From: "Phil Slater" <phil.slater@baesystems.com>
Subject: Impossible problem? A protected buffer to queue objects of a class-wide type
Date: Wed, 11 Apr 2007 12:34:15 +0100
Date: 2007-04-11T12:34:15+01:00 [thread overview]
Message-ID: <461cc46d$1_1@glkas0286.greenlnk.net> (raw)
I've hit a brick wall. Every strategy I try doesn't work.
I need to write a generic package that exports a protected queue type that
will act as a buffer for objects of a class-wide type. My strategy is that
when objects join the queue they are copied onto the heap, and an
access-to-class-wide value is put onto the queue (could be array-based or
linked-list-based - I don't mind). To retrieve an item from the queue, an
access-to-class-wide value is removed from the queue, the heap object it
designates is copied onto the stack, the heap object is deallocated, then
the copy is returned.
The crux of the problem goes like this:
(a) for the client code to retrieve an item, it needs to call a function; it
can't be done through a procedure or entry "out" parameter since I can't
declare an uninitialised variable of the class-wide type to pass as an
actual parameter. So the client code needs to do something like this:
declare
Next_In_Line : Item'class := My_Buffer.Dequeue; -- My_Buffer is the
protected queue
(b) To support this call, Dequeue must be written as a function. As such, it
cannot change the protected queue. What I need is the "entry" functionality,
since I want the Dequeue to wait if the queue is empty, and I want the item
to be removed from the queue as well as retrieved.
(c) In a non-concurrent environment, I would simply write two separate
operations on the queue: a function Read_Next_Item and a procedure
Delete_Next_Item, which the client code would call in turn. However, that's
no good in a concurrent environment - two tasks could both call
Read_Next_Item before the first has had a chance to call Delete_Next_Item,
so the same item is read twice and the following item is skipped.
I really do want all the heap operations through the access-to-class-wide
type to be hidden away in the package - I did consider writing a protected
buffer with an entry whose out parameter was an access-to-class-wide type,
but this is highly unsatisfactory as it puts the onus for deallocating heap
space into the client code - very messy.
I cannot see a way through this problem, and am astounded that a language of
this calibre seems to have rules that conspire together to make it appear
impossible. What have I missed?
next reply other threads:[~2007-04-11 11:34 UTC|newest]
Thread overview: 10+ messages / expand[flat|nested] mbox.gz Atom feed top
2007-04-11 11:34 Phil Slater [this message]
2007-04-11 12:25 ` Impossible problem? A protected buffer to queue objects of a class-wide type Rob Norris
2007-04-11 13:02 ` Phil Slater
2007-04-12 8:36 ` Stephen Leake
2007-04-11 13:14 ` Dmitry A. Kazakov
2007-04-11 14:01 ` Jean-Pierre Rosen
2007-04-11 17:18 ` tmoran
2007-04-13 17:02 ` Matthew Heaney
2007-04-13 17:26 ` Matthew Heaney
2007-04-13 17:36 ` Ed Falis
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