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Subject: Re: [boost] [endian] swap_in_place use case
From: Terry Golubiewski (tjgolubi_at_[hidden])
Date: 2010-06-03 14:57:39

Vicente Botet wrote:

I'm looking for a use case where the use of swap_in_place (or whatever is
renamed) is not followed/preceded by a copy.

When I need to send a message I need to copy the information to send in a
buffer. I gues that in this case the use of swap (copy) is the function to

Can somene help me to identify a real use case on which the swap_in_place
can be used to send a message that do not need to be initialized before

When I receive a message, I will need to store some of the messages
information on application context, store them on local variables or pass
them to other functions. All this suppose a copy from the buffer to the
different kinds of native storage. I gues that swap (copy) should be used
in this case also.

The example of Terry reveives a array of ints and makes a swap_in_place, but
do nothing with. Someone has proposed to sum all these ints, but in this
case, we don't need to swap_in_place and the count. With swap approach, we
can count at the same time as we do the conversion, so no need to swap in

Can somene help me to identify a real use case on which the swap_in_place
can be used to reveive a message, without needing to do any additional copy
from the swapped buffer to native data?

Terry replied:

Yes. Image/video decompression. In these cases, each video frame is stored
on a physical medium or is sent streaming across a network. The image is
usually represented as a matrix of "coefficients" that are in either big- or
little-endian format. During the decompression, the coefficients may be
read several times, so it is beneficial to store these coefficients in
native format in memory. Doing computation in place and transforming data
is also quite common in signal-processing, e.g. the Fourier transform.
Doing computation in place also provide a performance advantage because of
memory caching. That said though, such high-performance applications
usually involve (for me) exploitation of the underlying hardware and
processor instruction set for the performance critical parts, and therefore,
don't fall in the realm of generic C++ implementation.

But... these applications usually also eventually involve some form or
error-correction and/or compression close the the communication/storage
layer. Once that happens, then the data is necessarily copied to another
format. Frequently, these operations access blocks of the input stream and
cannot be done in place (an in-place decompression algorithm doesn't make
much sense). Once you need to do the copy, the advantage of
in-place-conversion is lost.


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