Re: Generic type parameters

[dommo1234@my-deja.com]

In article <m37lp6cm5h.fsf@mheaney.ni.net>,
  Matthew Heaney <matthew_heaney@acm.org> wrote:
>
>
> I don't know about "neater," but you can create another generic
package
> in which to declare the access types, and import an instantiation of
> that package as a generic formal package:
>
> generic
>   type TypeX is private;
> package TypeX_Access_Types_G is
>
>   type TypeX_A is access all TypeX;
>
>   type TypeX_AC is access constant TypeX;
>
> end;
>
> with TypeX_Access_Types_G;
>
> generic
>
>   with package TypeX_Access_Types is
>     new TypeX_Access_Types_G (<>);
>
>   use TypeX_Access_Types;
>
> package Example is
>
>   <as above>
>
> end;
>

I think this is quite neat, but having done some experimentation, I''ve
hit another problem that seems to relate to accesibility levels, a
subject that I find very confusing. Here's my test program :-

generic
    type TypeX is private;
package TestGeneric is
    type TypeX_A is access all TypeX;
    type TypeX_AC is access constant TypeX;
end TestGeneric;

with TestGeneric;

generic
    type TypeX is private;
package OtherGeneric is
    package TypeXAccessTypes is new TestGeneric(TypeX);
    use TypeXAccessTypes;
    type TestType is
        record
            variable: aliased TypeX;
        end record;
    function returnNonConst(aa: access TestType) return TypeX_A;
end OtherGeneric;

package body OtherGeneric is
    function returnNonConst(aa: access TestType) return TypeX_A is
    begin
        -- The following causes a Program_Error at runtime
        -- unless an Unchecked_Access is passed in!!!
        return aa.variable'Access;
    end returnNonConst;
end OtherGeneric;

with OtherGeneric; package OthGenInst is new OtherGeneric(Integer);
with OthGenInst; use OthGenInst;
procedure Main is
    testVar2: aliased TestType;
begin
    returnNonConst(testVar2'Access).all := 6;            -- ERROR!
    returnNonConst(testVar2'Unchecked_Access).all := 6;  -- OK!
end Main;

Can someone explain exactly what accessibility levels for testVar2,
tsetVar2.variable etc. are in the various scopes so I can understand
the rules better ...

Thanks,
Dom.


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