Re: [Boost-users] Compare boost::system::error_category

Subject: Re: [Boost-users] Compare boost::system::error_category
From: Niall Douglas (s_sourceforge_at_[hidden])
Date: 2017-03-25 12:45:56

>> Boost's quality of implementation is the same as libstdc++ or libc++,
>> you can get multiple instantiations in certain circumstances and those
>> can have differing addresses.
> 
> The various error categories are implemented (for header-only libraries)
> as inlined Meyer singletons. Does that mean that we have a general
> problem with such singletons?

The short answer is yes, the error category comparison operators are
unreliable on all major STLs apart from Dinkumware's and in Boost.

The longer answer is that the hack of strcmp()ing the name() works
surprisingly well in practice, and it might be wise if Boost's error
categories implemented that, followed by libstdc++ and libc++.

I expect after Charley Bay bangs the drum on this stuff at C++ Now
followed very shortly thereafter by me banging the drum at ACCU that we
might draw some attention to this problem by the STL maintainers. The
only one who really gets the problem is Stephan, but his solution for
Dinkumware does bring in an atomic fence which he himself admits is a
bit heavy. It certainly causes a lot of code bloat when using Outcome
with the Dinkumware STL, as every possible error code construction point
requires an atomic fence.

I've done some remedial evil hackery in Outcome to bypass Stephan's
atomic fence some of the time on VS. It's the best I can do given the
constraints.

Niall

-- 
ned Productions Limited Consulting
http://www.nedproductions.biz/ http://ie.linkedin.com/in/nialldouglas/