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Subject: Re: [boost] [boost::endian] Summary of discussion #1
From: Stewart, Robert (Robert.Stewart_at_[hidden])
Date: 2010-06-03 13:59:51

Scott McMurray wrote:
> On 3 June 2010 12:30, Terry Golubiewski <tjgolubi_at_[hidden]> wrote:
> > Tom wrote:
> >>
> >> endian::cast<big_to_host>() //explicit direction
> >> endian::cast<from_litle>() //"to_host" is implicit
> >> endian::cast<to_big>() //"from_host" is implicit
> >
> > Is it imporant that these names are templated? What are
> > the advantages of this templated-tagged design over..
> I also find it odd that the direction is mixed in with the endianness.

We discussed this previously. I had suggested to<>() and from<>() as well as endian_cast<>. Tomas noted the symmetry of the cast notation for all three cases, but that necessarily requires the direction in the template argument.

> I'd expect something like to_host_endian and from_host_endian, with a
> policy type for the endianness.

to_host_endian<little>() just barely implies conversion from little endian to host endianness to my mind. endian_cast<from_little>() is clearer to me. from<little>() is perfectly clear, but doesn't express that a copy is returned (versus in-place reordering) while the new-style cast syntax more readily implies it.

> The only advantage I see of the explicit one is that it theoretically
> allows cast<big_to_little>, but that would mean defining a number of
> types quadratic in the number of endiannesses. It clearly has no
> speed benefit over to_host then from_host on typical machines. I'm
> also not certain that it's even a useful operation.

As discussed previously, there are occasions when the reordering is for translation without the need to modify or inspect the data in the process. For those cases, going from a specific endianness to a specific endianness is appropriate. I don't imagine those uses are frequent, but it is rather nice that they can use the same syntax as the others.

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

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