Re: Q: How to declare object with known tag?

[stt@henning.camb.inmet.com (Tucker Taft)]

John Woodruff (woodruff@tanana.llnl.gov) wrote:
: I hope one of the Ada experts will advise me how to write the following
: code fragment.

: I would like to declare an object of a particular type, where the type
: is a tagged record, and I can specify which tag at run-time.  Here is
: the fragment I would like to build:


: declare 
:    Trial_Tag : Ada.Tags.Tag ;
:    type Trial_Type is access all Some_Tag.Base'Class ;
:    Trial : Trial_Type ;
: begin

:    Trial_Tag := Ada.Tags.Internal_Tag ("SOME_TAG.EXTEND21") ; 
: -- Let user input specify the string argument to Internal_Tag --

:    Trial := new SOMETHING -- I dont know what ; 
: -- Trial.all is to be an instance of some_tag.extend21

: end ;

: The goal: 

: I will declare a "heterogenous list" composed of various sub-types of
: the type declared in some_tag.  I wish to append a newly-created member
: onto the list.

: I want to read a user's input to decide which subtype is to be
: appended. If I have my way, I'll be able to write the procedure for
: recognizing the user's selection *before* all the subtypes have been
: declared, because I want to defer the definition of "the last" legitimate
: subtype until after the code to accept user selection has been written and
: tested.  Of course if the argument to Tags.Internal_Tag is wrong, I'll
: remonstrate with my user.

: I recognize that the user will need to create the object of (the new
: extension) of some_tag.extend21.  But to my thinking, after I have
: successfully declared Trial.all in the example, THEN I can dispatch to
: the newly extended type to receive the user's input.

: Is this a legitimate wish?  

Yes, and you can do this approximately by using some_tag.base'Class'Input.
This will read an external tag from a stream, convert it to the
internal tag, and then dispatch to the appropriate some_tag.extendXX'Read 
to read the remainder of the data.  It will raise Constraint_Error if
the external tag is not recognized.

You can override 'Read for the various type extensions if you want 
to recognize a special input format, rather than the normal input format 
used by 'Write/'Read.

See "Stream-Oriented Attributes" in RM95 13.13.

: John Woodruff	                                          N I F   \ ^ /
: Lawrence Livermore National Lab                         =====---- < 0 >
: 510 422 4661                                                      / v \

--
-Tucker Taft   stt@inmet.com   http://www.inmet.com/~stt/
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