9.4 Protected Units and Protected Objects
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A protected object provides coordinated access to shared data, through calls on its visible protected operations, which can be protected subprograms or protected entries. A protected unit is declared by a protected declaration, which has a corresponding
protected_body. A protected declaration may be a
protected_type_declaration, in which case it declares a named protected type; alternatively, it may be a
single_protected_declaration, in which case it defines an anonymous protected type, as well as declaring a named protected object of that type.
Paragraph 10 was deleted.
protected_definition defines a protected type and its first subtype. The list of
protected_operation_declarations of a
protected_definition, together with the
known_discriminant_part, if any, is called the visible part of the protected unit. [ The optional list of
protected_element_declarations after the reserved word private is called the private part of the protected unit.]
For a protected declaration with an
interface_list, the protected type inherits user-defined primitive subprograms from each progenitor type (see 3.9.4), in the same way that a derived type inherits user-defined primitive subprograms from its progenitor types (see 3.4). If the first parameter of a primitive inherited subprogram is of the protected type or an access parameter designating the protected type, and there is a
protected_operation_declaration for a protected subprogram or single entry with the same identifier within the protected declaration, whose profile is type conformant with the prefixed view profile of the inherited subprogram, the inherited subprogram is said to be implemented by the conforming protected subprogram or entry using an implicitly declared nonabstract subprogram which has the same profile as the inherited subprogram and which overrides it.
interface_listonly name interface types, and limits the descendants of the various kinds of interface types. Only a limited, protected, or synchronized interface can have a protected type descendant. Nonlimited or task interfaces are not allowed, as they offer operations that a protected type does not have.
The prefixed view profile of an explicitly declared primitive subprogram of a tagged protected type shall not be type conformant with any protected operation of the protected type, if the subprogram has the same defining name as the protected operation and the first parameter of the subprogram is of the protected type or is an access parameter designating the protected type.
For each primitive subprogram inherited by the type declared by a protected declaration, at most one of the following shall apply:
- the inherited subprogram is overridden with a primitive subprogram of the protected type, in which case the overriding subprogram shall be subtype conformant with the inherited subprogram and not abstract; or
- the inherited subprogram is implemented by a protected subprogram or single entry of the protected type, in which case its prefixed view profile shall be subtype conformant with that of the protected subprogram or entry.
If neither applies, the inherited subprogram shall be a null procedure. In addition to the places where Legality Rules normally apply (see 12.3), these rules also apply in the private part of an instance of a generic unit.
If an inherited subprogram is implemented by a protected procedure or an entry, then the first parameter of the inherited subprogram shall be of mode out or in out, or an access-to-variable parameter. If an inherited subprogram is implemented by a protected function, then the first parameter of the inherited subprogram shall be of mode in, but not an access-to-variable parameter.
If a protected subprogram declaration has an
overriding_indicator, then at the point of the declaration:
- if the
overriding_indicatoris overriding, then the subprogram shall implement an inherited subprogram; 11.12/2
- if the
overriding_indicatoris not overriding, then the subprogram shall not implement any inherited subprogram.
In addition to the places where Legality Rules normally apply (see 12.3), these rules also apply in the private part of an instance of a generic unit.
[The elaboration of a
protected_definition creates the protected type and its first subtype;] it also includes the elaboration of the
protected_operation_declarations in the given order.
[As part of the initialization of a protected object, any per-object constraints (see 3.8) are elaborated.]
The elaboration of a
protected_body has no other effect than to establish that protected operations of the type can from then on be called without failing the Elaboration_Check.
The content of an object of a given protected type includes:
- The values of the components of the protected object, including (implicitly) an entry queue for each entry declared for the protected object;
- A representation of the state of the execution resource associated with the protected object (one such resource is associated with each protected object).
[The execution resource associated with a protected object has to be acquired to read or update any components of the protected object; it can be acquired (as part of a protected action — see 9.5.1) either for concurrent read-only access, or for exclusive read-write access.]
As the first step of the finalization of a protected object, each call remaining on any entry queue of the object is removed from its queue and Program_Error is raised at the place of the corresponding
Bounded (Run-Time) Errors20.1/2
It is a bounded error to call an entry or subprogram of a protected object after that object is finalized. If the error is detected, Program_Error is raised. Otherwise, the call proceeds normally, which may leave a task queued forever.
with Ada.Finalization.Controlled; package Window_Manager is ... type Root_Window is new Ada.Finalization.Controlled with private; type Any_Window is access all Root_Window; ... private ... procedure Finalize (Object : in out Root_Window); ... end Window_Manager; 20.j/2package body Window_Manager is protected type Lock is entry Get_Lock; procedure Free_Lock; ... end Lock; 20.k/2Window_Lock : Lock; 20.l/2procedure Finalize (Object : in out Root_Window) is begin Window_Lock.Get_Lock; ... Window_Lock.Free_Lock; end Finalize; ... A_Window : Any_Window := new Root_Window; end Window_Manager;
protected_bodydefine the actions that take place upon calls to the protected operations.
Component_declarations are disallowed in a
protected_bodybecause, for efficiency, we wish to allow the compiler to determine the size of protected objects (when not dynamic); the compiler cannot necessarily see the body. Furthermore, the semantics of initialization of such objects would be problematic — we do not wish to give protected objects complex initialization semantics similar to task activation.
entry_declarations, since an entry involves an implicit component — the entry queue.
Example of declaration of protected type and corresponding body:
protected type Resource is entry Seize; procedure Release; private Busy : Boolean := False; end Resource; 28protected body Resource is entry Seize when not Busy is begin Busy := True; end Seize; 29procedure Release is begin Busy := False; end Release; end Resource;
Example of a single protected declaration and corresponding body:
protected Shared_Array is -- Index, Item, and Item_Array are global types function Component (N : in Index) return Item; procedure Set_Component(N : in Index; E : in Item); private Table : Item_Array(Index) := (others => Null_Item); end Shared_Array; 32protected body Shared_Array is function Component(N : in Index) return Item is begin return Table(N); end Component; 33procedure Set_Component(N : in Index; E : in Item) is begin Table(N) := E; end Set_Component; end Shared_Array;
Examples of protected objects:
Control : Resource;
Flags : array(1 .. 100) of Resource;
Extensions to Ada 83
Extensions to Ada 95
Overriding_indicators can be used to specify whether or not a protected operation implements a primitive operation.
Wording Changes from Ada 95