Re: [Boost-users] [serialization] Multiple archives in same stream

Subject: Re: [Boost-users] [serialization] Multiple archives in same stream
From: Kenny Riddile (kfriddile_at_[hidden])
Date: 2009-07-01 02:46:43

Chard wrote:
> Pardon me if this issue has been discussed earlier, but is there a 
> particular technical issue why multiple archives cannot be serialized to 
> (or, specifically, de-serialized from) the same stream?
> 
> (I'm still on 1.38, so slap me if this works with 1.39+).
> 
> Given the following example:
> 
> #include <boost/archive/text_oarchive.hpp>
> #include <boost/archive/text_iarchive.hpp>
> #include <boost/serialization/serialization.hpp>
> 
> void test_ser_app()
> {
>     {
>         {
>             std::ofstream file("ser.txt");
>             archive::text_oarchive arch(file);
>             std::string s("Hello World");
>             arch & s;
>             size_t pos = file.tellp();  // [1]
>         }
>         {
>             std::ofstream file("ser.txt", std::ios::app | std::ios::ate);
>             archive::text_oarchive arch(file);
>             std::string s("Goodbye World");
>             arch & s;
>         }
>     } // [2]
>     {
>         std::string s1, s2;
>         std::ifstream file("ser.txt");
>         {
>             archive::text_iarchive arch(file);
>             arch & s1;
>         }
>         size_t pos = file.tellg();  // [3]
>         try
>         {
>             archive::text_iarchive arch(file);
>             arch & s2;
>         }
>         catch (archive::archive_exception &e)
>         {
>             std::cout << e.what() << std::endl;
>         }
>     }
> }
> 
> The stream position reported at [1] is (correctly) 42, and examination of 
> the text file (at [2]) shows the two archives serialized okay.
> 
> However, the stream position at [3] appears to be at the end of the stream. 
> Does the serialization read in a big chunk, regardless of whether it will be 
> processing it all?
> 
> If I hack in a set stream position at [3], then the library quite happily 
> reads the second archive.  Is it possible to have the library leave the 
> stream pointer at the correct end position?

This doesn't directly answer your question, but I'm using some code that 
  reads individual archives from a single file containing hundreds of 
archives.  Basically I just use the iostreams library to grab the 
section of the stream I need by passing the stream through a 
boost::iostreams::restriction prior to constructing the archive.  In an 
unrelated note, the iostreams library's compression filters are also 
handy here...text and xml archives compress quite nicely :)