Re: [boost] [proto] transforms && rvalue refs

Subject: Re: [boost] [proto] transforms && rvalue refs
From: troy d. straszheim (troy_at_[hidden])
Date: 2009-03-30 13:17:17

Eric Niebler wrote:
> 
> Using expression templates to eliminate unnecessary temporaries is a 
> well understood idiom; it's why expression templates were invented in 
> the first place. You don't need rvalue references at all.
> 
> Could you say more about what you are trying to do and why you think 
> rvalue references are needed?
> 

Maybe I've missed something stupid or have organized things so as to 
cause myself trouble.  In an expression like

    // these contain pointers to memory out on the GPU
    array<float> A, B(10,10), C(10,10);

    A = exp(B) * C;

When it is time to evaluate the expression, the grammar fires the 
transform for exp(B) first.  It creates a temporary T with the same 
dimensions as B and and a function is called to compute the result into 
T.  Now 'T * C' happens.  In this case there is an available temporary, 
'T', so the result can go right in to T.  T is returned and moved in to 
A.  Done...  at least, that's the idea.

So how should these transforms know whether their arguments are 
overwritable or not?    I didn't see how to get the 'slicing through the 
expression' approach, as in the calc examples, with contexts and 
overloaded operator[], to work here:  each step of the calculation has 
to get done all at once, via a function call.   But maybe it is time to 
go back and look again.

-t