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From: "Warren W. Gay VE3WWG" <ve3wwg@home.com>
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Subject: In Plain Text: Tags.Internal_Tag(String'Input(Stream)) ??
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Date: Thu, 28 Dec 2000 15:02:55 GMT
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Date: 2000-12-28T15:02:55+00:00
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(Apologies re: the prior post in HTML, here again in plain:)

I want to read in WAVE file chunks using the S'Class'Input() facility. 
Each WAVE chunk starts with a String(1..4) that identifies the chunk. 
I have set up the external representation of each tagged record to 
match this chunk ID, for example: 

   for Riff_Form'External_Tag use "RIFF"; 

However, the difficulty now is that I want to override the way the 
Tag is read in (the Tag length is ALWAYS 4 bytes here). The Ada 
implementation attempts to read in the lower bound and upper 
bound of the Tag string first [ See ref to the note about 
Tags.Internal_Tag(String'Input(Stream))  below ]. 

I would prefer not to override String'Input since this would affect 
it's behavour for all strings. 

This suggests that I have to define my own S'Class'Input() 
instead. This would allow me to read the tag in, in any way I 
please, but then how do I proceed from there? How does the 
Ada S'Class'Input() dispatch once the external tag is known? 

I.E. how do I "dispatch to the subprogram denoted by the Input 
attribute of the specific type identified by the internal tag; returning 
that result."???? 

I tried to find the GNAT implementation of this, but so far have been 
unsuccessful. 

Thanks in advance... 

>From the ARM : 
http://www.adapower.com/rm95/arm95_214.html#SEC214 

1.S'Class'Input 

   S'Class'Input denotes a function with the following specification: 

   1.function S'Class'Input( 
             Stream : access Ada.Streams.Root_Stream_Type'Class) 
             return T'Class 

    2.First reads the external tag from Stream and determines the corresponding 
       internal tag (by calling Tags.Internal_Tag(String'Input(Stream)) -- See 
       section 3.9 Tagged Types and Type Extensions.) and then dispatches to 
       the subprogram denoted by the Input attribute of the specific type identified by 
       the internal tag; returns that result. 

-- 
Warren W. Gay VE3WWG
http://members.home.net/ve3wwg