Re: Alignment

From:

"Bo Persson" <bop@gmb.dk>

Newsgroups:

comp.lang.c++

Date:

Thu, 26 Feb 2009 20:50:28 +0100

Message-ID:

<70oa3pFgcc6hU1@mid.individual.net>

better_cs_now@yahoo.com wrote:

On Feb 26, 12:11 pm, Greg Herlihy <gre...@mac.com> wrote:

On Feb 26, 10:36 am, better_cs_...@yahoo.com wrote:

On Feb 26, 11:18 am, Gert-Jan de Vos <gert-

To force the unsigned char array to have the same alignment as
the Foo (which is my goal), I'd like to put these together in a
union. However, Foo has a non-trivial constructor, so it may not
be a member of a union. Do I have any othre options?

Allocate the char array dynamically:

char * fooArray = new char[sizeof(Foo)];

Now fooArray will be suitably aligned for a Foo type.

Greg

In addition to my prvious response a minute ago, your comment brings
up a side issue.

In my original post, I showed the Foo and the char array being
constructed off of the heap, and the response was that the
alignment I seek is not guaranteed. However, your response clearly
states that the alignment I seek *will* be guaranteed if they are
constructed on the heap. So, I'd like to reconfirm -- are alignment
issues indeed treated differently on the heap than off?

Thanks for the response!

No, the "trick" is the use of the global operator new,

void* operator new(std::size_t size) throw(std::bad_alloc);

which is required to allocate bytes "suitably aligned to represent any
object of that size".

Note that the operator doesn't know the type of the object, but must
still obey the rule. We don't know how!

Bo Persson