Re: Looping over a tagged record?

[tmoran@acm.org]

Note that doing a 'read or 'write of a record makes the compiler
generate code to sequentially visit each component of that record,
calling a 'read/write routine for each.  You get to supply those
'read/write routines if you want - they don't actually have to
do IO at all, but could do something else entirely.  Is that
what you meant by "looping over a [tagged] record"?

>Ada.Streams_IO, however, seems to be a built in method for doing
>most of what I need.  The only problem is that when I'm reading
>the stuff back in I won't be able to use the file tags to tell
>Ada what kind of data I'm reading.  That won't be known at
>compile time.  So I'll need to build the kind of case switch
  You can use 'Class'Read or 'Class'Input to have dispatching on the
particular type at run time.  You don't need to know about all the
possible descendants of a type when you compile your input routine.
You can later create a new descendant with an overriding input
routine to be called for that new type.  If your input data was
externally supplied, you'll of course have to write code to translate
to an Ada tag from however it decides what "type" an object is,
and any time you add a new input possibility you'll have to update
that code to handle it.  But no language, compiler, or computer can
tell what to do with a brand new, unexpected, type-of-object
indicator, until you tell it.  A case statement or the equivalent
is how you tell it, and the case statement will need a new branch
to understand what to do with a new record type.