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.
incomplete_type_declaration ::= type defining_identifier [discriminant_part];
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.
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.
The only allowed uses of a name that denotes an incomplete_type_declaration are 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;
- as the subtype_mark defining the subtype of a parameter or result of an access_to_subprogram_definition;
- as the subtype_mark in an access_definition;
- as the prefix of an attribute_reference whose attribute_designator is Class; such an attribute_reference is similarly restricted to the uses allowed here; when used in this way, the corresponding full_type_declaration shall declare a tagged type, and the attribute_reference shall occur in the same library unit as the incomplete_type_declaration.
A dereference (whether implicit or explicit -- see 4.1) shall not be of an incomplete type.
An incomplete_type_declaration declares an incomplete type and its first subtype; the first subtype is unconstrained if a known_discriminant_part appears.
The elaboration of an incomplete_type_declaration has no effect.
80 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).
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:
type Person(<>); -- incomplete type declaration type Car; -- incomplete type declaration
type Person_Name is access Person; type Car_Name is access all Car;
type Car is 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;
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Copyright © 2000 The MITRE Corporation, Inc. Ada Reference Manual