Ada Reference Manual — Legal Information

3.10.1 Incomplete Type Declarations

There are no particular limitations on the designated type of an access type. In particular, the type of a component of the designated type can be another access type, or even the same access type. This permits mutually dependent and recursive access types. An incomplete_type_declaration can be used to introduce a type to be used as a designated type, while deferring its full definition to a subsequent full_type_declaration.

Syntax

2/2

incomplete_type_declaration ::= type defining_identifier [discriminant_part] [is tagged];

Static Semantics

2.1/2

An incomplete_type_declaration declares an incomplete view of a type and its first subtype; the first subtype is unconstrained if a discriminant_part appears. If the incomplete_type_declaration includes the reserved word tagged, it declares a tagged incomplete view. An incomplete view of a type is a limited view of the type (see 7.5).

2.2/2

Given an access type A whose designated type T is an incomplete view, a dereference of a value of type A also has this incomplete view except when:

2.3/2

it occurs within the immediate scope of the completion of T, or

2.4/2

it occurs within the scope of a nonlimited_with_clause that mentions a library package in whose visible part the completion of T is declared.

2.5/2

In these cases, the dereference has the full view of T.

2.6/2

Similarly, if a subtype_mark denotes a subtype_declaration defining a subtype of an incomplete view T, the subtype_mark denotes an incomplete view except under the same two circumstances given above, in which case it denotes the full view of T.

Legality Rules

An incomplete_type_declaration requires a completion, which shall be a full_type_declaration. If the incomplete_type_declaration occurs immediately within either the visible part of a package_specification or a declarative_part, then the full_type_declaration shall occur later and immediately within this visible part or declarative_part. If the incomplete_type_declaration occurs immediately within the private part of a given package_specification, then the full_type_declaration shall occur later and immediately within either the private part itself, or the declarative_part of the corresponding package_body.

4/2

If an incomplete_type_declaration includes the reserved word tagged, then a full_type_declaration that completes it shall declare a tagged type. If an incomplete_type_declaration has a known_discriminant_part, then a full_type_declaration that completes it shall have a fully conforming (explicit) known_discriminant_part (see 6.3.1). If an incomplete_type_declaration has no discriminant_part (or an unknown_discriminant_part), then a corresponding full_type_declaration is nevertheless allowed to have discriminants, either explicitly, or inherited via derivation.

5/2

A name that denotes an incomplete view of a type may be used as follows:

as the subtype_mark in the subtype_indication of an access_to_object_definition; the only form of constraint allowed in this subtype_indication is a discriminant_constraint;

7/2

as the subtype_mark in the subtype_indication of a subtype_declaration; the subtype_indication shall not have a null_exclusion or a constraint;

8/2

as the subtype_mark in an access_definition.

8.1/2

If such a name denotes a tagged incomplete view, it may also be used:

8.2/2

as the subtype_mark defining the subtype of a parameter in a formal_part;

9/2

as the prefix of an attribute_reference whose attribute_designator is Class; such an attribute_reference is restricted to the uses allowed here; it denotes a tagged incomplete view.

9.1/2

If such a name occurs within the declaration list containing the completion of the incomplete view, it may also be used:

9.2/2

as the subtype_mark defining the subtype of a parameter or result of an access_to_subprogram_definition.

9.3/2

If any of the above uses occurs as part of the declaration of a primitive subprogram of the incomplete view, and the declaration occurs immediately within the private part of a package, then the completion of the incomplete view shall also occur immediately within the private part; it shall not be deferred to the package body.

9.4/2

No other uses of a name that denotes an incomplete view of a type are allowed.

10/2

A prefix that denotes an object shall not be of an incomplete view.

Static Semantics

11/2

This paragraph was deleted.

Dynamic Semantics

The elaboration of an incomplete_type_declaration has no effect.

NOTES

83 Within a declarative_part, an incomplete_type_declaration and a corresponding full_type_declaration cannot be separated by an intervening body. This is because a type has to be completely defined before it is frozen, and a body freezes all types declared prior to it in the same declarative_part (see 13.14).

Examples

Example of a recursive type:

type Cell; -- incomplete type declaration
type Link is access Cell;

type Cell is
   record
      Value  : Integer;
      Succ   : Link;
      Pred   : Link;
   end record;

Head : Link := new Cell'(0, null, null);
Next : Link := Head.Succ;

Examples of mutually dependent access types:

19/2

type Person(<>); -- incomplete type declaration
type Car is tagged; -- incomplete type declaration

20/2

type Person_Name is access Person;
type Car_Name is access all Car'Class;

21/2

type Car is tagged
   record
      Number  : Integer;
      Owner   : Person_Name;
   end record;

type Person(Sex : Gender) is
   record
      Name     : String(1 .. 20);
      Birth    : Date;
      Age      : Integer range 0 .. 130;
      Vehicle  : Car_Name;
      case Sex is
         when M => Wife           : Person_Name(Sex => F);
         when F => Husband        : Person_Name(Sex => M);
      end case;
   end record;

My_Car, Your_Car, Next_Car : Car_Name := new Car; -- see 4.8
George : Person_Name := new Person(M);
...
George.Vehicle := Your_Car;