Re: dynamic_cast is ugly!

James Kanze <>
Thu, 13 Mar 2008 05:37:39 -0700 (PDT)
On Mar 12, 1:23 pm, wrote:

On Mar 12, 6:38 am, Nick Keighley <>

On 12 Mar, 10:45, "Daniel T." <> wrote:

Juha Nieminen <nos...@thanks.invalid> wrote:

Daniel T. wrote:

Juha Nieminen <nos...@thanks.invalid> wrote:

It's not really that you need dynamic cast to *know* if the
shape is a Square. In this case you need it if you want to *do*
something to the object if it's a Square, and this operation is
not supported by Shape, only by Square.

And therein lies the problem. "you want to *do*
something to the object if it's a Square". If you find
yourself in that situation, the design has already
slid downhill. IMHO.

And what is the alternative you propose?

Don't try to *do* things to objects. Objects are supposed
to do for themselves, clients *notify* the objects of
things, they don't order them around.

I can only think of one possibility: Clutter the 'Shape'
base class with Square-specific virtual functions. This
defies all good OO design.

What is it you are trying to tell squares that you don't
want any other shapes to know about? Why the big secret?

"colour all the squares yellow"

If the shapes example is a bit fanciful make it a UML editor.
I want all the interfaces to stand out by changeing their
colour and my model is huge.

"colour all the interfaces yellow"

You can solve that by putting homogeneous elements in their
own container for just such operations If you want to
globally change the line style of connectors, operate on the
Connector collection, don't rifle through the generic one
interrogating each object.

Which means that the lower level managing the collections has to
know about the possible higher level types. Sounds like a
serious breach of encapsulation to me.

James Kanze (GABI Software)
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