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3.10.1 Incomplete Type Declarations

1
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

2.1/4
 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. If T denotes a tagged incomplete view, then T'Class denotes 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
2.4/3
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, or
2.5/3
it occurs within the scope of the completion of T and T is an incomplete view declared by an incomplete_type_declaration.
2.6/3
 In these cases, the dereference has the view of T visible at the point of the dereference. 
2.7/3
 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 three circumstances given above, in which case it denotes the view of T visible at the point of the subtype_mark.

Legality Rules

3/3
An incomplete_type_declaration requires a completion, which shall be a type_declaration other than an incomplete_type_declaration. If the incomplete_type_declaration occurs immediately within either the visible part of a package_specification or a declarative_part, then the 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 type_declaration shall occur later and immediately within either the private part itself, or the declarative_part of the corresponding package_body.
4/3
If an incomplete_type_declaration includes the reserved word tagged, then a type_declaration that completes it shall declare a tagged type. If an incomplete_type_declaration has a known_discriminant_part, then a 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 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: 
6/3
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 (a null_exclusion is not allowed); 
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;
8/3
as the subtype_mark in an access_definition for an access-to-object type; 
8.1/3
as the subtype_mark defining the subtype of a parameter or result in a profile occurring within a basic_declaration;
8.2/3
as a generic actual parameter whose corresponding generic formal parameter is a formal incomplete type (see 12.5.1).
8.3/2
 If such a name denotes a tagged incomplete view, it may also be used:
8.4/3
as the subtype_mark defining the subtype of a parameter in the profile for a subprogram_body, entry_body, or accept_statement;
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. 
9.1/3
 This paragraph was deleted.
9.2/3
This paragraph was deleted.
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.
10/3
A prefix that denotes an object shall not be of an incomplete view. An actual parameter in a call shall not be of an untagged incomplete view. The result object of a function call shall not be of an incomplete view. A prefix shall not denote a subprogram having a formal parameter of an untagged incomplete view, nor a return type that is an incomplete view. 
Paragraph 11 was deleted. 

Dynamic Semantics

12
The elaboration of an incomplete_type_declaration has no effect. 
NOTES
13
89  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).
13.1/3
90  A name that denotes an object of an incomplete view is defined to be of a limited type. Hence, the target of an assignment statement cannot be of an incomplete view.

Examples

14
Example of a recursive type: 
15
type Cell;  --  incomplete type declaration
type Link is access Cell;
16
type Cell is
   record
      Value  : Integer;
      Succ   : Link;
      Pred   : Link;
   end record;
17
Head   : Link  := new Cell'(0, null, null);
Next   : Link  := Head.Succ;
18
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;
22
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;
23
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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