Re: limited agregate and limited components default initialization

[Jere <jhb.chat@gmail.com>]

On Saturday, March 31, 2018 at 10:07:20 PM UTC-4, Jean-Claude Rostaing wrote:
> --
> -- Item_type and ITEM_RECORD are NOT meant to be declared uninitialized, ever.
> -- So how do I tell this to GNAT so that Item: aliased ITEM_TYPE as a component is allowed ?
> -- Implementing List_type with common access types and "type Bla, type Blabla is access Bla, complete declaration" would have been easy, but the private type Smart_Pointers made a hell of it.
> 
> with Pointers;
> 
> generic
>    type Item_Type(<>) is limited private;
> package Lists is
>    type List_Type is limited private;
>    type Item_Accessor (Item: not null access Item_type) is limited private with Implicit_Dereference => Item;
>       function Get(Position: in List_Iterator) return Item_Accessor;
> private
>    type ITEM_ACCESS;
>    type Item_Record is limited
>       record
>          Item: aliased Item_type;
>          Next : Item_Access;
>          Pred : Item_Access;
>       end record;
>    package Record_Pointers is new Pointers (Item_type => Item_Record);
>    type Item_Access is new Record_Pointers.Smart_Pointers;
> 
>    type Item_Accessor (Item: not null access Item_type) is limited record
>       REF: ITEM_ACCESS := SET(ITEM_RECORD'(ITEM.all, others => <>));
>    end record;
> 
>    type List_Type is limited record
>       First: Item_Access;
>       Last : Item_Access;
>       Count : Natural := 1;
>    end record;
> end Lists;
> 
> And what I was pointing your attention at was:
> lists.ads:39:49: initialization not allowed for limited types
> for: REF: ITEM_ACCESS := SET(ITEM_RECORD'(ITEM.all, others => <>));

Ok, your problems:
1.  For  your Item_Record type, the parameter Item, cannot be of type
    Item_Type as it is unconstrained.  That is what gives you the first
    error
2.  Item'(Item.all, others => <>) ==>  The problem here is "Item.all".
    You are essentially trying to initialize a limited component with
    a limited variable and copying a limited variable is not allowed.

you need a function that builds a limited type of type Item_Type and use
that in place of Set.  It has to be aggregate initialization.  What you
are trying to do is copy a limited variable to initialize another.
Essentially your Lists package needs to be modified:

generic
    type Item_Type is limited private;  --removed unconstrained
    with function Make_Item return Item_Type;
package Lists is

then your set call is:

REF: ITEM_ACCESS := SET(Make_Item);