45 Adam wrote:
> Anyway, I have my answer, and I know that D *does* have a reason for
> this implicitism.

Okay. Maybe I've been skimming over this thread too much, but I don't 
understand what forcing the programmer to mark an abstract class as abstract 
would do. The compiler knows that the class is abstract regardless. It's going 
to generate an error when you try and instantiate that class - whether or not 
you mark the class as abstract or not has no impact on that as long as there 
are abstract functions (the sole bizareness being able to mark a class as 
abstract when none of its functions are abstract).

It is perfectly legal and desirable to be able to have a class reference for 
an abstract class. e.g.

abstract class C
 abstract void func();

class D : C
 void func()
 writeln("hello world");

C c = new D();

The compiler has nothing to complain about with a reference to an abstract 
class until you try and instatiate it. So, how does explicitly marking the 
class abstract help with that?

I do think that it's a bit odd that D doesn't require that you mark classes as 
abstract if they have abstract functions and that it allows classes which 
don't have abstract functions to be marked abstract, but that should have no 
effect on whether the compiler can catch bugs related to abstract classes. The 
fact that the class has functions which are abstract is enough for the 
compiler to know that the class is abstract.

new C() is what needs to be disallowed for abstract classes, and that is an 
instantiation. It's the instantations that need to be checked, and they _are_ 
checked. It is a compilation error when you try and instantiate an abstract 

The only thing that I see that marking the class abstract does is give the 
programmer a visual indicator that the class is abstract. I do think that 
that's valuable, and I'd love it if it were required when any functions in the 
class are abstract and disallowed when none are, but I don't see how it can 
have any effect on what errors you're getting. It's instatiating an abstract 
class which as error, not declaring one, and unless you actually instantiate 
it, there's no way for the compiler to catch it, because there's nothing to 

- Jonathan M Davis

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