Annotated Ada Reference ManualLegal Information
Table of Contents   Index   References   Search   Previous   Next 

 7.5 Limited Types

{AI95-00287-01} [A limited type is (a view of) a type for which copying (such as for an assignment_statement) the assignment operation is not allowed. A nonlimited type is a (view of a) type for which copying the assignment operation is allowed.] 
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.
{AI95-00318-02} 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. In Ada 2005, most limited types are built-in-place upon return, rather than returned by reference. Thus the object “identity” is part of the logical value of most limited types. 
To be honest: {AI95-00287-01} {AI95-00419-01} For a limited partial view whose full view is nonlimited, copying assignment is possible on parameter passing and function return. To prevent any copying whatsoever, one should make both the partial and full views limited. 
Glossary entry: {Limited type} A limited type is (a view of) a type for which copying (such as in an assignment_statement) the assignment operation is not allowed. A nonlimited type is a (view of a) type for which copying the assignment operation is allowed.

Legality Rules

{AI95-00419-01} 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.]
Proof: {AI95-00419-01} The rule about the parent type being required to be limited can be found in 3.4. Rules about progenitor interfaces can be found in 3.9.4, specifically, a nonlimited interface can appear only on a nonlimited type. We repeat these rules here to gather these scattered rules in one obvious place. 
Reason: This prevents tagged limited types from becoming nonlimited. Otherwise, the following could happen: 
package P is
    type T is limited private;
    type R is tagged
        record -- Illegal!
               -- This should say “limited record”.
            X : T;
        end record;
    type T is new Integer; -- R becomes nonlimited here.
end P;
package Q is
    type R2(Access_Discrim : access ...) is new R with
            Y : Some_Task_Type;
        end record;
end Q;
{AI95-00230-01} 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. 
  {AI95-00287-01} {AI95-00318-02} In the following contexts, an expression of a limited type is not permitted unless it is an aggregate, a function_call, or a parenthesized expression or qualified_expression whose operand is permitted by this rule: 
Discussion: All of these contexts normally require copying; by restricting the uses as above, we can require the new object to be built-in-place. 

Static Semantics

{AI95-00419-01} {limited type} A type is limited if it is a descendant of one of the following: 
Ramification: Note that there is always a “definition,” conceptually, even if there is no syntactic category called “..._definition”.
{AI95-00419-01} This includes interfaces of the above kinds, derived types with the reserved word limited, as well as task and protected types. 
Ramification: {AI95-00419-01} Limitedness is not inherited from interfaces; it must be explicitly specified when the parent is an interface.
To be honest: {AI95-00419-01} A derived type can become nonlimited if limited does not appear and the derivation takes place in the visible part of a child package, and the parent type is nonlimited as viewed from the private part or body of the child package. 
Reason: {AI95-00419-01} We considered a rule where limitedness was always inherited from the parent for derived types, but in the case of a type whose parent is an interface, this meant that the first interface is treated differently than other interfaces. It also would have forced users to declare dummy nonlimited interfaces just to get the limitedness right. We also considered a syntax like not limited to specify nonlimitedness when the parent was limited, but that was unsavory. The rule given is more uniform and simpler to understand.
{AI95-00419-01} The rules for interfaces are asymmetrical, but the language is not: if the parent interface is limited, the presence of the word limited determines the limitedness, and nonlimited progenitors are illegal by the rules in 3.9.4. If the parent interface is nonlimited, the word limited is illegal by the rules in 3.4. The net effect is that the order of the interfaces doesn't matter. 
{nonlimited type} Otherwise, the type is nonlimited.
[There are no predefined equality operators for a limited type.] 

Implementation Requirements

  {AI95-00287-01} {AI95-00318-02} For an aggregate of a limited type used to initialize an object as allowed above, the implementation shall not create a separate anonymous object for the aggregate. For a function_call of a type with a part that is of a task, protected, or explicitly limited record type that is used to initialize an object as allowed above, the implementation shall not create a separate return object (see 6.5) for the function_call. The aggregate or function_call shall be constructed directly in the new object. 
Discussion: {AI95-00318-02} For a function_call, we only require build-in-place{build-in-place [partial]} for a limited type that would have been a return-by-reference type in Ada 95. We do this because we want to minimize disruption to Ada 95 implementations and users. 
14  {AI95-00287-01} {AI95-00318-02} 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. The following are consequences of the rules for limited types: 
To be honest: This isn't quite true if the type can become nonlimited (see below); function_calls only are required to be build-in-place for “really” limited types. 
Paragraphs 10 through 15 were deleted. 
15  {AI95-00287-01} Aggregates are not available for a limited composite type. Concatenation is not available for a limited array type.
16  {AI95-00287-01} 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.
17  {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. 
Ramification: Limited private types do not become nonlimited; instead, their full view can be nonlimited, which has a similar effect.
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.
Tagged types never become nonlimited. 


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);
   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;
end IO_Package;
18  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.
{AI-00318-02} 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 and function results will always be built directly in the result object (see 6.2 and 6.5).

Extensions to Ada 83

{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

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”. 

Extensions to Ada 95

{AI95-00287-01} {AI95-00318-02} {extensions to Ada 95} Limited types now have an assignment operation, but its use is restricted such that all uses are build-in-place. This is accomplished by restricting uses to aggregates and function_calls. Aggregates were not allowed to have a limited type in Ada 95, which causes a compatibility issue discussed in 4.3, “Aggregates”. Compatibility issues with return statements for limited function_calls are discussed in 6.5, “Return Statements”. 

Wording Changes from Ada 95

{AI95-00411-01} {AI95-00419-01} Rewrote the definition of limited to ensure that interfaces are covered, but that limitedness is not inherited from interfaces. Derived types that explicitly include limited are now also covered.

Table of Contents   Index   References   Search   Previous   Next 
Ada-Europe Sponsored by Ada-Europe