Re: A portable way of pointer alignment

From:

"Alf P. Steinbach" <alfps@start.no>

Newsgroups:

comp.lang.c++

Date:

Fri, 26 Feb 2010 00:53:54 +0100

Message-ID:

<hm72il$5eb$1@news.eternal-september.org>

* s_tec:

On Feb 24, 4:23 pm, "Chris M. Thomasson" <n...@spam.invalid> wrote:

FWIW, check out this simple macro:
_________________________________________________
#define ALIGN_OF(mp_type) \
    offsetof( \
        struct \
        { \
              char pad_ALIGN_OF; \
              mp_type type_ALIGN_OF; \
        }, \
        type_ALIGN_OF \
    )
_________________________________________________

This is really helpful. My code already uses the same basic technique,
but I did not realize there was a way to pack it into a single-line
macro like this. That will really simplify some stuff, so thanks!

I posted essentially the same solution but as a C++ function template and with
an additional alignment helper function as the very first response in this thread.

Then instead of ALIGN_OF( Type ) you write alignmentOf<Type>()...

Reposting that:

<code>
typedef ptrdiff_t Size;

template< typename T >
inline Size alignmentOf()
{
     struct X
     {
         char bah;
         T t;
     };

     return offsetof( X, t );
}

template <typename T>
T* align( char* const buf, Size const bufSize)
{
     Size const i = reinterpret_cast<Size>( buf );
     Size const j = i + alignmentOf<T>() - 1;
     Size const aligned = j - j % alignmentOf<T>();

     return (aligned - i >= bufSize - sizeof(T)? 0 : reinterpret_cast<T*>(
aligned ) );
}
</code>

Then I added a disclaimer about "not tested", but it was discussed and as I
recall it was OK.

Cheers,

- Alf