• Kris (27/27) Feb 25 2004 abstract class Foo
• Walter (4/8) Feb 25 2004 That's right. A class is abstract if and only if it has any abstract mem...
• Kris (7/15) Feb 25 2004 Note also that an abstract attribute on a method is ignored if said meth...
• Sean Kelly (14/16) Feb 25 2004 So this is an instance where "abstract" differs from the "=0" semantics
• Kris (8/25) Feb 25 2004 Exactly -- clean and elegant (although the 'abstract' attribute on AClas...
• Andres Rodriguez (7/15) Feb 25 2004 What is the point of declaring classes with the "abstract" keyword
• Stewart Gordon (22/33) Feb 26 2004 So, why have the abstract keyword if it means nothing?
• Ben Hinkle (47/47) Feb 26 2004 "Kris" wrote in message

"Kris" <someidiot earthlink.net> writes:

abstract class Foo
{
}

main()
{
    Foo foo = new Foo();
}

Shouldn't this example generate a compile error? I thought I'd try to get
around the "Partial-Interface Implementation" issue by attempting the
following wacky hack <g>:

interface Bar
{
    void bar();
}

abstract class Foo : Bar   // force class to be abstract?
{
    abstract void bar(){}; // hackety hack
}

main()
{
    Foo foo = new Foo();
    foo.bar();
}

This compiles and executes fine. Apparently it's perfectly legal to
instantiate a class explicitly declared as abstract. Only an abstract
method, sans body, causes the compiler to emit the expected error. I guess
the "abstract class" itself is just syntactically ignored?

"Walter" <walter digitalmars.com> writes:

"Kris" <someidiot earthlink.net> wrote in message
news:c1iu6b$2v8f$1 digitaldaemon.com...
 This compiles and executes fine. Apparently it's perfectly legal to
 instantiate a class explicitly declared as abstract. Only an abstract
 method, sans body, causes the compiler to emit the expected error. I guess
 the "abstract class" itself is just syntactically ignored?

That's right. A class is abstract if and only if it has any abstract member
functions. I should fix the documentation...

"Kris" <someidiot earthlink.net> writes:

Note also that an abstract attribute on a method is ignored if said method
has a body ...

- Kris

"Walter" <walter digitalmars.com> wrote in message
news:c1j4d8$8nd$1 digitaldaemon.com...
 "Kris" <someidiot earthlink.net> wrote in message
 news:c1iu6b$2v8f$1 digitaldaemon.com...
 This compiles and executes fine. Apparently it's perfectly legal to
 instantiate a class explicitly declared as abstract. Only an abstract
 method, sans body, causes the compiler to emit the expected error. I


guess
 the "abstract class" itself is just syntactically ignored?

 That's right. A class is abstract if and only if it has any abstract

member
 functions. I should fix the documentation...

Sean Kelly <sean ffwd.cx> writes:

Kris wrote:
 Note also that an abstract attribute on a method is ignored if said method
 has a body ...

So this is an instance where "abstract" differs from the "=0" semantics 
in C++.  If we already have abstract classes, why not allow for this:

interface IFace {
     void a();
     void b();
}

abstract class AClass : IFace {
     void a() {}
     abstract b();
}

class BClass : AClass {
     void b() {}
}

"Kris" <someidiot earthlink.net> writes:

Exactly -- clean and elegant (although the 'abstract' attribute on AClass
itself would be superfluous in D, and is indeed silently ignored).

Check out the nasty hack to achieve the same thing, in "Partial
implementation of Interfaces (Walter?)"

- Kris


"Sean Kelly" <sean ffwd.cx> wrote in message
news:c1j8i9$flq$1 digitaldaemon.com...
 Kris wrote:
  >
 Note also that an abstract attribute on a method is ignored if said


method
 has a body ...

