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From: "REH" <bogus@nowhere.net>
Newsgroups: comp.lang.ada,comp.lang.c++,comp.realtime,comp.software-eng
Subject: Re: [OT] Re: Teaching new tricks to an old dog (C++ -->Ada)
Date: Mon, 28 Mar 2005 11:43:43 -0500
Organization: Earth
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Date: 2005-03-28T11:43:43-05:00
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"Ioannis Vranos" <ivr@remove.this.grad.com> wrote in message
news:1112026376.920221@athnrd02...
> REH wrote:
>
> > These will mostly fail on systems with alignment constraints (SPARC,
PPC,
> > etc.).
>
> Why?
>
>
> -- 

Well, suppose your class contains a variable of type double, which on some
systems is 8 bytes and requires 8 byte alignment.  The array you are
defining to hold the class is a character array, which only requires 1 byte
alignment.  If it is created with a 1 byte alignment, when you create the
class in it using placement new, now the double is misaligned.  When you
access it nasty things can happen.  Intel processors can handled misaligned
variables, but take extra clock cycles to do so.  On a SPARC or PowerPC, you
will throw a hardware exception (segmentation fault or machine check most
likely).  A way around this problem is to either 1) make an array of
doubles, 2) union the array with a double, or 3) allocate from the heap.

REH