Re: [Libmesh-users] Parallel Mesh Generation in Libmesh

[Roy S. <roy...@ic...>]

On Mon, 13 Mar 2017, John Peterson wrote:

> On Mon, Mar 13, 2017 at 11:06 AM, Barna Becsek <bar...@gm...> wrote:
>
>       I was able to overcome the problem with a little help from a
>       friend.
>       Basically what I did is replace “this->qrule->get_weights()”
>       with “const_cast<const QBase *>(this->qrule)->get_weights()”
>       and everything worked fine!
>       I, however, still don’t understand why this would be
>       necessary. In theory qrule already is a QBase pointer.
> 
> 
> const_cast compiles down to literally no machine instructions IIRC.  This can't really have fixed the problem.

Technically this ought to cause the const get_weights() to get called
instead of the non-const, which would cause a const vector reference
to be returned instead of non-const... but then the vector reference
would have been cast to const regardless by the call to compute_map(),
so you're right in this case; no machine instructions in the end.

> Maybe there was not problem in the first place, or maybe you have a
> Heisenbug... the latter can sometimes be "fixed" with an innocuous
> change like adding a print statement...

This looks a *lot* like the one-line-of-magic-garbage I had in one of
my applications when first starting out with libMesh.  I never found a
bug associated with it, but when I stopped using the Red Hat "gcc
2.96" fork the problem went away for good.
---
Roy