Re: inheritance v passing object through the constructor

box ||= Box.new

class ErrorBox
def initialize(box)

end

end

That won’t do anything as written; the two ‘box’ local variables are
distinct.

I’m guessing you mean something like this:

class ErrorBox
def initialize(box)
@box = box
end
def draw
@box.draw
end
end

eb = ErrorBox.new(Box.new)
eb.draw

This is a simple example of the “delegator pattern”: (an instance of)
ErrorBox “has a” Box.

Over time I’ve found myself drawn more and more towards this pattern,
and
use class inheritance less and less.

For me, the biggest advantage is that you can implement composite
behaviour
easily. That is, your ErrorBox can contain multiple objects, either
creating
them itself or having them passed in. Its own method ‘x’ can in turn
call
A#y and B#z as appropriate. Composing systems in this way can make it
much
clearer what each piece is responsible for.

It allows cleaner decoupling: for example, the ‘foo’ method of Box may
not
make any sense when inside an ErrorBox, so you don’t delegate it.

Unit testing with mock objects is easy. Your ErrorBox object can have a
mock
Box, and you can easily check that it calls the correct methods on Box.
With
inheritance you’d end up stubbing or mocking methods on the same object,
which can get messy. Tests are also more robust because they don’t
depend on
implementation details of Box.

Finally, there’s more potential for code re-use. One ErrorBox object may
contain a TclBox, whilst another ErrorBox object may contain a QtBox. As
long as they have the same duck-type (e.g. in the above example they
both
respond usefully to “draw”) then they’ll both work, without code
changes,
whereas an ErrorBox which inherits from TclBox will only work with that.
The
decision as to which to use can even be left until runtime.

Does that answer your question?

Regards,

Brian.

2008/7/7 [email protected]:

class ErrorBox
def initialize(box)
@box = box
end
def draw
@box.draw
end
end
eb = ErrorBox.new(Box.new)
eb.draw

If ErrorBox really is a more specialized kind of Box, then subclassing
makes
sense in terms of design. If it’s not really a new kind of Box but wants
to
use some of Box’s behaviour then use delegation or consider mixins. Reg
Braithewaite has a decent article on this topic:

http://weblog.raganwald.com/2008/04/is-strictly-equivalent-to.html

Also, seek out the decorator in a book on design patterns. It’s usually
presented as a way of extending the behaviour of objects in many
orthogonal
ways in situations where subclassing would result in combinatorial
explosions of new subclasses. This is another case where mixins can also
make sense.

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