Re: [eigen] a few updates...

[Christoph Hertzberg <chtz@xxxxxxxxxxxxxxxxxxxxxxxx>]

On 22.12.2010 04:50, Benoit Jacob wrote:
> * There are 2 remaining places where I haven't fixed
> const-correctness: the Sparse module (leaving this to Gael) and
> swap(). I have improved the situation with swap() so that e.g. you
> can't swap const Matrix's anymore, but there remains a much trickier
> issue: swapping temporary expression as in:
> 
>     m.row(i).swap(m.row(j));
> 
> This is incredibly tricky because C++ doesn't allow one to take
> non-const references to temporaries, so one has to pass const
> references and const_cast, which loses the information of whether the
> expression was const in the first place.
> 
> There are 2 things we can do here:
> a) leave it there with c++98 and consider using rvalue references in c++0x mode
> b) actually there is a crazy aspect of c++98 that allows to fix this
> problem. As I said, the fundamental issue is that C++ doesn't allow
> one to take non-const references to temporaries... but that's not
> entirely true, there's one case in which this is allowed: that's when
> taking a reference to *this. At least insofar as GCC's behavior is
> compliant with the spec. So, while the API we have for swap() can't be
> fixed wrt const-correctness within c++98, this different API:
> 
>    xpr1.swap() = xpr2.swap()
> 
> can be made const-correct, by implementing swap() as returning a
> SwapWrapper<T>, and implementing swap() const as returning a
> SwapWrapper<const T>.

Seriously this looks a bit ugly ...
Why would you need a SwapWrapper<const T> anyways?

Another possibility would be:
c) Pass argument to swap() by value if it is a Map/Block-expression.
This should never be more than a pointer and a few integers and would
hopefully be optimized away in most cases.
Maybe not with the current implementation, when passing sth such as:
  m.block(...).block(...).transpose().row(...)
but that could be fixed when solving "direct access type unification".
Ideally, if every dimension is known at compile-time, passing by value
would then boil down to just passing a pointer to where the data starts.

Christoph

-- 
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Dipl.-Inf. Christoph Hertzberg
Cartesium 0.051
Universität Bremen
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28359 Bremen

Tel: (+49) 421-218-64252
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