Re: [boost] xml?

Subject: Re: [boost] xml?
From: Larry Evans (cppljevans_at_[hidden])
Date: 2010-04-09 12:34:38

On 04/09/10 10:48, Mathias Gaunard wrote:
> Larry Evans wrote:
> 
>> What is there about the XPath specification that makes any type
>> hierarchy for modelling it less suitable than using something
>> akin to boost.variant?
>>
>> You see, I'm wondering because using type hierarchies and
>> virtual functions has been touted as a great advantage of
>> OO programming; yet, it apparently lacks something which
>> you need.  I'd like to understand what that is.
> 
> Some could argue that the point of a base class is moot if you have to 
> downcast it to make anything useful with it,

But boost.variant has to do the equivalent of downcasting based on
discriminant (the value returned by which()).  You could argue
variant does that automatically, but then it might throw and
exception if the target of the assignment(e.g.) was the wrong
type.  Of course, you could check the discriminant before the
assignment, but then, how's that different than what has to be
done with virtual functions (using dynamic_cast).  Of course
one could argue that the variant library's apply_visitor
does all this checking for you before sending the correct
type to your actual visitor; however, the apply_visitor is
little different than the elements' (using the term from
visitor pattern, http://en.wikipedia.org/wiki/Visitor_pattern )
virtual accept functions.

> and that algebraic data 
> types (variant-like things) are a much more elegant solution when you 
> need to visit the different cases.

I'm still not seeing it :(  I thought algebraic data types were
one thing OO programming did well.  For example, a stack is
and ADT and the stl library has a stack.  AFAICT, every
component in boost.variant is like a element role in
the visitor_pattern.

-confusedly yours,
Larry