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From: tmoran@acm.org
Newsgroups: comp.lang.ada
Subject: Re: Access types and classwide programming
References: <09Oo7.12082$mj6.1852826@news6-win.server.ntlworld.com>
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Date: Sun, 16 Sep 2001 09:32:13 GMT
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Date: 2001-09-16T09:32:13+00:00
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> procedure test_x is
>
>     temp : aliased xxx;
>
> begin
>     init (temp);    -- this is ok!
>
>     put( get_something(temp'access)); -- error 1 here
> end x;
>
> error 1 is "non-local pointer cannot point to local object"
  Suppose function get_something put a copy of its parameter into a
global variable.  After exiting text_x, that global would be a pointer
to ???  Thus the error message.  There are two solutions: define
get_something to take as parameter a type which is declared inside
of text_x, in which case clearly no global could exist for get_something
to set, or else else temp'unchecked_access, thus telling the compiler
"I know this is dangerous but I've looked carefully and it's OK here".

> What I want to do is to have a variable of type xxx and to pass it to
> get_something and to init.
  The "init" case already works fine.  Why not change get_something to
    function get_something (sc : in xxx'class)
    return integer;
thus getting rid of the access type and the problem.

Suppose you have a tagged type ttype, and need an access type to ttype and
it's class.

> type ttype is tagged ...;
> type ttype_access is access all ttype;
> typ ttype_caccess is access all ttype'class;  -- [surely you meant this]
>
> How do you
>
> a) get "access" to a variable of ttype?
  x : aliased ttype;
  p : ttype_access;
  ...
  p := x'access;
  p := new ttype;

> b) create a new instance of a ttype which is pointed to by a variable of
> type ttype_caccess?
  y : aliased ttype;
  p : ttype_caccess;
  ...
  p := x'access;
  p := new ttype;