7.5 Limited Types
A limited type is (a view of) a type for which copying
(such as for an
assignment_statement)
is not allowed. A nonlimited type is a (view of a) type for which copying
is allowed.
Legality Rules
If a tagged record type has any limited components,
then the reserved word
limited shall appear in its
record_type_definition.
If the reserved word
limited appears in the definition of a
derived_type_definition,
its parent type and any progenitor interfaces shall be limited.
Static Semantics
A
view of a type is
limited if it is one of the following:
a type with the reserved word limited, synchronized,
task, or protected in its definition;
a class-wide type whose specific type is limited;
a composite type with a limited component;
an incomplete view;
a derived type whose parent is limited and is not
an interface.
Otherwise, the type is nonlimited.
There are no predefined equality operators for a
limited type.
A type is
immutably
limited if it is one of the following:
An explicitly limited record type;
A record extension with the reserved word limited;
A nonformal limited private type that is tagged
or has at least one access discriminant with a
default_expression;
A task type, a protected type, or a synchronized
interface;
A type derived from an immutably limited type.
A descendant of a generic formal limited private
type is presumed to be immutably limited except within the body of a
generic unit or a body declared within the declarative region of a generic
unit, if the formal type is declared within the formal part of the generic
unit.
15 While it is allowed to write initializations
of limited objects, such initializations never copy a limited object.
The source of such an assignment operation must be an
aggregate
or
function_call,
and such
aggregates
and
function_calls
must be built directly in the target object (see
7.6).
Paragraphs 10 through
15 were deleted.
16
As
illustrated in
7.3.1, an untagged limited
type can become nonlimited under certain circumstances.
Examples
Example of a package
with a limited type:
package IO_Package is
type File_Name is limited private;
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;
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;
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.
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.
The fact that the full view of File_Name is explicitly
declared
limited means that parameter passing will always be by
reference and function results will always be built directly in the result
object (see
6.2 and
6.5).
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