Strange behaviour of abstract method

Hello list,

in our OOP course we noticed some behaviour of an abstract method, which
I cannot explain. Maybe you can help me:

If I do the following

     SequenceableCollection new add: 123

I would expect an Error. The method add: is not implemented in
SequenceableCollectionbut inherited from collection, where it's only
statement is

     self subclassResponsibility

But in reality, Pharo seems to enter an endless loop, consuming more and
more memory.

Can someone please explain to me, what happens here?

Greetings,
Michael

Michael,

On 31 Oct 2013, at 11:00, Michael Paap <[hidden email]> wrote:

Obviously, abstract classes should not be instanciated, but in teaching this is bound to happen.

Now, the actual error happens in the environment (debugger, inspector, printString mechanisms): they see a collection and try to work with it, since it is an incomplete/degenerate one, things break/loop.

The following does work for example:

  SequenceableCollection new isCollection.

The question is: should there be an explicit error message when an abstract class is instanciated, is that even possible ?

Sven


 

Am 31.10.2013 11:24, schrieb Sven Van Caekenberghe:

> Obviously, abstract classes should not be instanciated,

This is of course known.

> Now, the actual error happens in the environment (debugger,
> inspector, printString mechanisms): they see a collection and try to
> work with it, since it is an incomplete/degenerate one, things
> break/loop.

Ok. This explains, why just "doing"

     SequenceableCollection new

is ok, while "printing" causes problems. Thank you for your explnation.

> The question is: should there be an explicit error message when an
> abstract class is instanciated, is that even possible ?

This may be difficult, because the instantiation of a not abstract
subclass normally /is/ an instantiation of the abstract super class,
isn't it?

And in Smalltalk we have no concept of abstractness in the language
itself, but only by convention.

Greetings,
Michael