Re: [boost] [system] Viral C++11 change would break dependent libraries:-(

Subject: Re: [boost] [system] Viral C++11 change would break dependent libraries:-(
From: Vicente J. Botet Escriba (vicente.botet_at_[hidden])
Date: 2012-08-27 17:33:21

Le 27/08/12 19:33, Beman Dawes a écrit :
> Ticket #7278 asks that Boost.System add C++11 noexcept support. A
> reasonable request, and I implemented it this morning by adding
> BOOST_NOEXCEPT at the appropriate places specified in the standard.
>
> But there is a problem.  noexcept is viral, so any classes that
> inherit from a class that uses noexcept must be changed to also use no
> except.
>
> An example: Boost.Filesystem has this class:
>
>    class codecvt_error_cat : public boost::system::error_category
>    {
>    public:
>      codecvt_error_cat(){}
>      const char*   name() const
>        {return "codecvt";}
>      std::string    message(int ev) const;
>    };
>
> Compiling with GCC 4.7 -std=c++0x results in this error:
>
> ..\..\../boost/system/error_code.hpp:187:32: error:   overriding
> 'virtual const char* boost::system::error_category::name() const
> noexcept (true)'
>
> To fix it, the filesystem class has to be changed:
>
>    class codecvt_error_cat : public boost::system::error_category
>    {
>    public:
>      codecvt_error_cat(){}
>      const char*   name() const BOOST_NOEXCEPT
>        {return "codecvt";}
>      std::string    message(int ev) const;
>    };
>
> I've held off committing the Boost.System changes to trunk until we've
> had a chance to discuss how to approach this potential break to an
> unknown number of Boost and user libraries.
>
> Ideas?
>
Hrr, this is really annoying. I have introduced a lot of BOOST_NOEXCEPT 
in Boost.Chrono and Boost.Thread without thinking that I was changing 
the interface. Apologies to any one these changes could break their code.

One possibility could be to use BOOST_SYSTEM_NOEXCEPT instead.

Boost.System could define it depending on the Boost.System version as

#if ! defined BOOST_SYSTEM_VERSION
#define BOOST_SYSTEM_VERSION 1
#endif

#if BOOST_SYSTEM_VERSION < 2
#define BOOST_SYSTEM_NOEXCEPT
#else
#define BOOST_SYSTEM_NOEXCEPT
#endif

and let the time to the others libraries to move to the new version.

Once all the dependent Boost libraries have made the change, 
Boost.System could increase the version

#if ! defined BOOST_SYSTEM_VERSION
#define BOOST_SYSTEM_VERSION 2
#endif

The dependent user could always force the Boost.System version.

HTH,
Vicente