"Philippe A. Bouchard" <[EMAIL PROTECTED]> writes:
> David Abrahams wrote:
>> "Philippe A. Bouchard" <[EMAIL PROTECTED]> writes:
>>
>>> Hi again,
>>>
>>> I am doing some helper class that will pad the space after a
>>> given type until it reaches a machine word boundary. This word
>>> boundary would be the maximum of (alignment_of<T>::value -
>>> sizeof(char)) where T is any 'typename', 'typename' is one of
>>> {is_class<U>::value == false} and U is any type.
>>
>> Even if what you wrote made sense, which I'm not sure it does,
>>
>> [to me at least - how can a word boundary be a number (max over
>> all types T of alignment_of<T> - sizeof(char)), rather than an
>> address? And then why subtract sizeof(char), i.e. 1, from the
>> maximal alignment?]
>
> Forget the [- sizeof(char)] it is erroneous, what I meant was something
> trivial [/ sizeof(char)].
OK, so what you really want is alignment_of<boost::detail::max_align>.
>> that algorithm relies on a big non-portable assumption, doesn't it?
>> Even if you enumerated all non-class types, there's no reason to think
>> that the maximal alignment has any relationship to a machine word.
>
> It is purely theoretical. max(alignment<T>) should reach its maximum
> somehow, which I cannot yet proove. STL has simplified this with a simple
> constant set to 8.
By "STL" I presume you mean your particular STL implementation.
>> Since I can't figure out what any of this is actually supposed to
>> mean, I guess I can't help with that.
>
> It consist of determining the number of bytes required to reach the next
> word boundary.
>
> Ex.:
>
> - The processor aligns each character to 8 bits.
> - The processor aligns each integer to 32 bits.
>
> 0x8000 character
> 0x8001 padding
> 0x8002 padding
> 0x8003 padding
> 0x8004 integer
> 0x8005 integer
> 0x8006 integer
> 0x8007 integer
template <class T>
struct padding_of
{
BOOST_STATIC_CONSTANT(
std::size_t, value = alignment_of<max_align>::value
- sizeof(T) % alignment_of<T>::value
);
};
or, if you don't trust max_align, then:
template <class T>
struct padding_of
{
BOOST_STATIC_CONSTANT(
std::size_t, value = alignment_of<max_align>::value
% (sizeof(T) % alignment_of<T>::value)
);
};
I guess.
HTH,
--
Dave Abrahams
Boost Consulting
www.boost-consulting.com
_______________________________________________
Unsubscribe & other changes: http://lists.boost.org/mailman/listinfo.cgi/boost
