Design Patterns discussion

[Russ Rufer <russ AT pentad.com>]

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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