Re: [eigen] Lazy evaluation bug, feature, or PEBKAC?

[Dave Steffen <dave.steffen@xxxxxxxxxxx>]

On Wednesday, March 02, 2011 11:07:19 am Benoit Jacob wrote:
> 2011/3/2 Benoit Jacob <jacob.benoit.1@xxxxxxxxx>:
> > 2011/3/2 Dave Steffen <dave.steffen@xxxxxxxxxxx>:
> >> On Wednesday, March 02, 2011 02:31:40 am Gael Guennebaud wrote:

[...]

> >>  In my application, I really *do* need to add two matrices, which
> >>  might or might not happen to have 1 column, and then operate
> >>  (read-only) on individual columns.  That doesn't strike me as a
> >>  terribly unusual thing to do.
> > 
> > The unusual thing here is to want to construct the expression object
> > once and then reuse it.
> > 
> > Constructing these expression objects has zero runtime cost in
> > optimized builds, because we make it very easy for the compiler to
> > optimize away the expression objects completely.
> > 
> > Therefore, there is no overhead in doing "a+b" everytime you need this
> > sum expression: zero overhead.
> > 
> > That's why practically nobody is trying to do what you're doing here :)
> 
> Also, note that your code only looks "natural" because it's using the
> C++0x auto keyword. The C++03 version would have to name explicitly
> the (very abstruse) expression type, which is another reason why
> practically nobody bothers.

  Riiiight.  That is, I shouldn't be stashing a ref to an
  *expression*, I need to be evaluating that expression into a Matrix.

  Being new to Eigen, it didn't occur to me that keeping refs to
  expressions could wreak so much havoc.

  So: the actual answer to my question is both "bug" and "PEBKAC",
  the latter problem to be resolved by the usual statement: "don't do
  that".

  :-)

-- 
Dave Steffen, Ph.D. - Software Engineer

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