Merged value_init.hpp from trunk, including new boost::initialized_value object. (Was discussed beforehand with Fernando Cacciola.)

[SVN r46464]

include/boost/utility/value_init.hpp

@@ -1,4 +1,4 @@
-// (C) Copyright 2002-2007, Fernando Luis Cacciola Carballal.
+// (C) Copyright 2002-2008, Fernando Luis Cacciola Carballal.
 //
 // Distributed under the Boost Software License, Version 1.0. (See
 // accompanying file LICENSE_1_0.txt or copy at
@@ -6,29 +6,20 @@
 //
 // 21 Ago 2002 (Created) Fernando Cacciola
 // 24 Dec 2007 (Refactored and worked around various compiler bugs) Fernando Cacciola, Niels Dekker
+// 23 May 2008 (Fixed operator= const issue, added initialized_value) Niels Dekker, Fernando Cacciola
 //
 #ifndef BOOST_UTILITY_VALUE_INIT_21AGO2002_HPP
 #define BOOST_UTILITY_VALUE_INIT_21AGO2002_HPP
 
 // Note: The implementation of boost::value_initialized had to deal with the
-// fact that various compilers haven't fully implemented value-initialization:
+// fact that various compilers haven't fully implemented value-initialization.
-// Microsoft Feedback ID 100744 - Value-initialization in new-expression
+// The constructor of boost::value_initialized<T> works around these compiler
-// Reported by Pavel Kuznetsov (MetaCommunications Engineering), 2005-07-28
+// issues, by clearing the bytes of T, before constructing the T object it
-// https://connect.microsoft.com/VisualStudio/feedback/ViewFeedback.aspx?FeedbackID=100744
+// contains. More details on these issues are at libs/utility/value_init.htm
-// GCC Bug 30111 - Value-initialization of POD base class doesn't initialize members
-// Reported by Jonathan Wakely, 2006-12-07 
-// http://gcc.gnu.org/bugzilla/show_bug.cgi?id=30111
-// GCC Bug 33916 - Default constructor fails to initialize array members
-// Reported by Michael Elizabeth Chastain, 2007-10-26
-// http://gcc.gnu.org/bugzilla/show_bug.cgi?id=33916
-// Borland Report 51854 - Value-initialization: POD struct should be zero-initialized
-// Reported by Niels Dekker (LKEB, Leiden University Medical Center), 2007-11-09
-// http://qc.codegear.com/wc/qcmain.aspx?d=51854
-// The constructor of boost::value_initialized<T> works around these issues, by
-// clearing the bytes of T, before constructing the T object it contains. 
 
 #include <boost/aligned_storage.hpp>
 #include <boost/detail/workaround.hpp>
+#include <boost/static_assert.hpp>
 #include <boost/type_traits/cv_traits.hpp>
 #include <boost/type_traits/alignment_of.hpp>
 #include <cstring>
@@ -48,23 +39,32 @@ class value_initialized
       remove_const<T>::type data;
     };
 
-    mutable aligned_storage<sizeof(wrapper), alignment_of<wrapper>::value> x;
+    mutable
+#if !BOOST_WORKAROUND(__BORLANDC__, BOOST_TESTED_AT(0x592))
+      typename
+#endif 
+      aligned_storage<sizeof(wrapper), alignment_of<wrapper>::value>::type x;
+
+    wrapper * wrapper_address() const
+    {
+      return static_cast<wrapper *>( static_cast<void*>(&x));
+    }
 
   public :
 
     value_initialized()
     {
-      std::memset(x.address(), 0, sizeof(x));
+      std::memset(&x, 0, sizeof(x));
 #ifdef BOOST_MSVC
 #pragma warning(push)
 #if _MSC_VER >= 1310
 // When using MSVC 7.1 or higher, the following placement new expression may trigger warning C4345:
 // "behavior change: an object of POD type constructed with an initializer of the form ()
-// will be default-initialized".  There is no need to worry about this, though.
+// will be default-initialized".  It is safe to ignore this warning when using value_initialized.
 #pragma warning(disable: 4345)
 #endif
 #endif
-      new (x.address()) wrapper();
+      new (wrapper_address()) wrapper();
 #ifdef BOOST_MSVC
 #pragma warning(pop)
 #endif
@@ -72,25 +72,25 @@ class value_initialized
 
     value_initialized(value_initialized const & arg)
     {
-      new (x.address()) wrapper( *static_cast<wrapper const *>(arg.x.address()) );
+      new (wrapper_address()) wrapper( static_cast<wrapper const &>(*(arg.wrapper_address())));
     }
 
     value_initialized & operator=(value_initialized const & arg)
     {
-      T & this_data = this->data();
+      // Assignment is only allowed when T is non-const.
-      T const & arg_data = arg.data();
+      BOOST_STATIC_ASSERT( ! is_const<T>::value );
-      this_data = arg_data;
+      *wrapper_address() = static_cast<wrapper const &>(*(arg.wrapper_address()));
       return *this;
     }
 
     ~value_initialized()
     {
-      static_cast<wrapper *>(x.address())->wrapper::~wrapper();
+      wrapper_address()->wrapper::~wrapper();
     }
 
     T& data() const
     {
-      return static_cast<wrapper *>(x.address())->data;
+      return wrapper_address()->data;
     }
 
     operator T&() const { return this->data(); }
@@ -110,6 +110,20 @@ T& get ( value_initialized<T>& x )
   return x.data() ;
 }
 
+
+class initialized_value_t
+{
+  public :
+    
+    template <class T> operator T() const
+    {
+      return get( value_initialized<T>() );
+    }
+};
+
+initialized_value_t const initialized_value = {} ;
+
+
 } // namespace boost