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3.9.1 Type Extensions

1/2
{AI95-00345-01} [ 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.] 

Language Design Principles

1.a
We want to make sure that we can extend a generic formal tagged type, without knowing its discriminants.
1.b
We don't want to allow components in an extension aggregate to depend on discriminants inherited from the parent value, since such dependence requires staticness in aggregates, at least for variants. 

Syntax

2
record_extension_part ::= with record_definition

Legality Rules

3/2
{AI95-00344-01} {AI95-00345-01} {AI95-00419-01} 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. 
3.a
Reason: If the parent is a limited formal type, then the actual might be nonlimited.
3.b/2
{AI95-00344-01} Ada 95 required the record extensions to be the same level as the parent type. Now we use accessibility checks on class-wide allocators and return statements to prevent objects from living longer than their type.
3.c/2
{AI95-00345-01} Synchronized tagged types cannot be extended. We have this limitation so that all of the data of a task or protected type is defined within the type. Data defined outside of the type wouldn't be subject to the mutual exclusion properties of a protected type, and couldn't be used by a task, and thus doesn't seem to be worth the potential impact on implementations. 
4/2
{AI95-00344-01} 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.
4.a
Reason: This paragraph ensures that a dispatching call will never attempt to execute an inaccessible subprogram body.
4.a.1/2
{AI95-00344-01} The convoluted wording (“formal type declared within the formal part”) is necessary to include tagged types that are formal parameters of formal packages of the generic unit, as well as formal tagged and tagged formal derived types of the generic unit.
4.b/2
{AI95-00344-01} This rule is necessary in order to preserve the contract model.
4.c/2
{AI05-0005-1} {AI95-00344-01} If an ancestor is a formal of the generic unit , we have a problem because it might have an unknown number of subprograms that require overriding, as in the following example: 
4.d/2
package P is
    type T is tagged null record;
    function F return T; -- Inherited versions will require overriding.
end P;
4.e
generic
    type TT is tagged private;
package Gp is
    type NT is abstract new TT with null record;
    procedure Q(X : in NT) is abstract;
end Gp;
4.f/2
package body Gp is
    type NT2 is new NT with null record; -- Illegal!
    procedure Q(X : in NT2) is begin nullend Q;
    -- Is this legal or not? Can't decide because
    -- we don't know whether TT had any functions that require
    -- overriding on extension.
end Gp;
4.g
package I is new Gp(TT => P.T);
4.h/2
I.NT is an abstract type with two abstract subprograms: F (inherited as abstract) and Q (explicitly declared as abstract). But the generic body doesn't know about F, so we don't know that it needs to be overridden to make a nonabstract extension of NT. Hence, we have to disallow this case.
4.h.1/2
Similarly, since the actual type for a formal tagged limited private type can be a nonlimited type, we would have a problem if a type extension of a limited private formal type could be declared in a generic body. Such an extension could have a task component, for example, and an object of that type could be passed to a dispatching operation of a nonlimited ancestor type. That operation could try to copy the object with the task component. That would be bad. So we disallow this as well.
4.i
If TT were declared as abstract, then we could have the same problem with abstract procedures.
4.j
We considered disallowing all tagged types in a generic body, for simplicity. We decided not to go that far, in order to avoid unnecessary restrictions.
4.k
We also considered trying make the accessibility level part of the contract; i.e. invent some way of saying (in the generic_declaration) “all instances of this generic unit will have the same accessibility level as the generic_declaration.” Unfortunately, that doesn't solve the part of the problem having to do with abstract types.
4.l/2
This paragraph was deleted.
4.m/2
Ramification: {AI95-00344} This rule applies to types with ancestors (directly or indirectly) of formal interface types (see 12.5.5), formal tagged private types (see 12.5.1), and formal derived private types whose ancestor type is tagged (see 12.5.1).

Static Semantics

4.1/2
  {AI95-00391-01} A record extension is a null extension if its declaration has no known_discriminant_part and its record_extension_part includes no component_declarations.

Dynamic Semantics

5
The elaboration of a record_extension_part consists of the elaboration of the record_definition.
NOTES
6
74  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. 
6.a
Discussion: We considered other terminology, such as “extended type.” However, the terms “private extended type” and “record extended type” did not convey the proper meaning. Hence, we have chosen to uniformly use the term “extension” as the type resulting from extending a type, with “private extension” being one produced by privately extending the type, and “record extension” being one produced by extending the type with an additional record-like set of components. Note also that the term “type extension” refers to the result of extending a type in the language Oberon as well (though there the term “extended type” is also used, interchangeably, perhaps because Oberon doesn't have the concept of a “private extension”).
7/2
75  {AI95-00344-01} When an extension is declared immediately within a body, primitive subprograms are inherited and are overridable, but new primitive subprograms cannot be added.
8
76  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.)
8.a
Reason: The restriction against depending on discriminants of the parent is to simplify the definition of extension aggregates. The restriction against using parent components in other ways is methodological; it presumably simplifies implementation as well.
9
77  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

10
Examples of record extensions (of types defined above in 3.9):
11
type Painted_Point is new Point with
  record
    Paint : Color := White;
  end record;
    -- Components X and Y are inherited
12
Origin : constant Painted_Point := (X | Y => 0.0, Paint => Black);
13
type Literal is new Expression with
  record                 -- a leaf in an Expression tree
    Value : Real;
  end record;
14
type Expr_Ptr is access all Expression'Class;
                               -- see 3.10
15
type Binary_Operation is new Expression with
  record                 -- an internal node in an Expression tree
    Left, Right : Expr_Ptr;
  end record;
16
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
17
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)));

Extensions to Ada 83

17.a
Type extension is a new concept. 

Extensions to Ada 95

17.b/2
{AI95-00344-01} Type extensions now can be declared in more nested scopes than their parent types. Additional accessibility checks on allocators and return statements prevent objects from outliving their type. 

Wording Changes from Ada 95

17.c/2
{AI95-00345-01} Added wording to prevent extending synchronized tagged types.
17.d/2
{AI95-00391-01} Defined null extension for use elsewhere. 

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