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7.5 Limited Types
1
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.
Legality Rules
2
If a tagged record type has any limited components,
then the reserved word limited shall appear in its record_type_definition.
Static Semantics
3
A
type is
limited if it is a descendant of one of the following:
4
- a type with the reserved word limited
in its definition;
5
- a task or protected type;
6
- a composite type with a limited component.
7
Otherwise, the type is nonlimited.
8
There are no predefined equality operators for
a limited type.
9
13 The
following are consequences of the rules for limited types:
10
- An initialization
expression is not allowed in an object_declaration
if the type of the object is limited.
11
- A default expression
is not allowed in a component_declaration
if the type of the record component is limited.
12
- An initialized
allocator is not allowed if the designated type is limited.
13
- A generic formal
parameter of mode in must not be of a limited type.
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 As
illustrated in 7.3.1, an untagged limited
type can become nonlimited under certain circumstances.
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;
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).
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