A.18.30 The Generic Package Containers.Unbounded_Priority_Queues
Static Semantics
The language-defined generic package Containers.Unbounded_Priority_Queues
provides type Queue, which implements the interface type Containers.Synchronized_Queue_Interfaces.Queue.
with System;
with Ada.Containers.Synchronized_Queue_Interfaces;
generic
with package Queue_Interfaces
is new Ada.Containers.Synchronized_Queue_Interfaces (<>);
type Queue_Priority
is private;
with function Get_Priority
(Element : Queue_Interfaces.Element_Type)
return Queue_Priority
is <>;
with function Before
(Left, Right : Queue_Priority)
return Boolean
is <>;
Default_Ceiling : System.Any_Priority := System.Priority'Last;
package Ada.Containers.Unbounded_Priority_Queues
is
pragma Preelaborate(Unbounded_Priority_Queues);
package Implementation is
... -- not specified by the language
end Implementation;
protected type Queue
(Ceiling : System.Any_Priority := Default_Ceiling)
with Priority => Ceiling
is
new Queue_Interfaces.Queue
with
overriding
entry Enqueue (New_Item :
in Queue_Interfaces.Element_Type);
overriding
entry Dequeue (Element :
out Queue_Interfaces.Element_Type);
not overriding
procedure Dequeue_Only_High_Priority
(At_Least :
in Queue_Priority;
Element :
in out Queue_Interfaces.Element_Type;
Success :
out Boolean);
overriding
function Current_Use
return Count_Type;
overriding
function Peak_Use
return Count_Type;
private
... -- not specified by the language
end Queue;
private
... -- not specified by the language
end Ada.Containers.Unbounded_Priority_Queues;
The type Queue is used to represent task-safe priority
queues.
The capacity for instances of type Queue is unbounded.
Two elements E1 and E2 are equivalent
if Before(Get_Priority(E1), Get_Priority(E2)) and Before(Get_Priority(E2),
Get_Priority(E1)) both return False.
The actual functions for Get_Priority and Before
are expected to return the same value each time they are called with
the same actuals, and should not modify their actuals. Before should
define a strict weak ordering relationship (see
A.18).
If the actual functions behave in some other manner, the behavior of
Unbounded_Priority_Queues is unspecified.
Enqueue inserts an item according to the order specified
by the Before function on the result of Get_Priority on the elements;
Before should return True if Left is to be inserted before Right. If
the queue already contains elements equivalent to New_Item, then it is
inserted after the existing equivalent elements.
For a call on Dequeue_Only_High_Priority, if the
head of the nonempty queue is E, and the function Before(At_Least,
Get_Priority(E)) returns False, then E is assigned to Element
and then removed from the queue, and Success is set to True; otherwise,
Success is set to False and Element is unchanged.
Ada 2005 and 2012 Editions sponsored in part by Ada-Europe
