Re: [boost] [boost::endian] Request for comments/interest

Subject: Re: [boost] [boost::endian] Request for comments/interest
From: Stewart, Robert (Robert.Stewart_at_[hidden])
Date: 2010-06-01 08:38:36

Terry Golubiewski wrote:
> Tomas Puverle wrote:
>
> > I definitely see the elegance of this code and as I said
> > before, I am not opposed to implementing the typed
> > interface.  Having said that, I do have several concerns
> > with how this gets actually read from/sent to a device
> > and additionally, as mentioned in another post, I think
> > this "neat" code may be a little opaque for someone not
> > familiar with it.
>
> I think the "opaqueness" is a major feature of this approach.

Opacity is good when you want high level abstract behavior, but shouldn't preclude the (potentially) higher efficiency low level API.

>  The user of the higher-level classes has no idea about the
> underlying endianness of the message fields or if they've
> been byte-swapped yet or not.

Either the typed endian values store an extra bit to determine whether the external to host swapping has already occurred, in which case they cannot be written out as is, or the swapping must occur *each time* the value is read.  I presume the latter to be the case.  When the external and host order match, this isn't a big deal, but imagine code that ignorantly loops over such objects repeatedly reading values when the external and host don't match.  The overhead can be significant.  That's where the opacity can be problematic and the user of such a library must be warned clearly about this potential.

> It just works.

Yes, but it just works inefficiently in some uses cases, unless I missed something.

> And if the systems engineers later decided to change some of
> the endianness of some of the message fields, the message
> definition code would change, but none of the code that uses
> the messages would have to change at all.

I see the value of that, but presuming that the code that reads such a message from the external device is encapsulated, the reading and swapping likely is done together once per message, so changes to endianness are localized without the special types.  There is danger of writing code in which reading and swapping is not localized, so it would be appropriate to alter users to the value of doing so.

_____
Rob Stewart                           robert.stewart_at_[hidden]
Software Engineer, Core Software      using std::disclaimer;
Susquehanna International Group, LLP  http://www.sig.com

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