 So this is an instance where "abstract" differs from the "=0" semantics
 in C++.  If we already have abstract classes, why not allow for this:

 interface IFace {
      void a();
      void b();
 }

 abstract class AClass : IFace {
      void a() {}
      abstract b();
 }

 class BClass : AClass {
      void b() {}
 }

"Andres Rodriguez" <rodriguez ai.sri.com> writes:

What is the point of declaring classes with the "abstract" keyword
then?  And it is not a rethorical question, I really don't know the
answer.


"Walter" <walter digitalmars.com> wrote in message
news:c1j4d8$8nd$1 digitaldaemon.com...
 "Kris" <someidiot earthlink.net> wrote in message
 news:c1iu6b$2v8f$1 digitaldaemon.com...
 This compiles and executes fine. Apparently it's perfectly legal to
 instantiate a class explicitly declared as abstract. Only an abstract
 method, sans body, causes the compiler to emit the expected error. I


guess
 the "abstract class" itself is just syntactically ignored?

 That's right. A class is abstract if and only if it has any abstract

member
 functions. I should fix the documentation...

Stewart Gordon <smjg_1998 yahoo.com> writes:

Walter wrote:

 "Kris" <someidiot earthlink.net> wrote in message
 news:c1iu6b$2v8f$1 digitaldaemon.com...
 
This compiles and executes fine. Apparently it's perfectly legal to
instantiate a class explicitly declared as abstract. Only an abstract
method, sans body, causes the compiler to emit the expected error. I guess
the "abstract class" itself is just syntactically ignored?

 
 
 That's right. A class is abstract if and only if it has any abstract member
 functions. I should fix the documentation...

So, why have the abstract keyword if it means nothing?

Java's implementation is robust in my mind:

- declaring a method as 'abstract' and giving it a body at the same time 
is an error
- the 'abstract class' syntactic salt is required
- a class can be declared abstract, and consequently actually be 
abstract, even if it has no abstract methods

I suppose you *could* deprecate this lack of salt in D.  But even if 
not, the compiler ought to pinpoint the cause of a 'cannot instantiate 
abstract class' error.  I once drove myself mad trying to find one of 
these, when I had simply forgotten to implement an abstract method from 
the base class.  Borland C++ does this, with a double error like

Error: Cannot create instance of abstract class 'Qwert'.
Error: Class 'Qwert' is abstract because of pure virtual function 'yuiop'.

I forget if the second line referenced the declaration of the pure 
virtual function (abstract method)....

Stewart.

-- 
My e-mail is valid but not my primary mailbox, aside from its being the 
unfortunate victim of intensive mail-bombing at the moment.  Please keep 
replies on the 'group where everyone may benefit.

"Ben Hinkle" <bhinkle4 juno.com> writes:

"Kris" <someidiot earthlink.net> wrote in message
news:c1iu6b$2v8f$1 digitaldaemon.com...
| abstract class Foo
| {
| }
|
| main()
| {
|     Foo foo = new Foo();
| }
|
| Shouldn't this example generate a compile error? I thought I'd try to get
| around the "Partial-Interface Implementation" issue by attempting the
| following wacky hack <g>:
|
| interface Bar
| {
|     void bar();
| }
|
| abstract class Foo : Bar   // force class to be abstract?
| {
|     abstract void bar(){}; // hackety hack
| }
|
| main()
| {
|     Foo foo = new Foo();
|     foo.bar();
| }
|
| This compiles and executes fine. Apparently it's perfectly legal to
| instantiate a class explicitly declared as abstract. Only an abstract
| method, sans body, causes the compiler to emit the expected error. I guess
| the "abstract class" itself is just syntactically ignored?

I agree with you that if it is syntactically valid to mark a
class as abstract it should be abstract, and I assume Walter will
fix that one day, but in practice it's common to name abstract
classes with the "Abstract" prefix and so I actually hardly ever
accidentally try to instatiate an abstract class. In fact I
can't remember ever trying to instantiate something named
"AbstractFoo". I find abstract classes are rare enough that
people don't look for the abstract attribute and so the "Abstract"
prefix makes the class much more usable since people read the
class name every time. Perhaps that could be put in the D
style guide.

-Ben