Re: [Boost-users] Boost serialization storage question

From: Leon Mergen (leon_at_[hidden])
Date: 2005-09-04 14:08:47

On Sun, 2005-09-04 at 11:33 -0700, Robert Ramey wrote:
> > I wondered how the Boost.Serialization library determines how many
> > bytes to read from an input stream to "fill" a class with data.
> 
> > That is, does it read an entire input stream until the end of the stream
> > and use that as the data to fill a class with, or does it perform a
> > sizeof(class) and read that from the stream ?
> 
> Neither.  Each class has its serialize function. Within this function it 
> calls the operator << to copy the member data to archive.  There are two 
> cases:
> 
> a) the data type is a primitive - int, etc - in which case the archive has 
> method for storing it and knows the length of an int on the particular 
> platform.
> 
> b) the datatype is a class - in which case that class's serialize function 
> is called.
> 
> So eventually the serialization resolves to a string of calls to primitive 
> types - all of which are supported by the archive class.

Ok, sounds logical. What about objects from the stdlib, such as
std::string ? I assume this function does not have a serialize()
function, but in some prototypes I created I noticed that this worked
perfectly. 

> > If it is the latter, then out of curiosity, how does it determine the
> > size of an object with different class versions ( so if an object
> > version 1 has been saved and then tries to load version
> 
> The first piece of data stored with the class is a version number. On 
> loading, this is available to be used in conditional ar >> operations.  When 
> one adds a new member to the class - the version number should be updated so 
> that the loading of new member can be skipped if the archive is an older 
> version.

Yes, I saw that in an example on the documentation. Looks like a great
feature!

> 2 ) ? Does it write some sort of object size at the beginning of the file ?
> 
> no.
> 
> > If it reads the entire stream until the end,
> 
> None of the currently implmented archive classes do this.

Ok, but then, out of curiosity, how does it know when to stop reading ?

> > what would be a good approach to save multiple objects in a single file, 
> > and be able to
> > read just a few objects out of the file ?
> 
> Current archive classes don't support a good way to do this.  Actually this 
> is an iteresting application - but its not supported by current archives. 
> One might be able make a new archive class or a new derivation from one of 
> the existing ones to support this, but I have no idea how difficult it would 
> be.  At a minium it would require a pretty thorough understanding of the 
> library and archive classes.

Yes, and I assume this will be a very hard task making it portable. Ok,
guess I'll look further away from serializations for a solution... 

Thanks a lot for your lengthy reply, at least I'm all out of hopes for
the boost serialization library now. :)

Regards,

Leon Mergen