[boost] Re: padding_of<> proposal

From: Philippe A. Bouchard (philippeb_at_[hidden])
Date: 2003-02-11 10:07:30

David Abrahams wrote:

[...]

> 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?

It is the result of arithmetics between sizeof(char)s, which is of type
size_t.  It can be converted back to a char *.

>       And then why subtract sizeof(char), i.e. 1, from the
>       maximal alignment?]
>
> that algorithm relies on a big non-portable assumption, doesn't it?

Yes, but simple examples are faster to understand.

> 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.

No exactly, but it will be a factor of the word boundary (wb = 4,
data_alignment = 8; 2 * 4 = 8).

[...]

Example is more consistant here:

- The processor aligns each character to 8 bits.
- The processor aligns each integer to 32 bits.
- BOOT_DATA_ALIGNMENT is set to 8

0x8000    character
0x8001    padding
0x8002    padding
0x8003    padding
0x8004    padding
0x8005    padding
0x8006    padding
0x8007    padding
0x8008    integer
0x800A    integer
0x800B    integer
0x800C    integer

In this case: padding_of<char> will be equal to 7.

Philippe A. Bouchard