Either that, or do what I suggested in my previous email: just write
2011/3/2 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
> Dave Steffen, Ph.D. - Software Engineer
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