Re: win32 design patterns

* Giovanni Dicanio:


I like his statement that "I'm not smart enough to handle [exceptions]" and
following up on that right away with "Yes, there are programming models like
RAII and transactions, but rarely do you see sample code that uses either.".

It may be so for Microsoft sample code, which then means it's low quality.

C++ exceptions are very tightly coupled to RAII. Without RAII, e.g. as in Java
and C# (the article seems to be mostly about C#, expressed in C++...),
exceptions can be a pain in the neck. Or some other body part. :-) Like a
coffee maker where you have to pour the water very very slowly into a small hole
at the top of the thing, at just the rate it's able to heat and drip it further
into the coffee grinds. Doing this support thing manually instead of automated
(which techniques the author doesn't employ), well, as I already wrote...

The author's example of subtly wrong exception based code, which seems to be C#
code:

   NotifyIcon CreateNotifyIcon()
   {
      NotifyIcon icon = new NotifyIcon();
      icon.Text = "Blah blah blah";
      icon.Visible = true;
      icon.Icon = new Icon(GetType(), "cool.ico");
      return icon;
   }

The author's "corrected" code intended to show subtlety of writing exception
safe code:

   NotifyIcon CreateNotifyIcon()
   {
       NotifyIcon icon = new NotifyIcon();
       icon.Text = "Blah blah blah";
       icon.Icon = new Icon(GetType(), "cool.ico");
       icon.Visible = true;
       return icon;
   }

Since the author doesn't discuss what he thought was wrong about the original,
we must guess, and it seems to be that he thought the Visible property
assignment could throw an exception, i.e. could fail if the Icon property hadn't
yet been assigned. In that case the original code was simply /wrong/, whether
that failure is signaled by exceptions or some other way. And the alleged
subtlety of the exception safe code just what was needed for correct code, again
no matter how the operations signal failure -- but with failure signalled via
exceptions, at least you get a failure indication instead of an invalid object.

The problem with both versions is that NotifyIcon is badly designed, with
multi-phase initialization that must be performed in a specific order, instead
of having a decent constructor that establishes the class invariant, or at least
allowing any order of initialization of independent internal parts.


This illustrates very forcefully the need for RAII, and how the lack of RAII in
languages like Java and C# makes exception handling like, uh, I lack the words.

For some reading about RAII and exceptions in C++, I like to direct people to
<url: http://www.research.att.com/~bs/3rd_safe0.html>.

Cheers,

- Alf

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