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
Static Semantics
2.1/2
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
3
4/2
5/2
A
name
that denotes an incomplete view of a type may be used as follows:
6
7/2
8/2
8.1/2
If such a
name
denotes a tagged incomplete view, it may also be used:
8.2/2
9/2
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
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
12
13
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;