id,summary,reporter,owner,description,type,status,milestone,component,version,severity,resolution,keywords,cc
1217,"value_initialized leaves data uninitialized, when using MSVC",Niels Dekker,Fernando Cacciola,"The data of an instance of boost::value_initialized<T> may not be initialized properly, when using Microsoft Visual C++. I already contacted Fernando Cacciola, the author of value_initialized, and we have developed a workaround, that I would like to propose.

== The problem ==
There are two kinds of types, for which the issue is relevant, for different reasons:

When T is an aggregate class or struct that contains both POD and non-POD members, the constructor of value_initialized<T> will typically leave the POD members of T uninitialized, when using MSVC:
{{{
  #include <boost/utility/value_init.hpp>

  struct A {
    std::string s;
    int i;
  };

  boost::value_initialized<A> a;
  BOOST_CHECK( get(a).i == 0 );  // Will fail on MSVC!
}}}

This is caused by a compiler bug, reported by Pavel at the Microsoft Connect website, 7/28/2005, Feedback ID 100744, [https://connect.microsoft.com/VisualStudio/feedback/ViewFeedback.aspx?FeedbackID=100744 Value-initialization in new-expression]  The report was closed at 11/14/2006, and its status was set to ""Closed (Won't Fix)"".  The bug appears relevant to all versions of MSVC, including the next one (MSVC 2008, code name ""Orcas"").

When T is an array of a POD type, MSVC 2005 provides an interesting warning, within the constructor of the base class of value_initialized<T>:
{{{
warning C4351: new behavior: elements of array 'T' will be default initialized.
}}}
This tells us that an older version of the compiler wouldn't properly initialize value_initialized<T>.  And indeed the following check fails, when using the 2003 version of MSVC:
{{{
  typedef int B[2];
  boost::value_initialized<B> b;  // MSVC 2005 warning C4351: new behavior!
  BOOST_CHECK( get(b)[1] == 0 ); // Fails on MSVC 2003 (BOOST_MSVC == 1310).
}}}

== The proposed workaround ==
A new version of value_init.hpp is attached to this report, which includes a workaround for MSVC. Basically, when constructing value_initialized<T> it clears the bytes of T, just before constructing T itself.  It does so by having the bytes of T stored in an instance of aligned_storage::type, and memsetting those bytes to zero before doing a placement new for type T, at the location of the bytes.

The workaround works well for MSVC, because for a built-in type, the value zero always has a bit pattern of only zero's, on any platform supported by this compiler, AFAIK.

Note that the workaround itself is a correct and standard compliant implementation of boost::value_initialized, but it may have a small performance penalty in comparison to the original version.

--
Niels Dekker
[[BR]]http://www.xs4all.nl/~nd/dekkerware
[[BR]]Scientific programmer at LKEB, Leiden University Medical Center
",Bugs,closed,Boost 1.35.0,None,Boost 1.34.1,Problem,fixed,,