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April 28, 2008
Make 'abstract' mandatory if the class is intended to be abstract, absence of 'abstract' means "not abstract"
A class can either be abstract or not abstract. Currently in D, if you 
don't mark a class as abstract, it can still be it if it contains an 
abstract method:

class Foo {
	
	abstract void someAbstract();
	
	void nonAbstract() {
	}
	
}

When designing a class, you have in mind whether the class is going to 
be abstract or not. If it's not going to be abstract, you want the 
compiler to help you by telling you "You made a mistake. This class is 
still abstract because you didn't implement method foo".

So I want to extend Foo with a class Bar, but I want Bar to be not abstract.

class Bar : Foo {
}

I compile, and it gives no error, of course. But I want there to be an 
error there. The only way I can get an error is by making a dummy 
function that instantiates Bar:

void blah() {
	Bar bar = new Bar();
}

main.d(14): Error: cannot create instance of abstract class Bar
main.d(14): Error: function someAbstract is abstract

The problems with this appraoch are two:
 - You have to make a dummy function to check whether you implemented 
Bar correctly.
 - You get two errors for each instantiation of Bar, if it's abstract 
(ugly).

Why not make "abstract" mandatory for a class if it's intended to be 
abstract, and the absence of "abstract" to mean "not abstract"? Java 
works this way, and I think it is for the reasons I mentioned.

Another advantage is that just by seeing the start of class definition 
you can tell whether a class is abstract or not. You don't have to see 
if any method is marked as abstract, or go to the superclasses to see if 
there is a method that is still not implemented.

(also, it would be nice if the compiler could tell you all the methods 
that still need an implementation, rather than just one)
April 29, 2008
Re: Make 'abstract' mandatory if the class is intended to be abstract, absence of 'abstract' means "not abstract"
Ary Borenszweig wrote:
> A class can either be abstract or not abstract. Currently in D, if you 
> don't mark a class as abstract, it can still be it if it contains an 
> abstract method:
> 
> class Foo {
>     
>     abstract void someAbstract();
>     
>     void nonAbstract() {
>     }
>     
> }
> 
> When designing a class, you have in mind whether the class is going to 
> be abstract or not. If it's not going to be abstract, you want the 
> compiler to help you by telling you "You made a mistake. This class is 
> still abstract because you didn't implement method foo".
> 
> So I want to extend Foo with a class Bar, but I want Bar to be not 
> abstract.
> 
> class Bar : Foo {
> }
> 
> I compile, and it gives no error, of course. But I want there to be an 
> error there.

You have my support. With the new hiding as an error thing, my faith in 
Walter thinking of catching stupid errors is shored up, and I have a 
reasonable hope that this suggestion will succeed.
April 29, 2008
Re: Make 'abstract' mandatory if the class is intended to be abstract, absence of 'abstract' means "not abstract"
I agree

On Mon, 28 Apr 2008 10:11:23 -0300, Ary Borenszweig wrote:

> A class can either be abstract or not abstract. Currently in D, if you
> don't mark a class as abstract, it can still be it if it contains an
> abstract method:
> 
> class Foo {
> 	
> 	abstract void someAbstract();
> 	
> 	void nonAbstract() {
> 	}
> 	
> }
> 
> When designing a class, you have in mind whether the class is going to
> be abstract or not. If it's not going to be abstract, you want the
> compiler to help you by telling you "You made a mistake. This class is
> still abstract because you didn't implement method foo".
> 
> So I want to extend Foo with a class Bar, but I want Bar to be not
> abstract.
> 
> class Bar : Foo {
> }
> 
> I compile, and it gives no error, of course. But I want there to be an
> error there. The only way I can get an error is by making a dummy
> function that instantiates Bar:
> 
> void blah() {
> 	Bar bar = new Bar();
> }
> 
> main.d(14): Error: cannot create instance of abstract class Bar
> main.d(14): Error: function someAbstract is abstract
> 
> The problems with this appraoch are two:
>   - You have to make a dummy function to check whether you implemented
> Bar correctly.
>   - You get two errors for each instantiation of Bar, if it's abstract
> (ugly).
> 
> Why not make "abstract" mandatory for a class if it's intended to be
> abstract, and the absence of "abstract" to mean "not abstract"? Java
> works this way, and I think it is for the reasons I mentioned.
> 
> Another advantage is that just by seeing the start of class definition
> you can tell whether a class is abstract or not. You don't have to see
> if any method is marked as abstract, or go to the superclasses to see if
> there is a method that is still not implemented.
> 
> (also, it would be nice if the compiler could tell you all the methods
> that still need an implementation, rather than just one)
April 29, 2008
Re: Make 'abstract' mandatory if the class is intended to be abstract, absence of 'abstract' means "not abstract"
vote++

On Mon, 28 Apr 2008 10:11:23 -0300, Ary Borenszweig
<ary@esperanto.org.ar> wrote:

>A class can either be abstract or not abstract. Currently in D, if you 
>don't mark a class as abstract, it can still be it if it contains an 
>abstract method:
>
>class Foo {
>	
>	abstract void someAbstract();
>	
>	void nonAbstract() {
>	}
>	
>}
>
>When designing a class, you have in mind whether the class is going to 
>be abstract or not. If it's not going to be abstract, you want the 
>compiler to help you by telling you "You made a mistake. This class is 
>still abstract because you didn't implement method foo".
>
>So I want to extend Foo with a class Bar, but I want Bar to be not abstract.
>
>class Bar : Foo {
>}
>
>I compile, and it gives no error, of course. But I want there to be an 
>error there. The only way I can get an error is by making a dummy 
>function that instantiates Bar:
>
>void blah() {
>	Bar bar = new Bar();
>}
>
>main.d(14): Error: cannot create instance of abstract class Bar
>main.d(14): Error: function someAbstract is abstract
>
>The problems with this appraoch are two:
>  - You have to make a dummy function to check whether you implemented 
>Bar correctly.
>  - You get two errors for each instantiation of Bar, if it's abstract 
>(ugly).
>
>Why not make "abstract" mandatory for a class if it's intended to be 
>abstract, and the absence of "abstract" to mean "not abstract"? Java 
>works this way, and I think it is for the reasons I mentioned.
>
>Another advantage is that just by seeing the start of class definition 
>you can tell whether a class is abstract or not. You don't have to see 
>if any method is marked as abstract, or go to the superclasses to see if 
>there is a method that is still not implemented.
>
>(also, it would be nice if the compiler could tell you all the methods 
>that still need an implementation, rather than just one)
April 29, 2008
Re: Make 'abstract' mandatory if the class is intended to be abstract, absence of 'abstract' means "not abstract"
vote +1

-- 
Bruno Medeiros - Software Developer, MSc. in CS/E graduate
http://www.prowiki.org/wiki4d/wiki.cgi?BrunoMedeiros#D
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