From: Martin Bonner (martin.bonner_at_[hidden])
Date: 2005-08-11 08:22:35
From: christopher diggins [mailto:cdiggins_at_[hidden]]
Sent: 11 August 2005 14:00
Subject: Re: [boost] alignment problem in proposed any alternative
> 3.9(2) says:
> For any complete POD object type T, whether or not the object holds a
> valid value of type T, the underlying
> bytes (1.7) making up the object can be copied into an array of char
> or unsigned char.36) If the content of the array of char or unsigned
> char is copied back into the object, the object shall subsequently
> hold its original value.
> This has nothing to do with void* and char*, but rather with all
> This clearly implies char arrays are stringently aligned.
Err, no it doesn't. It says that you can store the bits of a POD object
into a char array, and then load the bits back into the same (correctly
aligned) POD object.
It does *NOT* say that you can use the address of the char array as if it
were the address of the POD object.
long pod = 3141L;
memcpy( c, &pod, sizeof(pod) );
pod = 42L;
memcpy( &pod, c, sizeof(pod) );
assert (pod == 3141L); // 3.9(2) requires this assert to pass.
// nothing requires THIS assert to pass, and on some systems it fail
// because c is not suitable aligned.
assert ( *reinterpret_cast<long*>(&c) == 3141L );
-- Martin Bonner Martin.Bonner_at_[hidden] Pi Technology, Milton Hall, Ely Road, Milton, Cambridge, CB4 6WZ, ENGLAND Tel: +44 (0)1223 441434
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