Ada Reference Manual (Ada 2022 Draft 34)Legal Information
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3.9.1 Type Extensions

Every type extension is a tagged type, and is a record extension or a private extension of some other tagged type, or a noninterface synchronized tagged type. 


record_extension_part ::= with record_definition

Legality Rules

The parent type of a record extension shall not be a class-wide type nor shall it be a synchronized tagged type (see 3.9.4). If the parent type or any progenitor is nonlimited, then each of the components of the record_extension_part shall be nonlimited. In addition to the places where Legality Rules normally apply (see 12.3), these rules apply also in the private part of an instance of a generic unit. 
Within the body of a generic unit, or the body of any of its descendant library units, a tagged type shall not be declared as a descendant of a formal type declared within the formal part of the generic unit.

Static Semantics

 A record extension is a null extension if its declaration has no known_discriminant_part and its record_extension_part includes no component_declarations.
 In the case where the (compile-time) view of an object X is of a tagged type T1 or T1'Class and the (run-time) tag of X is T2'Tag, only the components (if any) of X that are components of T1 (or that are discriminants which correspond to a discriminant of T1) are said to be components of the nominal type of X. Similarly, only parts (respectively, subcomponents) of T1 are parts (respectively, subcomponents) of the nominal type of X.

Dynamic Semantics

The elaboration of a record_extension_part consists of the elaboration of the record_definition.
NOTE 1   The term “type extension” refers to a type as a whole. The term “extension part” refers to the piece of text that defines the additional components (if any) the type extension has relative to its specified ancestor type. 
NOTE 2   When an extension is declared immediately within a body, primitive subprograms are inherited and are overridable, but new primitive subprograms cannot be added.
NOTE 3   A name that denotes a component (including a discriminant) of the parent type is not allowed within the record_extension_part. Similarly, a name that denotes a component defined within the record_extension_part is not allowed within the record_extension_part. It is permissible to use a name that denotes a discriminant of the record extension, providing there is a new known_discriminant_part in the enclosing type declaration. (The full rule is given in 3.8.)
NOTE 4   Each visible component of a record extension has to have a unique name, whether the component is (visibly) inherited from the parent type or declared in the record_extension_part (see 8.3). 


Examples of record extensions (of types defined above in 3.9):
type Painted_Point is new Point with
    Paint : Color := White;
  end record;
    -- Components X and Y are inherited
Origin : constant Painted_Point := (X | Y => 0.0, Paint => Black);
type Literal is new Expression with
  record                 -- a leaf in an Expression tree
    Value : Real;
  end record;
type Expr_Ptr is access all Expression'Class;
                               -- see 3.9
type Binary_Operation is new Expression with
  record                 -- an internal node in an Expression tree
    Left, Right : Expr_Ptr;
  end record;
type Addition is new Binary_Operation with null record;
type Subtraction is new Binary_Operation with null record;
  -- No additional components needed for these extensions
Tree : Expr_Ptr :=         -- A tree representation of “5.0 + (13.0–7.0)”
   new Addition'(
      Left  => new Literal'(Value => 5.0),
      Right => new Subtraction'(
         Left  => new Literal'(Value => 13.0),
         Right => new Literal'(Value => 7.0)));

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