access: add adv chapter for idioms

- disabling deallocations
- type safe idiom: access to subtype

Author: leogermond

courses/fundamentals_of_ada/140_access_types-in_depth.rst

@@ -57,5 +57,6 @@ Access Types In Depth
 .. include:: 140_access_types/08-memory_management.rst
 .. include:: 140_access_types/09-memory_debugging.rst
 .. include:: 140_access_types/10-memory_control.rst
+.. include:: 140_access_types/11-type_safe_idioms.rst
 .. include:: labs/140_access_types-in_depth.lab.rst
 .. include:: 140_access_types/99-summary_with_pools.rst

courses/fundamentals_of_ada/140_access_types/10-memory_control.rst

@@ -123,4 +123,3 @@ System.Storage_Pools Example (Partial)
          end if;
       end loop;
    end Deallocate;
-

courses/fundamentals_of_ada/140_access_types/11-idiom_constant_pointer.rst

@@ -0,0 +1,98 @@
+===========================
+Advanced Access Type Safety
+===========================
+
+-----------------------------------
+Elaboration-Only Dynamic Allocation
+-----------------------------------
+
+* Common in critical contexts
+* Rationale
+
+1. We (might) need dynamically allocated date
+
+    - e.g. loading configuration data of unknown size
+
+2. Deallocations can cause leaks, corruption
+
+    - |rightarrow| **Disallow** them entirely
+
+3. A dynamically allocated object will needs deallocation
+
+    - |rightarrow| Unless it never goes out of **scope**
+
+* |rightarrow| Allow only allocation onto globals
+
+.. tip::
+
+    And restrict allocations to program elaboration
+
+--------------------------
+Prevent Heap Deallocations
+--------------------------
+
+* :ada:`Ada.Unchecked_Deallocation` cannot be used anymore
+* No heap deallocation is possible
+
+    - The total number of allocations should be bounded
+    - e.g. elaboration-only allocations
+
+.. code:: Ada
+
+    pragma Restrictions
+        (No_Dependence => Unchecked_Deallocation);
+
+--------------------------------
+Constant Access at Library Level
+--------------------------------
+
+.. code:: Ada
+
+   type Acc is access T;
+   procedure Free is new Ada.Unchecked_Deallocation (T, Acc);
+
+   A : constant Acc := new T;
+
+* :ada:`A` is :ada:`constant`
+
+    * Cannot be deallocated
+
+-------------------------------
+Constant Access as Discriminant
+-------------------------------
+
+.. code:: Ada
+
+   type R (A : access T) is limited record
+
+* :ada:`A` is :ada:`constant`
+
+    * Cannot be deallocated
+
+* :ada:`R` is :ada:`limited`
+
+    * Cannot be copied
+
+------------------------
+Idiom: Access to Subtype
+------------------------
+
+.. tip::
+
+   :ada:`subtype` improves access-related code safety
+
+* Subtype constraints still apply through the access type
+
+.. code:: Ada
+
+   type Values_T is array (Positive range <>) of Integer;
+   subtype Two_Values_T is Values_T (1 .. 2);
+   type Two_Values_A is access all Two_Values_T;
+
+   function Get return Values_T is (1 => 10);
+
+   -- O : aliased Two_Values_T := Get;
+   -- Runtime FAIL: Constraint check
+   O : aliased Values_T := Get; -- Single value, bounds are 1 .. 1
+   -- P : Two_Values_A := O'Access;
+   -- Compile-time FAIL: Bounds must statically match

courses/fundamentals_of_ada/140_access_types/11-type_safe_idioms.rst

@@ -37,7 +37,6 @@ Expert Resource Management
 .. container:: PRELUDE END
 
 .. include:: 110_private_types/07-limited_private.rst
-.. include:: 140_access_types/11-idiom_constant_pointer.rst
 .. include:: 260_controlled_types/10-idiom_refcounting.rst
 .. include:: 260_controlled_types/11-example_logger.rst
 .. include:: 230_interfacing_with_c/10-example_refcount_wrap.rst