Re: Self-referential types

["Matthew Heaney" <matthew_heaney@acm.org>]

In article <s04ql7cqh1s45@corp.supernews.com> , "Vladimir Olensky" 
<vladimir_olensky@yahoo.com> wrote:

> How it is  possible to initialize access variable
> by the initial value that is access attribute to type ?

Yes, of course it's possible to initialize an access object that is an
access to a type!

For a limited type T, then inside the declaration of the type, the
expression T'Access refers to the "current instance" of the type.

This is something a lot of programmers haven't learned yet.

This is the basis for programming with access discriminants, which is
how you do MI in Ada95.

For example:

  type Handle_Type (Stack : access Stack_Type) is
    limited null record;

  type Stack_Type is
    limited record
      Handle : Handle_Type (Stack_Type'Access);  --!!!
      <other stuff>
    end record;


Here's another example:

  type T is tagged limited private;
...
private

  function Do_Get_Default (O : T) return T2;

  function Get_Default (O : T'Class) return T2;

  type T is
    tagged limited record
       O : T2 := Get_Default (T'Access);  --!!!
    end record;


Where

  function Get_Default (O : T'Class) return T2 is
  begin
    return Do_Get_Default (O);
  end;


This is the idiom you use if you want to let a descendent type change
the default of component declared by the ancestor.



> Access attributes can not be applied to type.

Wrong.

> It may only be  applied to some object but not to its description.

Wrong.

> Also compiles are probably giving  misleading error messages.

No, the compiler was correct.


--
The new standards [for science curricula in Kansas] do not forbid the
teaching of evolution, but the subject will no longer be included in
statewide tests for evaluating students--a virtual guarantee, given the
realities of education, that this central concept of biology will be
diluted or eliminated, thus reducing courses to something like chemistry
without the periodic table, or American history without Lincoln.

Stephen Jay Gould, Time, 23 Aug 1999