Re: [Libmesh-devel] libMesh/DistributedMesh Work

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

On Wed, 2 May 2018, Derek Gaston wrote:

> I'll add one more reason to hate std::vector<bool>: threading.

Yikes, I forgot that!  Glad I added you all to the Cc; thanks!

> std::vector<bool> is not at _all_ thread safe: multiple processors working on the
> same vector will clobber each other's values.

Assuming by "working on" you mean "writing to", then yeah:

"Notwithstanding (17.6.5.9), implementations are required to avoid
data races when the contents of the contained object in different
elements in the same container, excepting vector<bool>, are modified
concurrently."

You're supposed to still be able to read in multiple threads
concurrently just fine, though.

> I personally wish they had made std::vector<bool> take 8 times more
> memory... and then let you use something else if you needed the
> optimization... but that's just me.

You, and Hinnant... and honestly I agree too, just not quite as
strongly.  I'm deeply opposed to the "resizeable bitsets shouldn't
exist" attitude I see sometimes, but "resizeable bitsets shouldn't
have been named std::vector" is pretty hard to argue with.
---
Roy