Re: [eigen] aliasing system

[Andre Krause <post@xxxxxxxxxxxxxxxx>]

Benoît Jacob wrote:
> On Friday 07 September 2007 10:44:18 Andre Krause wrote:
> > > > The point of my example with rows was that if we want to determine if
> > > > rows/columns/blocks/minors overlap in memory, we're trying to do
> > > > something that can't be done at compile-time (because the arguments that
> > > > you pass to these functions is not necessary a compile-time constant).
> > > > Hence that will have a cost, and if you want to handle all cases it'll
> > > > be a big cost (you'll have to check for each entry in the matrix if
> > > > overlapping occurs at that entry).
>
> > BUT i still would insist to have that check in _DEBUG mode only at
> > runtime ! if it is possible without too much coding. it is very
> > important to be able to somehow know that my code does what i assume it
> > to do. and newbies and those who usually skip the FAQ and DOCs will be
> > faced with strange errors if they try what seems to be a naturally
> > equation: m=m*m.
>
> OK, seems like a good idea. I'll try to do it.

ok, that would be really great. but please dont put any priority on it - 
i dont want to distract you from the main goal, a working eigen2 release!

> > but i have another idea:
>
> Umm, no. The expression templates are not just an optimization, they are the 
> central building block and the API would not be possible without them. 
> Without expression templates there is no way that we could allow adding one 
> row into another one by doing
>
> matrix.xpr().row(0) += matrix.row(1).
>
> So it's not possible to add/remove Ets depending on an ifdef.
>
> Cheers,
> Benoit

but it would be possible to add alias() inside the operator=(...) 
function(s) ???
so the system would always generate a temporary ? so one could cross 
validate one's code with and without temporaries.

something ala

operator=(const Matrix<T,d1,d2>&m)
{
	#ifdef _NO_EXPTEMPLATE_OPTIMIZE
	this->alias()=m;
	#else
	???
	#endif
}

??