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From: Dmitry A. Kazakov <mailbox@dmitry-kazakov.de>
Newsgroups: comp.lang.ada
Subject: Re: new_line in a put_line
Date: Wed, 04 Dec 2002 15:10:59 +0100
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Date: 2002-12-04T15:10:59+01:00
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On 04 Dec 2002 10:31:15 +0100, Fraser Wilson
<newsfraser@blancolioni.org> wrote:

>Dmitry A. Kazakov <mailbox@dmitry-kazakov.de> writes:
>
>> Just do not do I/O from them. Define a mutex as a protected object and
>> a spin lock as a controlled type (less troubles with releasing a
>> mutex):
>
>[..]
>
>>    procedure Put_Line (Text : String) is
>>       Get_It : Lock (Write_Mutex'Access);
>>    begin
>>       Ada.Text_IO.Put_Line (Text);
>>    end Put_Line;
>
>Something inside me rebels at using side-effects from a declaration
>like that. 

Right, right. It is also my opinion [there was a thread regarding this
subject in c.l.a] that using unused (:-)) objects probably indicates a
design problem. A task-based design might be better.

> I can't really talk; my database library unlocks objects
>as they go out of scope, but ... well, I know that code gets executed
>in declarations, but I don't necessarily want to be this aware of it.
>
>On the other hand, it's just a monitor, I shouldn't be so scared :)

You might also pack seizing the mutex into a "proxy" object
(File_Channel):

   type File_Channel is limited private;
   procedure Put (File : in out File_Channel; Text : String);
   procedure Put_Line (File : in out File_Channel; Text : String);
   procedure New_Line (File : in out File_Channel; Text : String);
...
private
   type File_Channel is
      new Ada.Finalization.Limited_Controlled with null record;
   procedure Initialize (File : in out File_Channel);
   procedure Finalize (File : in out File_Channel);

Initialize would seize the mutex, Finalize would release it. Thus only
one instance of File_Channel may exist at a time.

---
Regards,
Dmitry Kazakov
www.dmitry-kazakov.de