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From: "David C. Hoos, Sr." <david.c.hoos.sr@ada95.com>
Subject: Attribute definition clause for 'External_Tag
Date: 1999/05/16
Message-ID: <8b1tUD#n#GA.208@newstoo.hiwaay.net>#1/1
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Date: 1999-05-16T00:00:00+00:00
List-Id: <comp.lang.ada>

Here's one for all the language lawyers out there...

I wanted to relate the tags of a group of types all
derived from a common abstract ancestor to the values
an enumeration type whose values relate to the
tagged types.

To do this I used an attribute_definition_clause to
specify the External_Tag attribute for each of the
tagged types.

My first approach was to use an clause of the form

for Tagged_Type'External_Tag use
   Enum_Type'Image (Enum_Value);

I was surprised that the compiler complained that
the expression must be a static string.

Why is such an expression not static?

My next approach (the one I'm now using) was to
replace the clause with one of the form

for Tagged_Type'External_Tag use "ENUM_VALUE";

This is fine, as long as the string exactly
matches the 'Image attribute, and I can simply
relate Tags to enumeration values.

E.g., I can say

Enum_Var := Enum_Type'Value
  (Ada.Tags.External_Tag
   (Tagged_Type_Item'Tag));

However, I was even more surprised than I was about
the staticness issue when I saw that the compiler
did not complain if the same External_Tag attribute
is defined for two different types of the same class
-- i.e., descended from a common tagged ancestor.

What say ye?

1.  Should the 'Image attribute be allowed to define
    an external tag?

2.  Should the compiler reject identical External_Tag 
    attributes for types with a common ancestor?

David C. Hoos, Sr.