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Subject: Re: [boost] [boost::endian] Request for comments/interest
From: Stewart, Robert (Robert.Stewart_at_[hidden])
Date: 2010-05-27 10:04:53

vicente.botet wrote:
> Rob Stewart wrote:
> > Terry Golubiewski wrote:
> >
> >> With Beman's approach there is no extra overhead if the
> >> native type == endian type.
> >
> > In many cases, write format is big endian and the host is
> > little, so that assertion is small comfort.
> What the OP intend to say is that when we convert from the
> same endian there is no extra overload.

I understood that. My point was that the swapping scenario is very common, so the overhead will also be common.

> But even this is not true, as Beman's implementation will do
> a copy as there is no in-place operations, as you bellow.

I didn't realize I was bellowing about anything and I wasn't the one to introduce the idea of in-place swapping. Nevertheless, copying built-in types is trivial and should lend itself readily to optimization in many cases, so the difference should be small, if not zero.

> >> Beman's approach does provide several operators for
> >> endian-swappable types, but I just don't use those
> >> operations, preferring to extract to native,
> >> perform operations, and then store the result.
> >
> > That the operations are provided for types that look and
> > act like built-ins suggests that the operations are just as
> > efficient. In the big/wire, little/host scenario, simple
> > looking expressions can be rife with inefficiency.
> > Expression templates could be employed to reduce those
> > inefficiencies, of course, but as you note, when one is going
> > to do much with the values, it is better to swap once and
> > then work with the natively ordered value.
> We can remove all the operations of the Beman's typed
> approach, so no risk to over use of. Without no operations,
> the Beman's classes must be used just to convert from some
> endiannes to the native and vice-versa, and then operate on
> the native types.

At that point, however, the functional approach seems more straightforward.

> > Tomas' and my APIs are geared toward the latter approach.
> > Beman's design tends to encourage the former. Nevertheless,
> > I understand that less demanding applications would benefit
> > from the safety of Beman's approach, so both are worthwhile.
> IMO the single case when your and Tomas approach can be more
> efficient is when you use in_place conversions, as you are
> able to do nothing when the endianness is the same. The other
> case, i.e., conversion from one variable to another, is
> better handled by Beman's approach, as it is type safe and is
> should not be less efficient than yours.

If you remove the operators from Beman's types, then its a question of syntax and, possibly, safety. However, I don't see any type safety problems in my original or hybrid APIs.

Rob Stewart robert.stewart_at_[hidden]
Software Engineer, Core Software using std::disclaimer;
Susquehanna International Group, LLP

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