Re: Throwing Constructor Exceptions and cleaning up
"Richard Maher" <maher_rj@hotspamnotmail.com> wrote in message
news:gr45pi$hsf$1@news-01.bur.connect.com.au...
Hi,
If when an object's constructor throws an exception that object has not in
fact been intantiated, then what happens to the bits/objects/variables
that
were initialized/instantiated before the exception was thrown?
For example, if I open a file, or connect a socket while constructing an
object and then throw an exception before the constructor/instantiation
completes, then what happens to the file or socket connections that were
successfully instantiated?
a) Garbage Collected in the fullness of time as they're no longer
referenced?
b) Destroyed, and o/ride finalize() method that's called immediately?
c) It's considered extremely bad form to do this in a constructor. Have an
init() method instead?
Who considers it bad form for a constructor to throw an exception? I can
think of at least two reasons why a constructor might throw an exception:
1. Bad parameters
2. Inability to allocate a necessary resource (memory, database, socket,
file, etc.)
I think it is better that the constructor throw an exception, letting you
know immediately that instance creation failed, vs. creating a zombie
instance, requiring that you explicitly check it for validity, and all the
merry mixups that occur when you forget. An exception is harder to forget,
and more explicit, IMHO.
d) Make sure you close the file/Socket before throwing the exception
Absolutely. In some cases, the APIs that the constructor needs to call will
themselves throw exceptions. The constructor needs to catch, cleanup any
resources acquired earlier. If you decide to throw a new exception or
otherwise allow one to fly from inside the constructor, you first need to
undo what you have done so far. A well-placed "finally" block can be very
effective in cases where you just want to "let fly" with an exception thrown
by some API that you're using.
e) Something else. . .
I know there stuff on the web, and I am looking it up as we speak, but
there
is rather a lot of stuff on Exceptions so if someone here can help me out
I'd really appreciate it.
Cheers Richard Maher