XML OO Parametrization

[Michael Gauckler <ma...@gauckler.ch>]

Dear List, 

I am new to this list and XML in general.

I'd like to use XML to feed parameters in our modular C++ application. 

Modular in the sense that, depending on the configuration (determined at
program startup), different types of objects are created through a
factory.

Example:

Class A
{
	Helper* h;
} 

Class ColoredHelper : public Helper
{..}

Class StripedHelper : public Helper
{..}

On construction an instances of A the factory creates the helper
according to the configuration file and lets h point to it.

The problems we are facing are:

a) 

The configuration file must be made consistent. I am thinking of a XML
schema, which describes all possible classes that can be used as
Helper's. With the help of this xsd the configuration can be validated,
or maybe can be transformed into a GUI. Before starting the application
the configuration file can easily be checked against the schema.

b)

Parametrization of the Helpers. The different helpers might need
different parameters (ColorHelper maybe only one, whereas StripedHelper
needs two parameters). I am thinking of XML fragments, which are parsed
by the helper object to extract the appropriate parameters.

At the moment I am considering the Xerces parser in SAX mode. Is this
the right decision, or should I rather go for DOM?

Can XML fragments be passed to another object for parsing (upon
detection of the helpers token)? Or should I add new callback functions
according to the object generated?

Is the approach feasible, or am I missing something fundamentally?

Thank your for your help!

Yours, 
 Michael










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Re: XML OO Parametrization

[Axel Weiß <aw...@informatik.hu-berlin.de>]

Am Dienstag, 15. März 2005 19:05 schrieb Michael Gauckler:
> Dear List,
>
> I am new to this list and XML in general.
>
> I'd like to use XML to feed parameters in our modular C++ application.

Hi Michael,

this is a good idea - I had the same some months ago...

> Modular in the sense that, depending on the configuration (determined
> at program startup), different types of objects are created through a
> factory.
>
> Example:
>
> Class A
> {
> 	Helper* h;
> }
>
> Class ColoredHelper : public Helper
> {..}
>
> Class StripedHelper : public Helper
> {..}
>
> On construction an instances of A the factory creates the helper
> according to the configuration file and lets h point to it.

Seems to be the right concept.

> The problems we are facing are:
>
> a)
>
> The configuration file must be made consistent. I am thinking of a XML
> schema, which describes all possible classes that can be used as
> Helper's. With the help of this xsd the configuration can be
> validated, or maybe can be transformed into a GUI. Before starting the
> application the configuration file can easily be checked against the
> schema.

Ok, this is the purpose of xml-schema. What is the problem regarding 
this?

> b)
>
> Parametrization of the Helpers. The different helpers might need
> different parameters (ColorHelper maybe only one, whereas
> StripedHelper needs two parameters). I am thinking of XML fragments,
> which are parsed by the helper object to extract the appropriate
> parameters.

I usually map argument values to attribute values. They can be checked 
against their valid range during validation. Different arguments have 
different attribute names (as 'key'), so you can map each Helper class 
to an xml element that has as many attributes as the number of arguments 
the constructor takes.

> At the moment I am considering the Xerces parser in SAX mode. Is this
> the right decision, or should I rather go for DOM?
>
> Can XML fragments be passed to another object for parsing (upon
> detection of the helpers token)? Or should I add new callback
> functions according to the object generated?

It's maybe a matter of taste, but I prefer handling DOM-trees for the 
extraction of xml contents. Be sure to have something like a 
hierarchical desicion-tree and pick the argument values for your object 
generation at the last step.

> Is the approach feasible, or am I missing something fundamentally?

In my applications, most of the persistent configuration information is 
accessible by the user via menu operation or dialogs. The user settings 
are written back to the xml configuration file after change, for 
convenience.

Cheers,
			Axel

-- 
Humboldt-Universität zu Berlin
Institut für Informatik
Signalverarbeitung und Mustererkennung
Dipl.-Inf. Axel Weiß
Rudower Chaussee 25
12489 Berlin-Adlershof
+49-30-2093-3050
** www.freesp.de **

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