Zero size for access types are to emphasize that the access type shouldn't be used for anything
other than satisfying the need to have a named type (for the return type of
a function).
For example, in the stack-with-limited-items:
generic
type Item_Type is limited private;
package Stacks is
type Stack_Type is limited private;
type Item_Access is access all Item_Type;
for Item_Access'Storage_Size use 0;
function Push (Stack : access Stack_Type) return Item_Access;
function Set_Top (Stack : access Stack_Type) return Item_Access;
...
end Stacks;
An Ada function declaration must return a named type, so I have to declare
an access type for the return type. But I only want clients to do this:
Stack : aliased Stack_Type;
begin
Push (Stack'Access).all := Value;
...
Set_Top (Stack'Access).all := Value;
A client doesn't ever declare an object of type Item_Access -- that type
just happens to be the type returned by the function, which you always
dereference immediately.
Setting the Storage_Size for the access type ensures that no storage
resources are allocated for the type, and it prevents you from using it to
allocate, ie
IA : Item_Access := new Item_Type'(Value); -- you're being bad
A good compiler will give you a warning at compile-time that the allocator
will raise Storage_Error at run-time. (Sadly, there's no way to statically
prevent allocator use.)
The main thing is that the type is there just to satisfy the requirement for
a named type, and to ensure that no storage resources are consumed
unnecessarily.
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