boost/libs/type_traits/doc/aligned_storage.qbk

[/ 
  Copyright 2007 John Maddock.
  Distributed under the Boost Software License, Version 1.0.
  (See accompanying file LICENSE_1_0.txt or copy at
  http://www.boost.org/LICENSE_1_0.txt).
]

[section:aligned_storage aligned_storage]

   template <std::size_t Size, std::size_t Align>
   struct aligned_storage
   {
      typedef __below type;
   };
  
__type a built-in or POD type with size `Size` and an alignment 
that is a multiple of `Align`.  

__header ` #include <boost/type_traits/aligned_storage.hpp>` or ` #include <boost/type_traits.hpp>`

On the GCC and Visual C++ compilers (or compilers that are compatible with them), we support
requests for types with alignments greater than any built in type (up to 128-bit alignment).
Visual C++ users should note that such "extended" types can not be passed down the stack as
by-value function arguments.

[important
Visual C++ users should be aware that MSVC has an elastic definition of alignment, for
example consider the following code:

``
   typedef boost::aligned_storage<8,8>::type align_t;
   assert(boost::alignment_of<align_t>::value % 8 == 0);
   align_t a;
   assert(((std::uintptr_t)&a % 8) == 0);
   char c = 0;
   align_t a1;
   assert(((std::uintptr_t)&a1 % 8) == 0);
``

In this code the final assert will fail for a 32-bit build because variable `a1` is not
aligned on an 8-byte boundary.  Had we used the MSVC intrinsic `__alignof` in
place of `alignment_of` or `std::aligned_storage` in place of `boost::aligned_storage`
the result would have been no different.  In MSVC alignment requirements/promises only
really apply to variables on the heap, not on the stack.

Further, although MSVC has a mechanism for generating new types with arbitrary alignment 
requirements, such types cannot be passed as function arguments on the program stack.
Therefore had `boost::aligned_storage<8,8>::type` been a type declared with
`__declspec(align(8))` we would break a great deal of existing code that relies on
being able to pass such types through the program stack.
]

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