Re: [Boost-users] [serialization] appending elements

Subject: Re: [Boost-users] [serialization] appending elements
From: Robert Ramey (ramey_at_[hidden])
Date: 2009-03-09 00:47:41

Agustín K-ballo Bergé wrote:
> Is it possible to append serialized elements to an existing archive?

the serialization library doesn't currently address this situation.

>I
> need to keep a set of records on a file, and periodically add new
> entries to it (kind of like a database table). Reading the whole set,
> adding the new records, and serializing the set back is not an option
> since I expect the set to go pretty big.

I understand and sympathise.

> I have tried to keep several archives at the same stream (with
> no_header and no_tracking flags),

I believe this might be made to work.

> but then I don't know how to read
> them. Reading them as if they were a single archive does not work,
> should it?

no.

Here's an idea.

// append a bunch of archives to a file
main(..){
    ofstream os(..); // open for append
    {
        oarchive oa(os)
         oa << a bunch of stuff
    } // close archive leaving stream open
    {
        oarchive oa(os)
        oa << a bunch of other  stuff
    } // close second archive
} // now we have a closed stream with several self-contained archives in it

// read a bunch of archives from one file
main(..){
    ifstream is(...); // open the normal way
    {
        iarchive ia(..);    // open first archive
        ia >> first bunch of stuff
    } // close first archive leaving stream open
    {
        iarchive ia(..);    // open first archive
        ia >> second bunch of stuff
    } // close second archive leaving stream open
} // close input stream

exercise for the reader.  Figure out where to save the Number of archives 
inthe
file.  A couple of ideas
    1) a separate companion file
    2) save a few bytes at the beging of the file - call this archive count. 
Update this
after every "append" and read it duning load.

Good Luck with this.

Robert Ramey

main(..
>
> Agustín K-ballo Bergé.-