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7.5 Limited Types

1
   [{Limited type} [Glossary Entry]A limited type is (a view of) a type for which the assignment operation is not allowed. A nonlimited type is a (view of a) type for which the assignment operation is allowed.]
1.a
Discussion: The concept of the value of a limited type is difficult to define, since the abstract value of a limited type often extends beyond its physical representation. In some sense, values of a limited type cannot be divorced from their object. The value is the object.
1.b
In Ada 83, in the two places where limited types were defined by the language, namely tasks and files, an implicit level of indirection was implied by the semantics to avoid the separation of the value from an associated object. In Ada 95, most limited types are passed by reference, and even return-ed by reference.
1.c
To be honest: For a limited partial view whose full view is nonlimited, assignment is possible on parameter passing and function return. To prevent any copying whatsoever, one should make both the partial and full views limited.

Legality Rules

2
   If a tagged record type has any limited components, then the reserved word limited shall appear in its record_type_definition.
2.a
Reason: This prevents tagged limited types from becoming nonlimited. Otherwise, the following could happen:
2.b
package P is
    type T is limited private;
    type R is tagged
        record -- Illegal!
               -- This should say ``limited record''.
            X : T;
        end record;
private
    type T is new Integer; -- R becomes nonlimited here.
end P;
2.c
package Q is
    type R2(Access_Discrim : access ...) is new R with
        record
            Y : Some_Task_Type;
        end record;
end Q;
2.d
If the above were legal, then assignment would be defined for R'Class in the body of P, which is bad news, given the access discriminant and the task.

Static Semantics

3
   {limited type} A type is limited if it is a descendant of one of the following:
4
4.a
Ramification: Note that there is always a ``definition,'' conceptually, even if there is no syntactic category called ``..._definition''.
5
6
7
   {nonlimited type} Otherwise, the type is nonlimited.
8
   [There are no predefined equality operators for a limited type.]
NOTES
9
13  The following are consequences of the rules for limited types:
10
11
12
13
14
14  Aggregates are not available for a limited composite type. Concatenation is not available for a limited array type.
15
15  The rules do not exclude a default_expression for a formal parameter of a limited type; they do not exclude a deferred constant of a limited type if the full declaration of the constant is of a nonlimited type.
16
16  {become nonlimited} {nonlimited type (becoming nonlimited)} {limited type (becoming nonlimited)} As illustrated in 7.3.1, an untagged limited type can become nonlimited under certain circumstances.
16.a
Ramification: Limited private types do not become nonlimited; instead, their full view can be nonlimited, which has a similar effect.
16.b
It is important to remember that a single nonprivate type can be both limited and nonlimited in different parts of its scope. In other words, ``limited'' is a property that depends on where you are in the scope of the type. We don't call this a ``view property'' because there is no particular declaration to declare the nonlimited view.
16.c
Tagged types never become nonlimited.

Examples

17
    Example of a package with a limited type:
18
package IO_Package is
   type File_Name is limited private;
19
   procedure Open (F : in out File_Name);
   procedure Close(F : in out File_Name);
   procedure Read (F : in File_Name; Item : out Integer);
   procedure Write(F : in File_Name; Item : in  Integer);
private
   type File_Name is
      limited record
         Internal_Name : Integer := 0;
      end record;
end IO_Package;
20
package body IO_Package is
   Limit : constant := 200;
   type File_Descriptor is record  ...  end record;
   Directory : array (1 .. Limit) of File_Descriptor;
   ...
   procedure Open (F : in out File_Name) is  ...  end;
   procedure Close(F : in out File_Name) is  ...  end;
   procedure Read (F : in File_Name; Item : out Integer) is ... end;
   procedure Write(F : in File_Name; Item : in  Integer) is ... end;
begin
   ...
end IO_Package;
NOTES
21
17  Notes on the example: In the example above, an outside subprogram making use of IO_Package may obtain a file name by calling Open and later use it in calls to Read and Write. Thus, outside the package, a file name obtained from Open acts as a kind of password; its internal properties (such as containing a numeric value) are not known and no other operations (such as addition or comparison of internal names) can be performed on a file name. Most importantly, clients of the package cannot make copies of objects of type File_Name.
22
This example is characteristic of any case where complete control over the operations of a type is desired. Such packages serve a dual purpose. They prevent a user from making use of the internal structure of the type. They also implement the notion of an encapsulated data type where the only operations on the type are those given in the package specification.
23
The fact that the full view of File_Name is explicitly declared limited means that parameter passing and function return will always be by reference (see 6.2 and 6.5).

Extensions to Ada 83

23.a
{extensions to Ada 83} The restrictions in RM83-7.4.4(4), which disallowed out parameters of limited types in certain cases, are removed.

Wording Changes from Ada 83

23.b
Since limitedness and privateness are orthogonal in Ada 95 (and to some extent in Ada 83), this is now its own clause rather than being a subclause of 7.3, ``Private Types and Private Extensions''.

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