Re: [Boost-users] [Serialization] How to restore ref-counted strings

From: Robert Ramey (ramey_at_[hidden])
Date: 2007-03-09 23:29:02

Bill Lear wrote:
> On Friday, March 9, 2007 at 11:47:21 (-0800) Robert Ramey writes:
>>
>> Second I would make this a "helper class" .  Your
>> special "Lear Archive" would be derived from
>> an existing archive and the helper class using
>> multiple inheritance.  Look at the implemenation
>> of shared_ptr checked into the HEADof CVS
>> to see how to do this. ...
>
> I checked out the head of CVS, but did not see anything relating
> to a helper class in shared_ptr.hpp.  Not sure if I'm looking
> in the right place.  This is how I checked it out:
>
> % cvs -z3
> -d:pserver:anonymous_at_[hidden]:/cvsroot/boost
> checkout boost

Its in boost/archive/binary_iarchive.hpp

You don't have to check out, you can always use the web
browser interface to browser though the library.  Its much
better in that it gives a "3 dimensional" view so you can
see changes that have been made and differences between
versions.

The relevant section looks like:

   65 class binary_iarchive :
   66     public binary_iarchive_impl<
   67         boost::archive::binary_iarchive,
   68         std::istream::char_type,
   69         std::istream::traits_type
   70     >,
   71     public detail::shared_ptr_helper
   72 {
   73 public:
 ...

The shared_ptr_helper creates and destroy
an extra tracking object that shared_ptr
requires.  This is similar to your case.

> I'm not sure I need to use a cache of strings when saving, in fact,
> I think not, but I went ahead and more-or-less followed your
> instructions.

lol - well good luck with that!!!. Actually, now that I think
about it.  I guess we only need the cache of strings on input.

>
> I can't seem to get this to compile, but it's getting closer.
>
> The complete code is below my sig.
>
> The compiler (gcc) complains:
>
> error: 'class boost::archive::text_iarchive' has no member named
> 'cache'
> error: 'class boost::archive::text_oarchive' has no member named
> 'cache'

try changing
 > class ll_text_iarchive: public text_iarchive_impl<text_iarchive>,
>                        public ll_cache {
> public:
>    ll_text_iarchive(std::istream& is, unsigned int flags = 0)
>        : text_iarchive_impl<text_iarchive>(is, flags) {}
> };
>

to

> class ll_text_iarchive: public text_iarchive_impl<ll_text_iarchive>,
>                        public ll_cache {
> public:
>    ll_text_iarchive(std::istream& is, unsigned int flags = 0)
>        : text_iarchive_impl<ll_text_iarchive>(is, flags) {}
> };

Note that the argumetn to text_iarchive_impl should be the MOST
DERIVED class

> Also, for some reason, I can't use this idiom:
>
>    ar << ll_string(s);
>
> In fact, this won't work, either:
>
>   ar << string("foo");

My guess is that is the is the "const" trap which using is_wrapper
will get around.  But this only works in 1.35.  So for now do
one of the following:

use & instead of <<

or use a const_cast which will be somewhat complex.

The "const" trap is also explained in the documentation under
the section "rationale"

>
> Finally, I really have no idea how to use the reset_object_address()
> method.  I looked through the boost code, but couldn't decipher it.

Did you find anything about it in the documentation?

>
> Any help appreciated.

You're welcome.

Robert Ramey