Re: A suggestion for resource management

["Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>]

On Sat, 21 Aug 2010 21:47:05 +0200, Florian Weimer wrote:

> * Dmitry A. Kazakov:
> 
>> On Sat, 21 Aug 2010 18:20:29 +0200, Florian Weimer wrote:
>>
>>>           procedure Test_External_Formats is
>>>              ...
>>>              COBOL_File : File_Type;
>>>              ...
>>> 
>>>           begin
>>>              Open (COBOL_File, Name => "Some_File");
>>>              pragma Scoped (Close (COBOL_File));
>>> 
>>>              loop
>>>                ...
>>>           exception
>>>              when End_Error => ...
>>>           end Test_External_Formats;
>>
>> What is wrong with having a destructor for File_Type? If the language is to
>> be extended why not to fix that first?
> 
> You can't always change libraries in that way.  Adding destructors has
> both syntactic and run-time overhead.

I don't see why. Without class-wide objects there is no overhead,
everything is statically known. Neither there is any syntactic overhead.
File_Type is extended just once and then used everywhere. In fact it is
your solution that has distributed overhead (and error-prone too), because
the pragma has to be used everywhere a file is opened. BTW, file handle
File_Type should not be opened, it should be constructed. There should be
no invalid File_Type, at least the library should not encourage them as the
Ada library does and in your example follows.

> The run-time overhead can be
> minimized by a sufficiently advanced compiler (perhaps relying on
> whole-program optimization to detect the lack of task aborts).  I'm
> more concerned with the syntactic overhead.
> 
> There are some cases where the destructor solution doesn't really
> apply.  For instance, the cleanup operation might need two parameters,
> one denoting the object, and a second one decribing where to put it.

I would use mix-in in such rare cases. However, I try to avoid clean-ups.
It is a bad pattern.

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de