123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354 |
- [/
- 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.
- ]
- [endsect]
|