Ada Reference Manual (Ada 2022) — Legal Information

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

2/2

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.

2.1/5

In the following contexts, an expression of a limited type is permitted only if each of its operative constituents is newly constructed (see 4.4):

2.2/2

the initialization expression of an object_declaration (see 3.3.1)

2.3/2

the default_expression of a component_declaration (see 3.8)

2.4/2

the expression of a record_component_association (see 4.3.1)

2.5/2

the expression for an ancestor_part of an extension_aggregate (see 4.3.2)

2.6/2

an expression of a positional_array_aggregate or the expression of an array_component_association (see 4.3.3)

2.7/5

the base_expression of a record_delta_aggregate (see 4.3.4)

2.8/5

the qualified_expression of an initialized allocator (see 4.8)

2.9/5

the expression of a return statement (see 6.5)

2.10/5

the return expression of an expression function (see 6.8)

2.11/5

the default_expression or actual parameter for a formal object of mode in (see 12.4)

Static Semantics

3/3

A view of a type is limited if it is one of the following:

4/2

a type with the reserved word limited, synchronized, task, or protected in its definition;

5/3

a class-wide type whose specific type is limited;

6/2

a composite type with a limited component;

6.1/3

an incomplete view;

6.2/2

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.

8.1/3

A type is immutably limited if it is one of the following:

8.2/3

An explicitly limited record type;

8.3/3

A record extension with the reserved word limited;

8.4/3

A nonformal limited private type that is tagged or has at least one access discriminant with a default_expression;

8.5/3

A task type, a protected type, or a synchronized interface;

8.6/3

A type derived from an immutably limited type.

8.7/3

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.

9/5

NOTE 1 While it is allowed to write initializations of limited objects, such initializations never copy a limited object. The source of such an assignment operation will be an aggregate or function_call, and such aggregates and function_calls will be built directly in the target object (see 7.6).

Paragraphs 10 through 15 were deleted.

NOTE 2 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;

21/5

In the example, an outside subprogram making use of IO_Package can 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.

22/5

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.

23/5

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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