From: Hans-Bernhard Broeker <broeker AT physik DOT rwth-aachen DOT de>
Newsgroups: alt.comp.lang.learn.c-c++,comp.lang.c++,comp.os.msdos.djgpp
Subject: Re: Holes in structure
Followup-To: alt.comp.lang.learn.c-c++
Date: 6 Nov 2001 13:20:13 GMT
Organization: Aachen University of Technology (RWTH)
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[Crossposted without a proper F'up2 --- I restricted it to one group
as it should be]

In comp.os.msdos.djgpp Alex Vinokur <alexvn AT bigfoot DOT com> wrote:

> Here is some program that detects holes in structure.

A rather useless exercise, that is.  You don't need to know or detect
this kind of information.  If you think you have to, you should
re-think the arguments that brought you to this conlusion.  At least
one of them is flawed.

> Its output is not perfectly clear to me.

[...]
> Questions.
>  1. struct type0 (empty) :
>            Why is 'sizeof' = 1?

Why not?  An empty struct is a completely useless thing --- what on
earth would you care about its size for?

>  2. struct type1 :
>            Why is not 'Diff' (between 'sizeof' and 'All items sum')
>   equal 'All holes sum' ?

The thing you're investigating here is not called "hole"s, that's why.
It's called "padding", and that included possible padding at the *end*
of a structure, after the last element. That's exactcly where the
remaining three bytes of "holes" are, in this particular case.

>  3. struct type0 and type1 :
>            Why is not ('Diff' - 'All holes sum') the same value for
> type0 and type1.
>            Note. type0  : 'Diff' - 'All holes sum' = 1 - 0 = 1
>                  type1  : 'Diff' - 'All holes sum' = 9 - 6 = 3

Why should they be the same?


-- 
Hans-Bernhard Broeker (broeker AT physik DOT rwth-aachen DOT de)
Even if all the snow were burnt, ashes would remain.