abstract superclasses

[Giles Bowkett]

I sometimes use the Java thing of an abstract superclass in Ruby. I
usually just set up the abstract parent so that it'll break if used
directly. Is it better to just use Modules? Have other people run into
this?

--
Giles Bowkett

Blog: http://gilesbowkett.blogspot.com
Portfolio: http://www.gilesgoatboy.org
Tumblelog: http://giles.tumblr.com/

 

[Robert Klemme]

I sometimes use the Java thing of an abstract superclass in Ruby. I
usually just set up the abstract parent so that it'll break if used
directly. Is it better to just use Modules? Have other people run into
this?

Why bother? If your "abstract" class requires a method to be present
then just not defining it works pretty well - in modules as well as in
classes:

irb(main):001:0> o=Object.new.extend Enumerable
=> #<Object:0x1001a92c>
irb(main):002:0> o.map {|y| y.to_i}
NoMethodError: undefined method `each' for #<Object:0x1001a92c>
from (irb):3:in `map'
from (irb):3
from :0
irb(main):003:0>

Of course you need some documentation about the missing method - that
would probably be the only reason why I'd add a dysfunctional definition
of that method...

Kind regards

robert

 

[Thomas Adam]

I sometimes use the Java thing of an abstract superclass in Ruby. I
usually just set up the abstract parent so that it'll break if used
directly. Is it better to just use Modules? Have other people run into
this?

I'd use a Module, yes. Or a class:

class SomeClassMimickingAbstract
private_class_method :new
end

But you don't need to think this way in Ruby -- unlike Java, Ruby is
weakly typed.

-- Thomas Adam

 

[Yossef Mendelssohn]

I'd use a Module, yes. Or a class:

class SomeClassMimickingAbstract
private_class_method :new
end

But you don't need to think this way in Ruby -- unlike Java, Ruby is
weakly typed.

-- Thomas Adam

How do you suggest to use that class? I understanding keeping the
"abstract" class from being instantiated, but what about the concrete
classes, the ones you're going to use?

class RealThing < SomeClassMimickingAbstract
# now make .new public again
end

Ugly, isn't it? Of course, that's just more fuel for the "don't do
this in Ruby" fire.

 

[David A. Black]

Hi --

How do you suggest to use that class? I understanding keeping the
"abstract" class from being instantiated, but what about the concrete
classes, the ones you're going to use?

class RealThing < SomeClassMimickingAbstract
# now make .new public again
end

Ugly, isn't it? Of course, that's just more fuel for the "don't do
this in Ruby" fire.

You could do:

class C
private_class_method :new
def self.inherited(c)
class << c; public :new; end
end
end

I've actually never felt the need for abstract classes in Ruby
programs. I'm not a Java programmer, so it never would have occurred
to me in that context, and I haven't found myself inventing it for
Ruby.


David

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Both taught by David A. Black.
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[Rick DeNatale]

I've actually never felt the need for abstract classes in Ruby
programs. I'm not a Java programmer, so it never would have occurred
to me in that context, and I haven't found myself inventing it for
Ruby.

Note that ActiveRecord has a slightly different spin on the notion of
abstract class.

ActiveRecord::Base has a rw attribute called abstract_class and an
abstract_class? predicate method.

ActiveRecord::Base subclasses can be 'set' as abstract the meaning is
that such classes don't have a corresponding database table. They are
used as intermediate superclasses, typically to let a set of AR
classes to inherit a common database connection different from other
AR classes in the same application.

 

[Giles Bowkett]

It is a Java thing; it comes from "Refactoring," I think, the Martin
Fowler book. The idea is you handle excessive if/else/etc. statements
by creating classes and putting the different code branches in
subclasses. Since you'll have pieces which don't need to be different
in the subclasses, you keep those pieces in the superclass so you have
them in one place.

In practice I usually do the unimplemented method thing, so that
misusing the code just makes things blow up. The method hiding and
revealing is probably overkill, although if you extract it into a
module, you can have an AbstractSuperclass module pretty easily, which
is a gajillion times better than comments which say "abstract
superclass!! do not instantiate!!!".

As a refactoring it's generally pretty useful. The code becomes easier
to test and easier to read. But it's really not idiomatic at all, and
it's very inelegant.

--
Giles Bowkett

Blog: http://gilesbowkett.blogspot.com
Portfolio: http://www.gilesgoatboy.org
Tumblelog: http://giles.tumblr.com/

 

[Robert Dober]

I'd go for modules but Robert is right too, if a class just is
abstract by the way it is defined or refactored that's it, no way
checking it's instantiation, no please. If you really feel that an
entity shall not be instantiated than a Module seems to be the correct
Ruby device.

Cheers
Robert