Am 02.08.2010 16:50, schrieb Ryan W Sims:
On 8/2/10 1:56 AM, Jonathan M Davis wrote:
On Sunday 01 August 2010 21:59:42 Ryan W Sims wrote:
The following code fails with a "Bus error" (OSX speak for "Segfault,"
if I understand correctly).
// types.d
import std.stdio;
class A {
int x = 42;
}
void fail_sometimes(int n) {
A a;
if (n == 0) {
a = new A; // clearly a contrived example
}
assert(a.x == 42, "Wrong x value");
}
void main() {
fail_sometimes(1);
}
It's even worse if I do a 'dmd -run types.d', it just fails without even
the minimalistic "Bus error." Is this correct behavior? I searched the
archives& looked at the FAQ& found workarounds (registering a signal
handler), but not a justification, and the threads were from a couple
years ago. Wondering if maybe something has changed and there's a
problem with my system?
--
rwsims
You are getting a segmentation fault because you are dereferencing a null
reference. All references are default initialized to null. So, if you
fail to
explicitly initialize them or to assign to them, then they stay null,
and in
such a case, you will get a segfault if you try to dereference them.
Yes, I know *why* I'm getting a segfault, thank you - I set up the
example explicitly to defeat the compiler's null checking to test the
behavior. I was startled that there wasn't an exception thrown w/ a
stack trace.
[snip]
Unlike Java, there is no such thing as a NullPointerException in D.
You just get
segfaults - just like you would in C++. So, if you don't want
segfaults from
derefencing null references, you need to make sure that they aren't
null when
you dereference them.
- Jonathan M Davis
That was my question, thanks. It seemed like such an un-D thing to have
happen; I was surprised. I guess w/o the backing of a full virtual
machine, it's tricker to catch null dereferences on the fly, but boy
it'd be nice to have. Don't want to re-fire the debate here, though.
--
rwsims
If you want a NullPointerException as part of your program flow, you can
use enforce() (in std.contracts I think). I don't think catching a
NullPointerException in a big code block where you don't know which
dereferencing should fail is good style.
Mafi
