[boost] [utility] value_init workaround fixed [Re: value_init_test failure]

From: Niels Dekker - mail address until 2008-12-31 (nd_mail_address_valid_until_2008-12-31_at_[hidden])
Date: 2007-12-09 17:57:33

David Abrahams wrote:
> https://boost-consulting.com/trac/projects/boost/build/trunk reveals
> that
> https://boost-consulting.com/trac/projects/boost/changeset/41919
> introduced a failing test of the utility library.

Thanks for reminding me, Dave.  The test failure actually indicates that
the MSVC-workaround of value_initialized that I committed a while ago to
the trunk has a bug: when a value_initialized<T> object is copied, the
wrapped T object isn't copied properly.  If T has an accessible copy
constructor, it should be called when copy-constructing a
value_initialized<T>.  And if T has an accessible copy assignment
operator, it should be called when assigning a value_initialized<T>.   I
have just committed a fix for this bug to the trunk: 
http://svn.boost.org/trac/boost/changeset/41942

Luckily the bug wasn't yet released!  I mean, it isn't yet in Boost
release version 1.34.1, and I hope we can take it out of the release
branch.

The commit (changeset [41942]) aims to fix some other issues as well. 
Fernando Cacciola (the owner of value_init) and I also decided to no
longer distinguish between a "workaround version" and a non-workaround
version of value_initialized<T>.  Until now the workaround was only for
MSVC.  But it has appeared that various other compilers have
value-initialization related issues:

GCC Bug 30111 - Value-initialization of POD base class doesn't
initialize members
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=30111
GCC Bug 33916 - Default constructor fails to initialize array members
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=33916
Borland Report 51854 - Value-initialization: POD struct should be
zero-initialized
http://qc.codegear.com/wc/qcmain.aspx?d=51854

I recently added tests on these issues to value_init_test, and they
caused a test failures on a majority of the test platforms, at
http://beta.boost.org/development/tests/trunk/developer/utility_.html

In general, those compilers that don't support value-initialization well
forget to zero-initialize the POD-members of a type T, in some cases... 
The workaround is basically as follows:  The constructor of
value_initialized<T> stores the bytes of an instance of T in an
aligned_storage::type object, whose bytes are set to zero, followed by a
placement new.

Hopefully the new value_init.hpp will make an end to the test failures. 
If so, I hope we can still merge the trunk version with the release
version, 1.35.0.    

Kind regards,
-- 
Niels Dekker
http://www.xs4all.nl/~nd/dekkerware
Scientific programmer at LKEB, Leiden University Medical Center