Re: Persistence of limited tagged types

[Stephen Leake <Stephen.A.Leake@nasa.gov>]

Jano <nono@celes.unizar.es> writes:

> Hello,
> 
> I've read several past threads about this, but to reassure me I want to 
> bring it back again, or at least to know the typical workaround.
> 
> I have a heterogeneous collection by means of class wide access types, 
> where the accessed types itself are descendants of a 
> 
> type Object is abstract tagged limited private;
> 
> These objects are kind of state-dependent, and I have an abstract method 
> which serializes an Object to disk. So far, so good.
> 
> The problem comes when I want to reconstruct the collection from disk. I 
> can't think of a mean to obtain a valid allocated pointer initialized 
> with some dispatching call.
> 
> A neat solution could be a function that returned an allocated pointer 
> given a tag, but AFAIK there is not such a function.

The external representation of the type needs to be significantly
different than the internal representation. In particular, the
external representation needs to have a "readable" representation of
the tag.

You will have to write a function that reads the external
representation of the tag, allocates an object of the appropriate
type, and returns the pointer. Yes, this function must be changed
every time you add a new tag.

-- 
-- Stephe