Re: [boost] [Boost Review] Property Tree Library

From: Brian Allison (brian_at_[hidden])
Date: 2006-04-24 08:48:59

Thorsten Ottosen wrote:

>Daniel Walker wrote:
>  
>
>>On 4/23/06, Thorsten Ottosen <thorsten.ottosen_at_[hidden]> wrote:
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>>>>Schemas give you data types and type checking, which obviously is
>>>>nice to have when you're dealing with data. I think XML schema
>>>>validation is one of the most import features of XML for the same
>>>>reason that I like C++ templates and type-safe compile time
>>>>polymorphism: making sure your data types are correct before hand
>>>>gives you one less thing to worry about.
>>>>        
>>>>
>>>Why is that better than a run-time exception when loading the file?
>>>      
>>>
>>Why is what better? Maybe I wasn't clear. When an XML file includes a
>>schema and fails validation when loaded, you do get a run-time
>>exception. I was trying to say that's a good thing. An XML validating
>>parser is similar to a compiler for a strongly typed language: it
>>catches type errors (in addition to syntax errors) immediately before
>>you actually try to use the file.
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>Ok, why is that exception better than the one I generate if I don't meet 
>the tag I expect, of if the contents of a tag is not of the type I expect?
>
>-Thorsten
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It's not that the exception is atomically better, but the question seems 
similar to the old-time 2-phase construction programmer asking why it's 
better to avoid 2-phase construction. The parsing/validation of XML 
allows me to trust that my data structure has been constructed fully and 
as a valid foo - or that an exception was thrown for some appropriate 
reason. For XML, malformed XML itself is akin to a constructor call 
where it can only be determined to be malformed at runtime.

In fact, in each of the places where I've used XML to transmit data , it 
has been a representation of an invariant. This is no different than a 
class which properly represents an invariant, and I prefer not to be 
able to receive broken objects from factories.

Pardon me if I'm missing the point of your question...

Brian Allison