On Wed, 13 Jan 2010, Vijay S. Mahadevan wrote:
> But the fact is that I do not use EquationSystems directly but instead
> only rely on the derived member, say CustomEquationSystem, which is
> aware of the necessary calls to create, manipulate and delete
> SpecialSnowflakeSystem. Hence, CustomEquationSystem should override
> add_system(type, name) and will perform an action only if the type is
> SpecialSnowflakeSystem. Else, it will just forward the call to
> EquationSystems::add_system(type, name).
Right. So your code will work, because it's using
CustomEquationSystem. But when I try to open your file in my
meshdiff, meshplot, meshimage, etc. utilities, they'll crash, because
they're using EquationSystems. Likewise, if I'd used your method,
none of the files I'd written out would be openable in any code you've
written.
It's okay if you don't have such generic tools or don't need that
level of interoperability, but it's not cleaner or more flexible.
>> When they're read as restart files, however, and I actually really
>> *do* want to create a SpecialSnowflakeSystem, that's still easy
>> enough: the code that's reading them is SpecialSnowflakeSystem-aware,
>> so it calls es.add_system<SpecialSnowflakeSystem>("Special") itself.
>> Then when es.read() encounters a system named "Special" in the data
>> file, it fills in the data for the existing system of that name.
>
> Btw, this will not work as is.
When theory and practice give differing answers, it's time to
reexamine the theory...
> Because, the read() method calls add_system(type, name) immaterial
> of whether such a system exists currently or not. May be a call to
> has_system() before this would actually implement the behavior you
> specify.
Look at the add_system() implementation. Like with add_vector(), you
can add an already-existant name and you'll just get a reference to
the already-added object.
---
Roy
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