type Root_Storage_Pool is abstract
  new Ada.Finalization.Limited_Controlled with private
with Storage_Model_Type => (
   Allocate     => Allocate,
   Deallocate   => Deallocate,
   Storage_Size => Storage_Size
);
pragma Preelaborable_Initialization (Root_Storage_Pool);

procedure Allocate
  (Pool                     : in out Root_Storage_Pool;
   Storage_Address          : out System.Address;
   Size_In_Storage_Elements : System.Storage_Elements.Storage_Count;
   Alignment                : System.Storage_Elements.Storage_Count)
is abstract;

procedure Deallocate
  (Pool                     : in out Root_Storage_Pool;
   Storage_Address          : System.Address;
   Size_In_Storage_Elements : System.Storage_Elements.Storage_Count;
   Alignment                : System.Storage_Elements.Storage_Count)
is abstract;

function Storage_Size
  (Pool : Root_Storage_Pool)
   return System.Storage_Elements.Storage_Count
is abstract;

type My_Pools is new Root_Storage_Pool with record [...]

My_Pool_Instance : Storage_Model_Pool.Storage_Model :=
   My_Pools'(others => <>);

type Acc is access Integer_Array with Storage_Pool => My_Pool;