controlled types: idiom for refcounting

Author: leogermond

courses/fundamentals_of_ada/260_controlled_types/10-idiom_refcounting.rst

@@ -0,0 +1,75 @@
+=========================================
+Reference Counting Using Controlled Types
+=========================================
+
+---------------
+Global Overview
+---------------
+
+* Idiom for counting object references
+
+    - Safe deallocation
+    - No memory leak
+    - Efficient
+    - All :ada:`access` must then be using it
+
+* A refcounted type derives from :ada:`Refcounted`
+
+    - Tagged
+    - Get a :ada:`Ref` through :ada:`Set`
+    - Turn a :ada:`Ref` into an :ada:`access` through :ada:`Get`
+
+.. code:: Ada
+    
+    package Ref_Counter is
+       type Refcounted is abstract tagged private;
+       procedure Free (Self : in out Refcounted) is null;
+
+       type Refcounted_Access is access all Refcounted'Class;
+       type Ref is tagged private;
+
+       procedure Set (Self : in out Ref; Data : Refcounted'Class);
+       function Get (Self : Ref) return Refcounted_Access;
+       procedure Finalize (P : in out Ref);
+       procedure Adjust   (P : in out Ref);
+    private
+       type Refcounted is abstract tagged record
+          Refcount : Integer := 0;
+       end record;
+
+       type Ref is new Ada.Finalization.Controlled with record
+           Data : Refcounted_Access;
+       end record;
+
+----------------------
+Implementation Details
+----------------------
+
+* :ada:`Set` is safe
+    
+    - :ada:`Ref` default value is :ada:`null`
+    - Clears up any previously used :ada:`Ref`
+
+.. code:: Ada
+
+    procedure Set (Self : in out Ref; Data : Refcounted'Class) is
+      D : constant Refcounted_Access := new Refcounted'Class'(Data);
+    begin
+      if Self.Data /= null then
+          Finalize (Self); -- decrement old reference count
+      end if;
+
+      Self.Data := D;
+      Adjust (Self);  -- increment reference count (set to 1)
+    end Set;
+
+* :ada:`Adjust` called for all new references
+
+.. code:: Ada
+
+    overriding procedure Adjust (P : in out Ref) is
+    begin
+       if P.Data /= null then
+          P.Data.Refcount := P.Data.Refcount + 1;
+       end if;
+    end Adjust;

courses/fundamentals_of_ada/300_expert_resource_management.rst courses/fundamentals_of_ada/700_expert_resource_management.rst

@@ -34,3 +34,4 @@ Expert Resource Management
 .. container:: PRELUDE END
 
 .. include:: 110_private_types/07-limited-private.rst
+.. include:: 260_controlled_types/10-idiom_refcounting.rst