Re: Dynamically tagged expression required

[Martin Krischik <krischik@users.sourceforge.net>]

Maciej Sobczak wrote:

> Martin Krischik wrote:
> 
>>>    A : Shape;
>>>    B : Shape'Class := A;             -- (1)
>>>    C : Triangle := (SideLen => 7);
> 
>>>begin
>>>
>>>    A := C;                           -- (2)
>>>    B := C;                           -- (3)
>>>
>>>end Hello;
> 
>>>Why is (1) allowed?
>> 
>> because it is an initialization.
> 
> OK, so that explains why it's possible to use Shape'Class as a parameter
> in subroutine. Every time the subroutine is called, the parameter is
> *initialized* with the value, and therefore it can be different for each
> call:
> 
> procedure Draw(S : Shape'Class);

You know this procedure won't dispatch?

> T : Triangle;
> R : Rectangle;
> 
> and later:
> 
> Draw(T);   -- S initialized with T
> Draw(R);   -- S initialized with R
> 
> Right?

Yes. But it only look like the parameter is initialized. Internally the
compiler likely to use a pointer to pass the paramer to speed up things.
Remember "in" parameter are also constant.

And for the dispatching calls the compiler allways uses a pointer.

>>>Why is (3) not allowed?
>> 
>> because it is an assignment.
> 
> And this is where I want to dig deeper.
> 
>> http://en.wikibooks.org/wiki/Ada_Programming/Object_Orientation
> 
> This page says:
> 
> "The Class type is a real data type. You can declare variables of the
> class-wide type, assign values to them [...]"
> 
> "Assign values to them" is a problem here.
> As the subject of my original post says (and what compiler told me),
> "dynamically tagged expression required".

> What can I provide as a dynamically tagged expression?
> 
> It looks like some ShapeClassPtr.all is OK there, except that I get the
> CONSTRAINT_ERROR, if the new value has a different tag. This observation
> agrees with what Dmitry said - the tag of the class-wide object cannot
> be changed.

Once the variable is initialized it becomes constained - you can only
assigned object of the same constain to them.

Simpler example:

declare
  Text : String := "Hello"; - (A)
begin
 Text := "World"  - (B)
 Text := "World!" - (C)
end

With (A) String becomes constrained to (1 .. 5). The assignment (B) is ok as
well. However (C) will fail - with the exclamation mark the text is 6
characters and the assignment will fail.

> What other dynamically tagged expression can be provided as a right-hand
> side of the (default) assignment to the object of class-wide type?
> Is this useful in practice?

To tell the truth: Up until now I have only initialized 'Class variables but
never reassigned them with a new value. Even more I just rename them:

http://en.wikibooks.org/wiki/Ada_Programming/Subtypes#Rename_view

That saves copying the data.

'Class is also usefull as element type for container types which can store
indefinite elements.

Martin

-- 
mailto://krischik@users.sourceforge.net
Ada programming at: http://ada.krischik.com