Re: set_index and and end_of_file with just a stream reference

["Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>]

On 2021-02-20 20:08, Mehdi Saada wrote:
> Okay :-)
> what I wanted is:
> I read an acronyme in the stream file, if good I input the adjacent record type, otherwise I would advance on the stream until the next acronyme with set_index(stream_access, index(stream_access) + composite_type_stream_size) and read the next acronyme (unbounded_string).
> Now I just input both objects and verify the acronyme.
> But I don't like writing an object that maybe won't be used.

So, the problem is "compartmentalization" of a stream.

[As Simon said you better read/write everything as is]

If you really want to implement it, one technique is chained "virtual" 
stream. You create a stream type like this:

    type Compartment_Stream
         (  Source : not null access Root_Stream_Type'Class
         )  is new Root_Stream_Type with ...

The Read/Write operations call to Read/Write of Source, but add 
delimiters separating "compartments." E.g. the stream elements 0..254 
are encoded as is. The element 255 is encoded as a pair (255,255). The 
pairs (255,*) are treated as an "end of a compartment" and ignored. This 
way you can add a Skip operation to Compartment_Stream that reads 
everything until first (255,*). You can even create a nested structure 
of compartments using this schema. E.g. encode start of a compartment as 
(255,0) and its end as (255,1) etc.

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de