A.18.30 The Generic Package Containers.Unbounded_Priority_Queues
Static Semantics
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The language-defined generic package Containers.Unbounded_Priority_Queues
provides type Queue, which implements the interface type Containers.Synchronized_Queue_Interfaces.Queue.2/3
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);
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package Implementation is
... -- not specified by the language
end Implementation;
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protected type Queue
(Ceiling: System.Any_Priority := Default_Ceiling)
with Priority => Ceiling is
new Queue_Interfaces.Queue with
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overriding
entry Enqueue (New_Item : in Queue_Interfaces.Element_Type);
overriding
entry Dequeue (Element : out Queue_Interfaces.Element_Type);
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not overriding
procedure Dequeue_Only_High_Priority
(At_Least : in Queue_Priority;
Element : in out Queue_Interfaces.Element_Type;
Success : out Boolean);7/3
overriding
function Current_Use return Count_Type;
overriding
function Peak_Use return Count_Type;
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private
... -- not specified by the language
end Queue;
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private
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... -- not specified by the language
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end Ada.Containers.Unbounded_Priority_Queues;
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The type Queue is used to represent task-safe priority
queues.13/3
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The capacity for instances of type Queue is unbounded.14/3
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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.15/3
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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.16/3
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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.16.a/3
Ramification: Enqueue
never blocks; if more storage is needed for a new element, it is allocated
dynamically. We don't need to explicitly specify that Queue needs finalization,
because it is visibly protected.
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For a call on Dequeue_Only_High_Priority, if the
head of the non-empty 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.17.a/3
Ramification: {
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Unlike Dequeue, Dequeue_Only_High_Priority is not
blocking; it always returns immediately.
17.b/3
Reason: {
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The use of Before is "backwards" so that
it acts like ">=" (it is defined similarly to ">");
thus we dequeue only when it is False.
Extensions to Ada 2005
17.c/3
Ada 2005 and 2012 Editions sponsored in part by Ada-Europe
