Upcasting

Upcasting

[Matthew Baulch]

For the records:

...
type Base is tagged with
  record
    Base1, Base2 : Integer;
  end record;

type Derived is new Base with
  record
    Derived1, Derived2 : Float;
  end record;
...
Foo1 : Base;
Foo2 : Derived;
...

Obviously to assign Foo2 to Foo1 you would:
	Foo1 := Base(Foo2);

But if you wanted to assign Foo1 to Foo2:
	Foo2 := (Foo1 with 5.0, 10.0);
would work.

My question is: How would you do this 'upcasting' if varying
types existed? Obviously the compiler could tell where each
value must go based on their type if only one of their type
was needed in the transition. How does this type of thing work?
(or does the A95 compiler not allow it)

Re: Upcasting

[Lutz Donnerhacke]

* Matthew Baulch wrote:
> My question is: How would you do this 'upcasting' if varying
> types existed? Obviously the compiler could tell where each
> value must go based on their type if only one of their type
> was needed in the transition. How does this type of thing work?
> (or does the A95 compiler not allow it)

procedure t is
   type Base is tagged null record;
   type Derivated is new Base with null record;

   d  : Derivated;
   dc : Base'Class := d;
   b  : Base       := Base(d);
   bc : Base'Class := b;
begin
-- d := Base (b);       -- will not compile
   d := Derivated (dc);
   d := Derivated (bc); -- will raised CONSTRAINT_ERROR on run time
end t;

Re: Upcasting

[Stephen Leake]

"Matthew Baulch" <matt@greenroom.com.au> writes:

> My question is: How would you do this 'upcasting' if varying
> types existed? 

I don't know what you mean by "varying types". You might mean "variant
records", but I doubt it.

Can you give an example of what you mean?

-- 
-- Stephe

Re: Upcasting

[Matthew Baulch]

On Tue, 05 Nov 2002 12:33:51 +0000, Stephen Leake wrote:

> "Matthew Baulch" <matt@greenroom.com.au> writes:
> 
>> My question is: How would you do this 'upcasting' if varying
>> types existed? 
> 
> I don't know what you mean by "varying types". You might mean "variant
> records", but I doubt it.
> 
> Can you give an example of what you mean?

Not variant records. Varying types: eg. several vars with different types:
	Int1, Int2 : Integer;
	Flt1, Flt2 : Float;
	Str1, Str2 : String(1..10);

So, how would you use the: "x := (xxx with xxx, xxx, xxx)" statement
where multiple types are present.

Re: Upcasting

[Georg Bauhaus]

Matthew Baulch <matt@greenroom.com.au> wrote:
: But if you wanted to assign Foo1 to Foo2:
:        Foo2 := (Foo1 with 5.0, 10.0);
: would work.

Not sure I understand your question, because it is not obvious to me
what a varying type is. You could do

   foo2 := (foo1 with derived1 =>  5.0, derived2 => 10.0);

: 
: My question is: How would you do this 'upcasting' if varying
: types existed? Obviously the compiler could tell where each
: value must go based on their type if only one of their type
: was needed in the transition.