Re: A portable way of pointer alignment

From:

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

Newsgroups:

comp.lang.c++

Date:

Sun, 10 Jan 2010 08:06:16 +0100

Message-ID:

<hibu9e$9qd$1@news.eternal-september.org>

* Pavel:

Is there a portable C++-ish way to get a properly aligned pointer to an
object of given type from given arbitrary pointer-to-char? More
precisely, how to implement a template function with this prototype:

template <typename T>
T *
align(char *buf, size_t bufSize);

whose input is a memory buffer and output is either the lowest position
in the buffer at which one can placement-new an object of type T or a
null pointer if it is impossible?

It depends what you mean by portable.

If you mean "formally portable" then you can just wait a year or two or three
until C++0x, and then you have alignment as part of the standard library.

Until then I'm pretty sure that you'll find this functionality in the Boost
library (disclaimer 1: haven't checked).

But if you absolutely insist on doing it yourself, and the buffer does not stem
from an ordinary 'new' (which produces a pointer suitably aligned for any
ordinary type), and you're happy with a little formal UB, then you might try
something like

<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>

(disclaimer 2: not tested! :-) )

Cheers & hth.,

- Alf

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