Re: [boost] [integer] Promotion of endian library code from vault

From: Roland Schwarz (roland.schwarz_at_[hidden])
Date: 2008-05-23 08:19:32

dizzy wrote:
> On Friday 23 May 2008 14:05:56 Roland Schwarz wrote:
>>    one cannot predict the alignment of the members. (Or am I wrong
>>    in this respect?)
> 
> Correct, which is why protocol binary structures are never mapped directly in 
> memory (you can with some compiler extensions but you won't gain anything 
> since I/O is the bottleneck in such cases and not memory copy). Instead a 
> serialization aproach should solve such issues.

It seems I was wrong with my assumptions about endian.hpp:

The proposed lib actually does have an unaligned type, i.e. it
actually maps the types to char bytes[..] arrays. So the question
remains what the standard has to say about alignment of

struct foo {
   char bytes_1 [3];
   char bytes_2 [2];
   char bytes_3 [1];
   char bytes_4 [4];
};

Will such a struct be equivalent to
char bytes[3+2+1+4] ?

Hmm, and foo isn't necessarily a POD. Writing this out with
binary write... what is guaranteed? I fear not much.

> That kinda looks like reinventing boost.serialization although with a 
> different API (I did something similar in my code).

Indeed there are similarities. But there are differences as well.

1) I wanted a way to control layout on a per struct basis. I wanted
   to be able to go as low as bit position.

2) boost.serialization is fine when I want to make my in memory
   classes persistent, but it is of little help for the decoding
   of binary protocol packages. ( I do not claim it is not possible).

3) boost.serialization solves two orthogonal problems with a single
   implementation. One problem is mapping native types to portable
   types (partial overlap with standard lib << operators), the other
   is writing out a tree of objects (serialization). I was aiming
   only at the first problem, and so does endian.hpp I guess.
   Perhaps serialization could be refactored to allow more
   control over the layout within the serialize functions?

-- 
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