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### aligned_storage

    template <std::size_t Size, std::size_t Align>
    struct aligned_storage
    {
       typedef see-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.