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: 2017-03-24 23:32:58

On 23/03/2017 20:26, Christopher Pisz via Boost-users wrote:
> The following comparison fails for an error whom outputs "asio.misc" for
> errorCode.category().name() and "end of file" for errorCode.message()

The C++ 11 standard implies that each error category instance shall have
a globally unique address and comparisons of equality shall use that
address to compare. Unfortunately, this is not reliable in portable
code, only the Dinkumware STL implements true address uniqueness
anywhere in the process and it adds a full memory barrier to achieve
that i.e. it's expensive.

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.

In my own code, I do the comparison operator first, and if that fails I
do a strcmp() of the category's name(). To date, this has not bitten me.
I would personally consider this aspect of error categories to be a
defect in the standard, as specified it forces non-header-only
implementation if you want it to be conforming, and even that doesn't
cover RTLD_LOCAL which means you need to fall back onto named shared
memory or some hack.

Niall

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