Re: [eigen] sparse matrix wrapper flavour

[Benoît Jacob <jacob@xxxxxxxxxxxxxxx>]

Hi,

all this sounds very good,

On Wednesday 25 June 2008 23:43:20 Gael Guennebaud wrote:
> {
>     typename SparseWrapper<MyMatrixType, AccessPattern>::type w(m);
>     // use w according to the access pattern you requested
> }
> // at the deletion of the wrapper, m is automatically updated.

I see, sounds good.

> To avoid redundancy, I see two approaches to implement such wrappers:
> 1 - they inherit the corresponding sparse matrix class and simply
> overload the constructor, destructor and store a reference to m;
> 2 - they store an object of the corresponding sparse matrix type and
> define operator* and operator-> such that they behave like a pointer.
>
> I think both approaches are more or less equivalent:
> (1) allows to overwrite some matrix functions if needed,
> (2) implies a pointer like style that emphasis the fact your dealing
> with something different

Given the choice, I tend to prefer storing a member object rather than 
inheriting.

In your particular case, if you go for inheritance, since your wrapper class 
will be a different type than the wrapped class, it will lead to more 
template instantiations when you construct expressions classes like e.g. 
wrapper.col(i).

So, I vote for (2). The only thing is, make sure that the wrapper class 
doesn't have a member (method or data) with the same name as a member of the 
wrapped class. So that if the user forgets to use "->" instead of "." he will 
get a compiler error. Otherwise the bug could be very hard to spot!

Cheers,

Benoit

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