Re: [boost] [serialization] Enhancing Archives with Archve Adaptors

From: Ian McCulloch (ianmcc_at_[hidden])
Date: 2005-11-27 11:10:14

Robert Ramey wrote:

> Ian McCulloch wrote:
>>  Is the bitwise_?archive_adaptor
>> supposed to be useful for anything other than a native binary
>> archive?
> 
> This is an alternative way of implementing the save_binary
> optimization.  Application of this adaptor to any existing archive class
> will create a new archive class that includes the save_binary
> optimization.
> So it could be applied to say a, text_oarchive.  But of course
> it would make no sense to do this as it would result in a slower
> archive (in text archives, save_binary renders binary output
> as base64 text).  Also, saving data as bitwise binary maintains
> the representation of the host machine so the application
> of this adaptor would result in an archive which would
> be non-portable accross machines.

Ok.

Would it be correct to describe the adaptor's function as overriding
serialization for a particular type (or set of types) and serializing it in
a different format, using facilities already available to the archive(s) ? 

It is clear that bitwise_serializable_adator, even when applied to a native
binary archive will, in general, produce an archive that is incompatible
with the base archive (say, if the user marks some POD type as
bitwise_serializable, and the POD type contains some padding).  Thus, I
agree this belongs firmly in the class 'adaptor' - the resulting archive is
distinct from the base archive.  I can even imagine that there are some
interesting uses for it.  So I apologise for some of the language I used in
my previous post, it was out of line.

As far as I can tell, what distinguishes the 'adaptor' case from the array
extensions that have been discussed at length, is

1. In the default case the save_array() function (**) reproduces the
existing behaviour.  That is, an array_adaptor<Base> would produce a
bit-for-bit identical archive to using the Base archive type itself, for
all Base archives that currently exist in the serialization library.

2. In the non-trivial case where save_array() does something different to
the default, it needs to invoke functionality that does _not_ already exist
in the serialization library, and is _archive_ _specific_.

Both of these points strongly suggest, to me at least, that the save_array()
extension is not properly an adaptor.  Do you have a different
interpretation?

(**) Just for the record, I agree with Peter Dimov that something like
save_sequence(Iterator, size) is better.  This is needed for archives that
directly support arrays as a distinct structure, if you want to serialize
say a deque using the archive array format.

Regards,
Ian