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3.9.4 Interface Types

An interface type is an abstract tagged type that provides a restricted form of multiple inheritance. A tagged type, task type, or protected type may have one or more interface types as ancestors.


interface_type_definition ::= 
    [limited | task | protected | synchronizedinterface [and interface_list]
interface_list ::= interface_subtype_mark {and interface_subtype_mark}

Static Semantics

An interface type (also called an interface) is a specific abstract tagged type that is defined by an interface_type_definition.
An interface with the reserved word limited, task, protected, or synchronized in its definition is termed, respectively, a limited interface, a task interface, a protected interface, or a synchronized interface. In addition, all task and protected interfaces are synchronized interfaces, and all synchronized interfaces are limited interfaces. 
A task or protected type derived from an interface is a tagged type. Such a tagged type is called a synchronized tagged type, as are synchronized interfaces and private extensions whose declaration includes the reserved word synchronized.
A task interface is an abstract task type. A protected interface is an abstract protected type. 
An interface type has no components. 
An interface_subtype_mark in an interface_list names a progenitor subtype; its type is the progenitor type. An interface type inherits user-defined primitive subprograms from each progenitor type in the same way that a derived type inherits user-defined primitive subprograms from its progenitor types (see 3.4).

Legality Rules

All user-defined primitive subprograms of an interface type shall be abstract subprograms or null procedures.
The type of a subtype named in an interface_list shall be an interface type.
A type derived from a nonlimited interface shall be nonlimited.
An interface derived from a task interface shall include the reserved word task in its definition; any other type derived from a task interface shall be a private extension or a task type declared by a task declaration (see 9.1).
An interface derived from a protected interface shall include the reserved word protected in its definition; any other type derived from a protected interface shall be a private extension or a protected type declared by a protected declaration (see 9.4).
An interface derived from a synchronized interface shall include one of the reserved words task, protected, or synchronized in its definition; any other type derived from a synchronized interface shall be a private extension, a task type declared by a task declaration, or a protected type declared by a protected declaration.
No type shall be derived from both a task interface and a protected interface.
In addition to the places where Legality Rules normally apply (see 12.3), these rules apply also in the private part of an instance of a generic unit.

Dynamic Semantics

The elaboration of an interface_type_definition creates the interface type and its first subtype. 
NOTE   Nonlimited interface types have predefined nonabstract equality operators. These can be overridden with user-defined abstract equality operators. Such operators will then require an explicit overriding for any nonabstract descendant of the interface.


Example of a limited interface and a synchronized interface extending it:
type Queue is limited interface;
procedure Append(Q : in out Queue; Person : in Person_Name) is abstract;
procedure Remove_First(Q      : in out Queue;
                       Person :    out Person_Name) is abstract;
function Cur_Count(Q : in Queue) return Natural is abstract;
function Max_Count(Q : in Queue) return Natural is abstract;
-- See 3.10.1 for Person_Name.
Queue_Error : exception;
-- Append raises Queue_Error if Cur_Count(Q) = Max_Count(Q)
-- Remove_First raises Queue_Error if Cur_Count(Q) = 0
type Synchronized_Queue is
                      synchronized interface and Queue; -- see 9.11
procedure Append_Wait(Q      : in out Synchronized_Queue;
                      Person : in     Person_Name) is abstract;
procedure Remove_First_Wait(Q      : in out Synchronized_Queue;
                            Person :    out Person_Name) is abstract;
procedure Transfer(From   : in out Queue'Class;
                   To     : in out Queue'Class;
                   Number : in     Natural := 1) is
   Person : Person_Name;
   for I in 1..Number loop
      Remove_First(From, Person);
      Append(To, Person);
   end loop;
end Transfer;
This defines a Queue interface defining a queue of people. (A similar design is possible to define any kind of queue simply by replacing Person_Name by an appropriate type.) The Queue interface has four dispatching operations, Append, Remove_First, Cur_Count, and Max_Count. The body of a class-wide operation, Transfer is also shown. Every nonabstract extension of Queue will provide implementations for at least its four dispatching operations, as they are abstract. Any object of a type derived from Queue can be passed to Transfer as either the From or the To operand. The two operands can be of different types in a given call.
The Synchronized_Queue interface inherits the four dispatching operations from Queue and adds two additional dispatching operations, which wait if necessary rather than raising the Queue_Error exception. This synchronized interface can only be implemented by a task or protected type, and as such ensures safe concurrent access.
Example use of the interface:
type Fast_Food_Queue is new Queue with record ...;
procedure Append(Q : in out Fast_Food_Queue; Person : in Person_Name);
procedure Remove_First(Q : in out Fast_Food_Queue; 
                       Person : out Person_Name);
function Cur_Count(Q : in Fast_Food_Queue) return Natural;
function Max_Count(Q : in Fast_Food_Queue) return Natural;
Cashier, Counter : Fast_Food_Queue;
-- Add Casey (see 3.10.1) to the cashier's queue:
Append (Cashier, Casey);
-- After payment, move Casey to the sandwich counter queue:
Transfer (Cashier, Counter);
An interface such as Queue can be used directly as the parent of a new type (as shown here), or can be used as a progenitor when a type is derived. In either case, the primitive operations of the interface are inherited. For Queue, the implementation of the four inherited routines will necessarily be provided. Inside the call of Transfer, calls will dispatch to the implementations of Append and Remove_First for type Fast_Food_Queue.
Example of a task interface:
type Serial_Device is task interface;  -- see 9.1
procedure Read (Dev : in Serial_Device; C : out Character) is abstract;
procedure Write(Dev : in Serial_Device; C : in  Character) is abstract;
The Serial_Device interface has two dispatching operations which are intended to be implemented by task entries (see 9.1).

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