Re: Please help a newbie with Booch/Ada.

[Stephen Leake <Stephen.Leake@gsfc.nasa.gov>]

Lengyel Sandor <hunnia@pacbell.net> writes:

> I am fooling around with the Booch components, since I think
> "Collections" are very usefull.

good idea.

>    PACKAGE String_References IS
>       TYPE String_ptr IS ACCESS ALL String;
>       TYPE Const_String_Ptr IS ACCESS CONSTANT String;
>    END String_References;
> 
> 
> Then I created a root_string_container.ads as:
> 
>    with Bc.Containers;
>    with String_References;
>    package Root_String_Container is new Bc.Containers
>       (String,String_References.String_ptr);
> 
> 
> (Coping the format of root_container.ads. :-) )
> 
> When I run a semanric check on this later code, I get the following
> message.
> 
> root_string_container.ads:4:08: actual for "Item" must be a definite
> subtype.

See RM 3.3 (23). Basically, an indefinite type is any unconstrained
type (in this case, String is an unconstrained array). A definite type
is a constrained type.

The problem is that the Ada 95 Booch components are modeled after the
C++ Booch components. Since there is no such thing as an "indefinite
type" in C++, the Ada 95 Booch components restrict you to definite
types. This is one of the advantages of Ada 95 over C++; you can give
more information about the type. On the other hand, a careless library
author can shoot you down.

As a work around, you can use Ada.Strings.Unbounded.Unbounded_String
instead of String.

Better would be to fix the Ada 95 Booch components to allow indefinite
types. Find the declaration of Bc.Containers, and add "(<>)" as a
discriminant to the object type. Be aware this might break lots of
stuff; I haven't tried it.

-- Stephe