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4.1.5 User-Defined References

Static Semantics

1/3
{AI05-0139-2} Given a discriminated type T, the following type-related operational aspect may be specified:
2/3
Implicit_Dereference

This aspect is specified by a name that denotes an access discriminant declared for the type T.
2.a/3
Aspect Description for Implicit_Dereference: Mechanism for user-defined implicit .all.
3/3
{AI05-0139-2} A (view of a) type with a specified Implicit_Dereference aspect is a reference type. A reference object is an object of a reference type. The discriminant named by the Implicit_Dereference aspect is the reference discriminant of the reference type or reference object. [A generalized_reference is a name that identifies a reference object, and denotes the object or subprogram designated by the reference discriminant of the reference object.]
3.a.1/3
Glossary entry: A reference type is one that has user-defined behavior for “.all”, defined by the Implicit_Dereference aspect.

Syntax

4/3
{AI05-0139-2} generalized_reference ::= reference_object_name

Name Resolution Rules

5/3
{AI05-0139-2} {AI05-0269-1} The expected type for the reference_object_name in a generalized_reference is any reference type. 

Static Semantics

6/3
{AI05-0139-2} A generalized_reference denotes a view equivalent to that of a dereference of the reference discriminant of the reference object.
7/3
{AI05-0139-2} Given a reference type T, the Implicit_Dereference aspect is inherited by descendants of type T if not overridden. If a descendant type constrains the value of the reference discriminant of T by a new discriminant, that new discriminant is the reference discriminant of the descendant. [If the descendant type constrains the value of the reference discriminant of T by an expression other than the name of a new discriminant, a generalized_reference that identifies an object of the descendant type denotes the object or subprogram designated by the value of this constraining expression.]

Dynamic Semantics

8/3
{AI05-0139-2} The evaluation of a generalized_reference consists of the evaluation of the reference_object_name and a determination of the object or subprogram designated by the reference discriminant of the named reference object. A check is made that the value of the reference discriminant is not the null access value. Constraint_Error is raised if this check fails. The generalized_reference denotes the object or subprogram designated by the value of the reference discriminant of the named reference object.

Examples

9/3
{AI05-0268-1} type Barrel is tagged ...  -- holds objects of type Element
10/3
{AI05-0139-2} {AI05-0299-2} type Ref_Element(Data : access Element) is limited private
   with Implicit_Dereference => Data;
      -- This Ref_Element type is a "reference" type.
      -- "Data" is its reference discriminant.
11/3
{AI05-0139-2} {AI05-0268-1} function Find (B : aliased in out Barrel; Key : String) return Ref_Element;
   -- Return a reference to an element of a barrel.
12/3
{AI05-0268-1} {AI05-0299-2} B: aliased Barrel;
13/3
{AI05-0139-2} ...
14/3
{AI05-0139-2} {AI05-0268-1} Find (B, "grape") := Element'(...);  -- Assign through a reference.
15/3
{AI05-0139-2} {AI05-0268-1} -- This is equivalent to:
Find (B, "grape").Data.all := Element'(...);

Extensions to Ada 2005

15.a/3
{AI05-0139-2} The aspect Implicit_Dereference and the generalized_reference are new. 

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