Re: Readonly variables in Ada?

[collard@software.org (David Collard)]

In article <1990Jun5.085654.920@lth.se> collberg@DNA.LTH.Se (Christian S. Collberg) writes:
> Is it possible to declare (the equivalent of) an exported readonly
> variable in Ada? I'm referring to something similar to the READ_ONLY
> attribute present in Mesa. I'm having a hard time reading the 

>    package P;
>        type T is limited private;
>        v : T;            -- We want a client of P to be able to
>                          -- perform any operation on v, except
>                          -- assignment.
>    private
>       type T is BOOLEAN;
>    end P;

I think, for a read only VARIABLE, there is an easier answer -- the type
can be visible, but declare the variable in the body of the package and
only provide an interface to read it.

package P is
  function T return Boolean;
end P;

package body P is
  Static_Package_Variable : Boolean;
  function T return Boolean is
  begin
    return Static_Package_Variable;
  end T;
end P;

with P;
procedure P_User is
begin
  if P.T then  --Legal
    null;
  end if;

  P.Static_Package_Variable := False; -- Illegal because the variable is not
                                         visible.

end P_User;
A
end P_User;


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D. Thor Collard                      Internet: collard@software.org
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