Re: [Patch, libstdc++/64584, libstdc++/64585] Clear basic_regex after imbue and make assign exception tolerant

[Jonathan Wakely <jwakely at redhat dot com>]

On 16/01/15 14:34 +0100, Marc Glisse wrote:
> On Fri, 16 Jan 2015, Jonathan Wakely wrote:
>
> > Am I missing something here, why are you moving the iterators?
> >
> > Iterators are cheap to copy, so moving them is not necessary.
>
> (I am not talking about this patch in particular)
>
> Iterators are supposed to be relatively cheap to copy. In practice, 
> that copy doesn't always come for free. When you implemented 
> filesystem iterators, IIRC you had at least a shared_ptr in your 
> iterators, so copying involved some atomic operations and those have a 
> cost (first a direct cost on the system, but also an optimization cost 
> because we can't simplify 5 consecutive copies to only one). I have 
> seen iterators that were even more expensive (and inconvenient to make 
> cheaper). If we can internally reduce the number of copies (move 
> instead, pass by reference, etc), that seems good to me. I wanted to 
> make a pass on predefined_ops.h but didn't find the time.

In general I agree, but for regex the iterators are _usually_ going to
be pointers, because we try to unwrap string iterators to get raw
pointers, so we only use the Foo<const char*> specializations and not
Foo<std::string::iterator> as well.

If std::move on a pointer definitely doesn't pessimize, then I suppose
it's no worse and the move is OK.