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From: vicente.botet (vicente.botet_at_[hidden])
Date: 2008-09-01 12:24:43


>From wikipedia
"a frame is a data packet of fixed or variable length which has been encoded
by a data link layer communications protocol for digital transmission over a
node-to-node link. Each frame consists of a header frame synchronization and
perhaps bit synchronization, payload (useful information, or a packet at
higher protocol layer) and trailer. Examples are Ethernet frames and
Point-to-point protocol (PPP) frames."

Is there any interest in a frame library which will be based on an extension
of the archive concept. The saving archive concept allows to save
serializable data at the end of the archive, and the loading archive concept
allows to read serializable data from the begining of the archive. The
saving frame concept will allows to save serializable data either at the end
or the *begin* of the frame, and the loading frame concept allows to read
serializable data from the begining or the *end* of the archive.

I'm not sure which syntax will be the more adequated. The serialization
library use the <<, >>, and & operators that are associative from left to
right. We need the equivalent operators from right to left. <<=, >>=, and &=
seams to be the more adequated candidates but I don't know if it is correct
in C++ to define this operators with this prototype

template <typename T> frame& operator<<=(const T&, frame&);
template <typename T> frame& operator>>=(const T&, frame&);
template <typename T> frame& operator&=(const T&, frame&);

h1 >>= h2 >>= sf << t2 << t1
(h1 >>= (h2 >>= ((sf << t2) << t1)))

sould be equivalent to

sa & h1 & h2 & t2 & t1

if sf and sa were empty.

The main difference is that we can do it hierarchically. The top layer will
create a frame, and serialize its own information elements.

frame sf;
h_n >>= sf << p_n << t_n;

Then this top layer will use a primitive of the lower level having as
parameter the frame as payload.

A primitive at the k level will add its own header and trailer information
element to the payload of the upper level

void primitive_k(frame& sf) {
    // ...
    h_k >>= sf << t_k;
    // ...
    // ...

So the frame allows to serialize top-down. With the archive concept the
serialization must be done bottom-up, needing to chain each one of the
information element in a list and only when we have all off them we can
start the serialization.
I think that the frame approach should be more efficient because it avoid to
store in dynamic memory the information elements to be serialized, instead
they are serialized directly.

Loading a frame works as loading an archive, except that we can load also at
the end of the frame. This avoids to load the complete archive to load the
trailer of the lower levels.

lf >>= h_1;
// ...
t_1 << lf;

In addition, it would be greate to have saving/loading frames (I'm not sure
but I think that an archive can not be saving and loading at the same time).
The same frame can be used as loading, analyzing only some lower levels, and
as saving in order to construct other lower levels. This will be very useful
for gateways and routers.

| L4 |<----------------------------->| L4 |
| L3 | | ADAPTATION | | L3 |
| L2 |<-->| L2 | | X2 | <--> | X2 |
| L1 |<-->| L1 | | X1 | <--> | X1 |

It would be greate that the same data that can be serialized on archives,
could be serializable on a frame using the same save function. But this
works only when we save at the end of the frame. Let me see this using a
little example:

Class C has 3 information elements to serialize (a, b and c). So the save
functions could be something like

template <typename ARCHIVE>
void C::save(ARCHIVE& sa) {
    sa & a & b & c;

This save function works well from left to right, but can not be used when
saving at the begining of a frame, because the expected result when saving
at the begining is

| a | b | c | sa |

but the result will be

| c | b | a | sa |

So unhappylly we need a different save function

template <typename FRAME>
void C::save(FRAME& sa, begin_tag&) {
    a >>= b >>= c >>= sa;
    // a >>= (b >>= (c >>= sa));

Please let me know if you are interested in such a library, and what are
your requirements.

Robert, do you think that this can be seen as an extension of the
serialization library?

Best regards,

Vicente Juan Botet Escriba

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