Re: Virtual functions in constructors. Is it possible?
Le 05/07/2011 11:09, Alf P. Steinbach /Usenet a ?crit :
* Leo Equinox Gaspard, on 04.07.2011 23:59:
Couldn't you [one] use, in ~Initializer :
try {
if (p)
p->Init();
} catch(...) {
p->InitFailed();
throw;
}
No.
This could handle exceptions from Init in the sense of reacting to them,
at the cost of creating a zombie object (unusable object) when such an
exception occurs.
But for this case it would still yield a memory leak, because this all
happens after the new-expression has been fully evaluated. When the
exception occurs in the constructor, the new-expression can clean up
(and does). When the exception occurs after the new-expression is fully
evaluated, which is the case with the call of Init, the new-expression
is already out of the picture, in the past.
And it can't handle the case where the construction of the object that p
is pointing to, throws an exception.
For then, by the time the ~Initializer runs (at the end of the
full-expression that instantiates the object), the exception has
propagated out of the new-expression and, due to the new-expression's
cleanup, the object that p points to has been destroyed, hence, the call
'p->Init()' is now calling a virtual function on a non-existent object,
which is Undefined Behavior. With g++ the error is caught by the
runtime, and you get a crash. With MSVC it's not caught, and you get
more subtle errors.
Oh. I thought the exception was in Init().
The run of the program I figured was :
=> new Derived()
=> Instantiation of an Initializer
=> Only do it, we don't want *any* exception in this constructor
=> End of new
=> ~Initializer()
=> Call to Init(), catching exceptions
=> Init() does the whole construction stuff, additionnally calling
Base's Init()
=> If any exception was caught, then clear the work done by Init()
using InitFailed()
=> InitFailed() should be exception-less and make the object ready to
be destructed.
=> Then rethrow the exception
=> End of ~Initializer
=> If any exception is thrown, then the object is not valid, and hence
should not be used, but immediately destructed. That's the default
behavior of the exception, which will make any scoped items to be deleted.
Of course, using a raw pointer will lead to memory leaks, but that's
always as it with them plus exceptions.
Where am I mistaking ?
Cheers,
Leo
And implement an InitFailed() virtual method that acts as the
countrary (not
sure of the word to use there) of Init ?
So the code should have no more memory leaks.
Sorry, no, see above.
Cheers & hth.,
- Alf