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

Subject: Re: [boost] [system] Viral C++11 change would break dependent libraries:-(
From: Beman Dawes (bdawes_at_[hidden])
Date: 2012-08-28 14:37:02

On Mon, Aug 27, 2012 at 2:43 PM, Olaf van der Spek <ml_at_[hidden]> wrote:
> On Mon, Aug 27, 2012 at 7:33 PM, Beman Dawes <bdawes_at_[hidden]> wrote:
>> Ideas?
>
> Can the derived classes be updated before the superclass is updated?
> If not, a flag day/release seems necessary which might be problematic
> for user libs & apps.

That worked OK for Boost.Filesystem's filesystem_error and a UDT
that's part of the Boost.System test suite.

So that will help with Boost's codebase; we can find and fix the
affected code before actually changing Boost.System.

But there is still a problem for user code.

Also, BOOST_NOEXCEPT is null for C++03 libraries and compilers. But in
some cases signature that has to be changed currently has a "throw()"
throw-specifier. So I guess we need a BOOST_NOEXCEPT_NOTHROW macro
defined as "noexcept" for C++11 and "throw()" for C++03. Otherwise we
would be changing the semantics for C++03 compilers.

That's similar to what libstdc++ does:

// Macro for noexcept, to support in mixed 03/0x mode.
#ifndef _GLIBCXX_NOEXCEPT
# ifdef __GXX_EXPERIMENTAL_CXX0X__
#  define _GLIBCXX_NOEXCEPT noexcept
#  define _GLIBCXX_USE_NOEXCEPT noexcept
#  define _GLIBCXX_THROW(_EXC)
# else
#  define _GLIBCXX_NOEXCEPT
#  define _GLIBCXX_USE_NOEXCEPT throw()
#  define _GLIBCXX_THROW(_EXC) throw(_EXC)
# endif
#endif

I guess that by defining _GLIBCXX_NOEXCEPT,  _GLIBCXX_USE_NOEXCEPT,
and _GLIBCXX_THROW(_EXC) yourself, it is possible to build the library
from source any way you want. But that runs a serious risk of your
library build getting out of sync with your program builds.

--Beman