[RFC] OOP - Destructors

considered/rfc-oop-destructors.rst

@@ -0,0 +1,96 @@
+- Feature Name: Standard OOP model
+- Start Date: May 5th 2020
+- RFC PR:
+- RFC Issue:
+
+Summary
+=======
+
+Motivation
+==========
+
+Guide-level explanation
+=======================
+
+Record, tagged records and class records can declare destructors. The
+destructor is identifed by the `Destructor` attribute, e.g.:
+
+.. code-block:: ada
+
+   package P is
+      type T is tagged null record;
+      for T'Destructor use My_Destructor;
+
+      procedure My_Destructor (Self : in out T);
+
+      type T2 is new T with null record
+      with Destructor => My_Destructor;
+
+      procedure My_Destructor (Self : in out T2);
+   end P;
+
+The destruction sequence works in the following way:
+- If a type has an explicit destructor, it is first called.
+- If a type has components hierarchy, wether or not it has an explicit
+  destructor, the destructor sequence is called on each components.
+- If a type is in a tagged hierarchy, wether or not it has an explicit
+  destructor, the parent destructor sequence is called.
+
+Destructors are called at the same place as when Ada finalization is run.
+
+
+Reference-level explanation
+===========================
+
+
+Rationale and alternatives
+==========================
+
+Drawbacks
+=========
+
+
+Prior art
+=========
+
+Unresolved questions
+====================
+
+Future possibilities
+====================
+
+We need a scoped syntax for the destructor. One option is to piggy back on
+a separate RFC being written that allows to define attributes directly in
+the form of type'attribute name. For example, specifying Write could be done
+in the following way:
+
+.. code-block:: ada
+
+   type T is null record;
+
+   procedure S'Write(
+      Stream : not null access Ada.Streams.Root_Stream_Type'Class;
+      Item : in T);
+
+Using this gives us a new un-scoped notation:
+
+.. code-block:: ada
+
+   package P is
+      type T is tagged null record;
+
+      procedure T'Destructor (Self : in out T);
+
+   end P;
+
+And this can be easily extended to a scoped notation for Destructor as well as
+other attributes:
+
+.. code-block:: ada
+
+   package P is
+      type T is tagged record
+          procedure T'Destructor (Self : in out T);
+      end record;
+   end P;
+