Re: Looping over a tagged record?

[Charles Hixson <charleshixson@earthling.net>]

tmoran@acm.org wrote in
news:ltrY6.111132$%i7.79731461@news1.rdc1.sfba.home.com: 

>> basically need to do is examine the incoming data and
>...
>  Look at packages Ada.Streams and Ada.Stream_IO
> Stream_IO lets you read an arbitrary number of bytes into a
> Stream_Element_Array, and Streams lets you conveniently
> convert those raw bytes into an object of type whatever.
>   Suppose your data stream has a series of objects.  Each
>   object 
> is preceded by a single character identifying code.  Then
> you could 
>   Character'Read((Stream, ID_Code);
>   case ID_Code is
>     when 'A' => An_Object_Type'Read(Stream, An_Object);
>     when 'B' => A_Different_Type'Read(Stream,
>     A_Different_Object); ...
> If An_Object_Type is a record, Ada will automatically read
> it by a series of 'Read, of the correct type, on each
> component - or you could override that and instead supply
> your own 'Read. 
> 
That looks quite interesting.  I would definitely need to supply 
my own read, and probably my own write too, since the objects 
will likely need to be stored in non-contiguous areas of the 
file. (If freed space can't be reused, the process would be 
unacceptably inefficient.  And when dealing with small chunks of 
data I would also need to be able to pack then into blocks.)

If I can make that work it will certainly be better than the 
alternatives that had occurred to me. (Address mapped storage, 
e.g., with arbitrary mappings needing to be defined.  Ugh!  Or 
tagged types, with all of the basic types defined for every 
possible offset into the block.  Ugh! again.)

 ...
After looking at the package:  Perhaps I need to rethink things 
to use arbitrary sized blocks rather than fixed size blocks.  
Otherwise it looks like just what I was looking for!


-- 
Charles Hixson

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