Design Patterns discussion

[Jesús Alonso <kenchoweb AT hotmail.com>]

Thanks Russ, that is a very interesting reading.


> From: Russ Rufer 
> <russ AT pentad.com>
> To: Jesús Alonso 
> <kenchoweb AT hotmail.com>
> CC: 
> patterns AT celticblues.com,
>   
> gang-of-4-patterns AT cs.uiuc.edu
> Subject: Re: [gang-of-4-patterns] Create object without know type of object
> Date: Thu, 03 May 2007 16:06:12 -0700
>
> Hi Ed and Jesús,
>
> The hints Jesús gave at the end of his message capture the essence of a 
> pattern named Product Trader. You can find more details here:
>
>    Product Trader
>    by Dirk Baumer and Dirk Riehle
>    
> http://www.riehle.org/computer-science/research/1996/plop-1996-product-trader.pdf
>
>
> - Russ
>
> Jesús Alonso wrote:
> > Hi Ed,
> >
> > The Factory Method is the way to go here in my opinion. You receive the 
> > filename and substract the extension to know the file type. Then you take 
> > the extension and use it as a filter to choose the right class to 
> > instance. It's probably the most lightweight and easy to implement 
> > solution, and it sure gives great results, so I suggest you to use it, yes 
> > :)
> >
> > The abstract factory will only be useful in case you need to instance many 
> > other classes depending on the file name. I guess you only want to 
> > instantiate a FileWhatever class, so Abstract Factory would be meaningless 
> > here.
> >
> > However, you expressed that you want to extend the supported file types in 
> > the future, so I think you should take an approach similar to the Abstract 
> > Factory, but simpler. On the application load, register a number of 
> > factories in the file reader, passing its discriminator as an extra 
> > parameter. Something like:
> > myFileReader->registerFileType ("jpg", new JpegFileTypeFactory ());
> > You would store the discriminator and the factory reference in a 
> > std::map<std::string,FileTypeFactory*>. That should allow you to 
> > instantiate the right file type doing:
> > BaseFile *newFile = factories[fileExtension]->load (filename);
> > Pretty straightforward and easy to read if you have some knowledge on the 
> > STL containers.
> >
> > Hope that helped!
> >
> >
> > > From: 
> > > patterns AT celticblues.com
> > > To: 
> > > gang-of-4-patterns AT cs.uiuc.edu
> > > Subject: [gang-of-4-patterns] Create object without know type of object
> > > Date: Thu, 03 May 2007 12:58:15 -0400
> > >
> > > Let's I am writing some code that will read two or more different file
> > > types.  Assume that I will have a BaseFile class, and two subclasses.
> > > With the load of specific file contents performed in the Load method.
> > >
> > > BaseFile
> > > {
> > > .
> > > .
> > > .
> > >     virtual bool Load() = 0;
> > > .
> > > .
> > > .
> > > };
> > >
> > > FileTypeOne: public BaseFile
> > > {
> > > .
> > > .
> > > .
> > >     bool Load();
> > > .
> > > .
> > > .
> > >
> > > };
> > >
> > > FileTypeTwo: public BaseFile
> > > {
> > > .
> > > .
> > > .
> > >     bool Load();
> > > .
> > > .
> > > .
> > >
> > > };
> > >
> > >
> > > What I need, is a way to create a new file object type without
> > > necessarily knowing which type of file is going to be read.  For
> > > example, if given the name of the file to be read, I pass it to some
> > > function/object and it will return a ptr to a new correct type of
> > > object.
> > >
> > > BaseFile *newFile = CreateFile(filename)
> > >
> > > Seems like this may be a job for a factory or factory method but I am
> > > not sure.  What pattern would best suit this functionality?
> > >
> > > To make matters more confusing, this is for a real time simulation, so
> > > it needs to be lean and mean, and preferrably simple to implement and
> > > update as new file types are defined.
> > >
> > > Thanks,
> > > Ed
> > >
> > >
> > > _______________________________________________
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> > > gang-of-4-patterns AT cs.uiuc.edu
> > > http://lists.cs.uiuc.edu/mailman/listinfo/gang-of-4-patterns
> >
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