7.5 Limited Types

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 is possible on parameter passing and function return. To prevent any copying whatsoever, one should make both the partial and full views limited.

Term entry: limited type — type for which copying (such as in an assignment_statement) is not allowed
Note: All types are either limited types or nonlimited types.

Term entry: nonlimited type — type for which copying is allowed

Proof: {AI95-00419-01} {AI12-0005-1} 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: 

{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 task. 

Ramification: {AI12-0127-1} We don't need to mention the base_expression of an array_delta_aggregate here, as its type cannot be limited (see 4.3.4), and thus neither can its base_expression. Similarly, we do not need any rules for components of a delta_aggregate nor the elements of a container_aggregate, as neither are allowed to be limited (see 4.3.4 and 4.3.5) 

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. 

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 limited is present. 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. 

Reason: The full type in both of these cases must necessarily be immutably limited. We need to include private types as much as possible so that we aren't unintentionally discouraging the use of private types. 

Discussion: An immutably limited type is a type that cannot become nonlimited subsequently in a private part or in a child unit. If a view of the type makes it immutably limited, then no copying (assignment) operations are ever available for objects of the type. This allows other properties; for instance, it is safe for such objects to have access discriminants that have defaults or designate other limited objects. 

Ramification: A nonsynchronized limited interface type is not immutably limited; a type derived from it can be nonlimited. 

Ramification: In an instance, a type is descended from the actual type corresponding to the formal, and all rules are rechecked in the specification. Bodies are excepted so that we assume the worst there; the complex wording is required to handle children of generics and unrelated bodies properly. 

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. 

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. 

The restrictions in RM83-7.4.4(4), which disallowed out parameters of limited types in certain cases, are removed. 

{AI05-0299-1} Since limitedness and privateness are orthogonal in Ada 95 (and to some extent in Ada 83), this is now its own subclause rather than being a subclause of 7.3, “Private Types and Private Extensions”. 

{AI95-00287-01} {AI95-00318-02} 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”. 

{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. 

{AI05-0052-1} {AI05-0217-1} Correction: Added a definition for immutably limited types, so that the fairly complex definition does not need to be repeated in rules elsewhere in the Reference Manual.

{AI05-0067-1} {AI05-0299-1} Correction: The built-in-place rules are consolidated in 7.6, and thus they are removed from this subclause.

{AI05-0087-1} Correction: Fixed an oversight: class-wide types were never defined to be limited, even if their associated specific type is. It is thought that this oversight was never implemented incorrectly by any compiler, thus we have not classified it as an incompatibility.

{AI05-0147-1} Allowed conditional_expressions in limited constructor contexts — we want to treat these as closely to parentheses as possible.

{AI05-0177-1} Added wording so that expression functions can return limited entities.

{AI05-0178-1} Correction: Added incomplete views to the list of reasons for a view of a type to be limited. This is not a change as the definition already was in 3.10.1. But it is much better to have all of the reasons for limitedness together. 

{AI12-0172-1} Correction: A raise_expression can be used in an expression used in a limited context. This is harmless (no object will be created if an exception is raised instead), useful, and thus appears to have been an omission when raise_expressions were added to the language. 

{AI12-0127-1} Added the base_expression of a delta_aggregate as a limited context.