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From: ncohen@watson.ibm.com (Norman H. Cohen)
Newsgroups: comp.lang.ada
Subject: Re: Ada 9X question: accessibility
Date: 15 Nov 1994 13:57:42 GMT
Organization: IBM T.J. Watson Research Center
Distribution: world
Message-ID: <3aaeom$qn9@watnews1.watson.ibm.com>
References: <3a0hob$h5f@hopper.acm.org> <CzA7v5.KIJ@inmet.camb.inmet.com>
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Date: 1994-11-15T13:57:42+00:00
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In article <CzA7v5.KIJ@inmet.camb.inmet.com>, stt@spock.camb.inmet.com
(Tucker Taft) writes: 

|> As you guessed, it makes no difference where Ptr_2 is declared,
|> what matters is where its type is declared.  In the new terminology,
|> the accessibility level of Int_2 is statically deeper than
|> the accessibility level of Access_Integer_Type, so the 'Access
|> is illegal.

It would be helpful to explain why the rule must be based on the depth of
the variable's type rather than on the depth of the variable itself:  A
value stored in a local variable of a global access type can be assigned
to a global variable of the same type, thus creating a dangling
reference.

     procedure Example is
        type Outer_Access_Type is access all Integer;
        Outer_Variable: Outer_Access_Type;
     begin
        declare
           Inner_Variable : Outer_Access_Type;
           Target         : aliased Integer;
        begin
           Inner_Variable := Target'Access;  -- ILLEGAL!
           Outer_Variable := Inner_Variable;
        end;
        Outer_Variable.all := Outer_Variable.all + 1;  -- dangling reference
     end Example;

--
Norman H. Cohen    ncohen@watson.ibm.com