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

[ Thread Index | Date Index | More Archives ]

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


Dave Steffen, Ph.D. - Software Engineer

Numerica Corporation <>
4850 Hahns Peak Drive, Suite 200
Loveland, Colorado 80538
main (970) 461-2000 x 227   direct (970) 612-2327
Email: dave.steffen@xxxxxxxxxxx
fax (970) 461-2004

This message and any attachments are intended only for the individual
or entity to which the message is addressed.  This is a private
message and may contain privileged information.  If you are neither
the intended recipient nor the agent responsible for delivering the
message to the intended recipient, you are hereby notified that any
review, retransmission, dissemination, or taking of any action in
reliance upon, the information in this communication is strictly
prohibited, and may be unlawful.  If you feel you have received this
communication in error, please notify me immediately by returning this
email to me and deleting it from your computer.

Mail converted by MHonArc 2.6.19+