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Subject: Re: [boost] [endian] swap_in_place use case
From: Terry Golubiewski (tjgolubi_at_[hidden])
Date: 2010-06-04 15:24:25

> Tom wrote:
>> Terry wrote
>> If you're only going to access each element of the file, then converting
>> endian on access should be faster than swapping in place and then
>> processing, without needing double the memory.
>> Could you please give a specific example (preferably with code) that I
>> could
>> play with?
> I was under the impression I already provided you with a use case in an
> earlier
> thread.

I'm looking for a specific application, (perhaps a specific video
compression algorithm), that I can try.
My previous test program read a large disk file, as you proposed. Now, I
need something

> Converting to endian on access certainly won't be faster than
> swap_in_place, as
> you cannot have a "zero cost" version. Additionally, in our scenarios the
> data
> gets accessed multiple times.

What is an example of one of these scenarios?

> But just to re-iterate:
> 1) swap_in_place<>() can be zero cost in the case of no swapping, which
> cannot
> be said for the copying approach. I.e. it has a much better best case
> behaviour
> and the same cost for the worst case behaviour.
> 2) It is one of the safer ways of handling floating point numbers.

I disagree. I have demonstrated that...

1) endian-on-access can be zero cost in the native-case.
2) swapping in place is not any safer because the C++ type system cannot
help to determine which portions of an object have been swapped, nor
document the endian properties of a data structure. endian<big, double>
can, and should, be defined to provided portable floating-point transfer and
persistent storage.


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