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: 2010-06-02 09:48:53

vicente.botet wrote:
> Rob Stewart wrote:
> > Terry Golubiewski wrote:
> >> Assuming you just read a TCP stream of big-endian T's into
> >> memory at a location 'char* buf', then you could endian-
> >> convert the T's, if necessary, like this...
> >>
> >> const endian<big, T>* src = reinterpret_cast<const
> >> endian<big, T>*>(buf);
> >> endian<native, T>* dst = reinterpret_cast<endian<native, T>*>(buf);
> >>
> >> (void) copy(src, src+numTs, dst);
> >
> > A call to swap_in_place() avoids the ugly reinterpret_casts
> > and looks far simpler.
>
> You need a cast to see a void* as a T*. This was already the
> case with Tom code:
>
> >void * data = ...;
> >read(fh, data, size);
> >MatrixHeader * mh = static_cast<MatrixHeader*>(data);

Quite right, but a static_cast suffices, it is to a simpler type, and there is only one.

> > Thus far, I'm not convinced that there is any value in the
> > endian types, but keep trying!
>
> If I'm not wrong, swap_in_place can be defined on top of the
> endian types as follows
>
> template <typename E, typename T>
> swap_in_place(T* data) {
>     const endian<E, T>* src = reinterpret_cast<const endian<E
> T>*>(data);
>     endian<native, T>* dst = reinterpret_cast<endian<native,
> T>*>(data);
>     copy(src, src+numTs, dst);
> }
>
> used as follows
>
> swap_in_place<big>(data);
>
> This swap_in_place is a no-op if native is big.

I'll accept that the casts are zero cost in this case, and copy() can be written better than Terry showed previously, but the extra complexity may thwart optimization, so performance testing or an examination of emitted object code would be necessary to verify your assertion.

_____
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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