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From: Ted Dennison <dennison@telepath.com>
Subject: Re: Variant record question
Date: 2000/01/21
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In article <4V0i4.622$dw3.29725@news.wenet.net>,
  "Mike Silva" <mjsilva@jps.net> wrote:
> I've been working on some proof-of-concept code, and I don't
> understand something about arrays of variant records.  Specifically,
> the following code works with a default discriminant defined for the
> type T_t, but if I remove the default discriminant it fails at the
> declaration of T_array_t.  Why must I put in a default discriminant
> when I can then override it in an aggregate (variable 'A' below)?

I have personally never written an Ada compiler, but below is roughly
the dialog that I imagine is going on between a developer and the
compiler when a discriminated type is declared. Hopefully the
experienced compiler writers in the crowd will correct anything that I
got a bit wrong.

When you declare a discriminated type without defaults for its
discriminants, you are effectively telling the compiler: "I'll tell you
how big objects of this type are when I create them." When the compiler
needs to know the objects' size (eg, so it can write the machine code to
perform an assignment) It may need to keep an associated dope record
with the size, or look at the discriminant and do a calculation.

Suppose I want an array of these things. Now suppose I come along and
ask for element 20. How does the compiler know where element 2000 is? It
would have to look up the sizes of every element from 1 to 1999. That's
not exactly the kind of effencicy we'd typically want out of an array!
That kind of indexing is what records are for.

When you give the type a default discriminant, you are effectively
telling the compiler: "I don't know how big objects of this type are
either; I may need to change it. So please just allocate enough space
for the maximum that my discriminant values could ever require. And
until I tell you otherwise, use this discriminant value."

Now since every object of this discriminated type is the same size (the
maximum it could ever need to be), array indexing can work just like it
would with any other object.

--
T.E.D.

http://www.telepath.com/~dennison/Ted/TED.html


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