Design Patterns discussion

[Lorenzo Bettini <bettini AT dsi.unifi.it>]

Hi there!

sorry for the delay of my answer but I experienced problems with my
e-mail lately.  I reply also to the list because it may be of interest.

any comments are welcome :-)

I attach a paragraph from a paper I'm writing about it:

We can benefit from dynamic overloading in implementing other design
patterns that involve two different class hierarchies, such as, e.g.,
the \emph{Strategy} pattern that ``defines a family of algorithms,
encapsulate each one, and make them interchangeable'' \cite{Gamma}.
The book \cite{Gamma} makes an example of use of this pattern for
encapsulating the text formatting strategy for their document editor
case study: one can use a simple text formatter for producing a draft
copy of the document, while, for the final version, a more complex
formatting policy can be adopted.  While the need of dynamic
overloading is not immediately evident from the structure of the
pattern, its usefulness suddenly appears when one has to actually
implement the formatting code: the document is stored in an aggregate
structure where simple elements (characters, figures, etc.)  have the
same interface of aggregate ones (row, columns, etc.).  This aggregate
structure is indeed the pattern \emph{Composite}, that is usually used
for implementing hierarchically structured information (recursive
composition) and exploited in conjunction with patterns such as
Strategy and Visitors and many others.  Thus, the formmatter will have
to behave differently according to the actual type of the document
element that has to be formatted.  Dynamic overloading will avoid
using dynamic type checking and type casts.

Yves Roy wrote:
> Lorenzo:
>
> Could you explain to me how dd could be use in implementing strategy?
> Visitor, I understand, but I am curious to see an example with Strategy.
>
> Yves
>
> Lorenzo Bettini wrote:
>
>
> > I found double dispatch quite useful in implementing many design
> > patterns (visitor, observer, strategy,...) so I'm posting this announce
> > here too:
> >
> > Hi
> >
> > I've just released doublecpp:
> >
> > Doublecpp is a preprocessor for C++ that handles a new linguistic
> > construct for defining branches of a multi-method.  The "right" branch
> > of such a method will be selected dynamically at run-time according to
> > the actual type of the object on which the method is invoked and to the
> > actual type of the first argument: double dispatch.
> >
> > http://www.lorenzobettini.it/software/doublecpp/index.html
> >
> > Doublecpp is free software; you are free to use, share and modify it
> > under the terms of the GNU General Public License (see COPYING).
> >
> > Doublecpp is delivered with sources that have to be compiled. It can be
> > downloaded from the following site:
> >
> > http://www.lorenzobettini.it/software/doublecpp/doublecpp-0.2.tar.gz
> >
> > Doublecpp has been developed using GNU gcc 3.2.2
> >
> > You can build doublecpp also under Windows, using cygwin
> > (http://www.cygwin.com).  If you have problems with that, you can
> > request the windows binaries directly to me.
> >
> > IMPORTANT: you have to use a 3.x version of gcc: earlier versions have
> > problems handling code generated by doublecpp.
> >
> > You can find the documentation in the file
> >
> > http://www.lorenzobettini.it/software/doublecpp/doublecpp.ps
> >
> > I look forward to receiving comments!


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|  Lorenzo Bettini          ICQ# lbetto, 16080134     |
|  PhD in Computer Science                            |
|  Dip. Sistemi e Informatica, Univ. di Firenze       |
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