Re: [boost] ublas notation

From: Michael Stevens (support-1_at_[hidden])
Date: 2002-07-16 06:17:21

>
>
>Date: Mon, 15 Jul 2002 10:38:08 -0400
>From: "David Abrahams" <david.abrahams_at_[hidden]>
>  
>
>>>> > One idea to consider:
>>>> >
>>>> >     x.assign() = blah blah blah
>>>> >
>>>> > would at least set the LHS off from the rhs a little better.
>>>      
>>>
>>>
>>> Or add some policy to the matrices and vectors to indicate if they are
>>> aliased. e.g. 'matrix< double, column_major, restrict > a(10,10);'
>>    
>>
This is very intersting. Similar to what I was thinking regarding syntax 
and alias problems. Providing/using more traits information to/in 
assigment operators could be very useful.At the moment we have the 
situation where if you use
        A = B;
then a temporay is created incase the right hand side is a expression 
with an alias of A. Of course we can use
    A.assign (B);
but it is rather ugle.
I believe it is true that if the expression (B in this case) is a 
container then direct assignment is safe; even if B is an alias for A. 
Similarly for vector proxies:
    v = row(B, 2)
is always safe if B is a container.

In the future this may become more important as more high level linear 
algerbra objects are added. At the moment (soon) we already have a 
temporary creating prod<xx>(a,b). If I implemented a Cholesky 
factorisation in uBLAS I would probably have the object constructed from 
a symmetric matix and store the factorisation in a triangular class 
member. I think it will be a common property of all factorisations that 
they cannot be efficiently evaluted by the index/iterated expression 
template mechanism without creating a temporary first. Therefore 
factorisation object will always have the safe direct assigment property 
as above. It would be nice to avoid the .assign() syntax and be able to use
        UT = cholesky(M);

There may already be enough traits information in uBLAS expressions to 
make operator= choose an appropriate (temporary free) assignment. If 
nobody can see any dangers in this I will try to develop some uBLAS 
patches to try this out.

>    a .noalias= x * y;
>
>would be nice, or possibly
>
Mmmm this looks very nice. Has the advantage that it works even when not 
using the uBLAS namespace.

>
>    a /noalias = x * y;
>
>or
>    a [noalias]= x * y;
>
>Mmm, that last one's tasty...
>
>-Dave
>
Or even
        a = noalias(x*y)
which could simply use the above traits mechanism and wrap the result of 
x*y with a trair to indicate safe direct assigment.

Michael