Re: [boost] [iostreams][io_state_savers] Request for interest in some ios_base basic utilities

10 Nov 2011

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From: Vicente Botet <vicente.botet@wanadoo.fr>
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  For example xalloc is thread safe in both MSVC and GCC libraries.
I don't like this kind of design, which forces the user to 
initialize the
 library. I will expect we can do better.
Global constructor?
I don't see what you mean.
I mean that all static ids are installed in global constructor like:

src/foo.cpp

int get_id()
{
  static int v = std::ios_base::xalloc();
  return v;
}

namespace details {
  struct installer {
    installer() { get_id(); }
  } instance;
}

This makes sure that if somebody links with the library the 
static ids (facet's ids) and static variables allocated
with xalloc are installed.

Of course this would not solve problem when two threads
try dynamically load the same library linked with this one.

But this is quite extreme case.
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And singleton can't be done in header only library under dll platform 
as
 well.
Isn't Boost.Flyweight a header only library?
I'm not familiar with it. So I don't know.
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So... It is problem. However it is not problem for standard
 submission as you always have library - standard c++ library which
 by no means can be header only.
Well for the standard submission there is thread or module issue if this
state is included directly on the ios state and the default facet is already
imbue with the associated facets ;-). Of course this is something I cannot
do with Boost.Chorno, or I don't know how to do it :'(.
But you can fallback to default if facet is not installed, i.e.:

   if(!have_facet<foo>(loc))
        // Do something as std::locale::classic dooes
   else
       use_facet<foo>(loc).bar(...)

Same for configuration. Ugly but useful for high level functions.
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Now ios state flags should not be thread safe as they stream specific
 and I don't expect from user to alter iostream flags from different
 threads.
(*) With ICU not all facets are immutable so in some places I use TLS to
     improve performance as for example with collator.
Why do you need mutable facets? Isn't this against the locale/facet design?
Best,
Vicente
It is... But when ICU does not provide immutable collator object and
cloning collator object is too damn heavy such that it become terribly
slow then I fallback to TLS and mutate TLS installed objects. Which 
allows be to give reasonable performance while keeping it invisible
for user and fully thread safe.

For non ICU based facets, there is no such problem.

Artyom Beilis
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