[Boost-commit] svn:boost r63311 - sandbox/variadic_templates/boost/composite_storage/alignment

Subject: [Boost-commit] svn:boost r63311 - sandbox/variadic_templates/boost/composite_storage/alignment
From: cppljevans_at_[hidden]
Date: 2010-06-25 12:43:02

Author: cppljevans
Date: 2010-06-25 12:43:01 EDT (Fri, 25 Jun 2010)
New Revision: 63311
URL: http://svn.boost.org/trac/boost/changeset/63311

Log:
alignment metaclasses
Added:
   sandbox/variadic_templates/boost/composite_storage/alignment/
   sandbox/variadic_templates/boost/composite_storage/alignment/aligned_offset.hpp   (contents, props changed)
   sandbox/variadic_templates/boost/composite_storage/alignment/compose.hpp   (contents, props changed)

Added: sandbox/variadic_templates/boost/composite_storage/alignment/aligned_offset.hpp
==============================================================================
--- (empty file)
+++ sandbox/variadic_templates/boost/composite_storage/alignment/aligned_offset.hpp	2010-06-25 12:43:01 EDT (Fri, 25 Jun 2010)
@@ -0,0 +1,51 @@
+//compile-time aligned offset.
+#ifndef BOOST_COMPOSITE_STORAGE_ALIGNMENT_ALIGNED_OFFSET_HPP_INCLUDED
+#define BOOST_COMPOSITE_STORAGE_ALIGNMENT_ALIGNED_OFFSET_HPP_INCLUDED
+
+namespace boost
+{
+namespace composite_storage
+{
+namespace alignment
+{
+  
+  template
+  < std::size_t Offset //minimum offset from start of some composite, C,
+  , std::size_t Alignment//for a component, Ci, whose alignment is Alignment.
+  >
+  struct
+aligned_offset
+{
+        static
+      std::size_t const
+    remainder
+    = Offset%Alignment
+    ;
+        static
+      std::size_t const
+    value
+    = remainder == 0
+    ? Offset
+    : Offset+(Alignment-remainder)
+    /**@brief
+     * value is minimum value > Offset such that
+     * value%Alignment == 0.
+     *
+     *Why is this useful?
+     * If:
+     *   1) the composite, C, is located in memory at Offset0
+     * and:
+     *   2) Offset0%Alignment == 0,
+     * then:
+     *   (Offset0+value)%Alignment == 0, 
+     * IOW, Ci will have the required alignment, Alignment.
+     *
+     */
+    ;
+    
+};
+
+}//exit alignment namespace
+}//exit composite_storage namespace
+}//exit boost namespace
+#endif

Added: sandbox/variadic_templates/boost/composite_storage/alignment/compose.hpp
==============================================================================
--- (empty file)
+++ sandbox/variadic_templates/boost/composite_storage/alignment/compose.hpp	2010-06-25 12:43:01 EDT (Fri, 25 Jun 2010)
@@ -0,0 +1,73 @@
+//compile-time composition of alignments
+#ifndef BOOST_COMPOSITE_STORAGE_ALIGNMENT_COMPOSE_HPP_INCLUDED
+#define BOOST_COMPOSITE_STORAGE_ALIGNMENT_COMPOSE_HPP_INCLUDED
+
+#ifdef COMPOSE_ALIGNMENTS_WITH_LCM
+  #include <boost/math/common_factor_ct.hpp>
+#endif
+
+namespace boost
+{
+namespace composite_storage
+{
+namespace alignment
+{
+  template
+  < std::size_t Left
+  , std::size_t Right
+  >
+  struct
+compose
+/**@brief
+ *  "Metafunction" returning alignment which is compatible with the 
+ *   metafunction argument alignments, Left and Right.
+ *
+ *  The real reason why this template was created instead of directly using
+ *  static_lcm was to provide the documentation shown below which justifies
+ *  static_lcm use.
+ */
+{
+        static
+      std::size_t const
+    value=
+    #ifdef COMPOSE_ALIGNMENTS_WITH_LCM
+      ::boost::math::static_lcm
+      < (unsigned long)Left
+      , (unsigned long)Right
+      >::value
+      //The use of static_lcm is based on the statement:
+      //
+      //  N is the least common multiple of all Alignments
+      //
+      //in paragraph 1 on page 10 of:
+      //
+      //  http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2007/n2165.pdf
+      //
+      //AFAICT, the reason lcm is used instead of just taking the
+      //maximum:
+      //
+      //  Left>Right?Left:Right
+      //
+      //is to account for "Extended alignments" mentioned in paragraph 3 
+      //on page 3 of the n2165 reference mentioned above.  OTOH, if only
+      //"Fundamental alignments" are used (which, I assume, are the
+      //"static alignments;powers of 2" in paragraph 4 on page 5 of:
+      //
+      //  http://www.open-std.org/JTC1/SC22/WG21/docs/papers/2006/n2140.pdf
+      //
+      //) then simply using max instead of lcm would work.
+    #else
+      Left>Right?Left:Right 
+      //2010-03-11:
+      //  this is what boost::variant does currently, 
+      //  although there's a pending fix:
+      //    https://svn.boost.org/trac/boost/ticket/993
+      //
+    #endif
+    ;
+};
+  
+}//exit alignment namespace
+}//exit composite_storage namespace
+}//exit boost namespace
+#endif