Why null reference not showing crash.

Aug 02

Brother Bill

Aug 02

monkyyy

Aug 02

Richard (Rikki) Andrew Cattermole

Aug 02

Andy Valencia

import std.stdio; import std.exception; void main() { MyClass variable; use(variable); } class MyClass { int member; } void use(MyClass variable) { writeln("variable: ", variable); try { writeln(variable.member); // ← BUG } catch (Exception ex) { writeln("Exception: ", ex); } }

August 02

Re: Why null reference not showing crash.

Posted by monkyyy
in reply to Brother Bill

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monkyyy

Posted in reply to Brother Bill

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On Saturday, 2 August 2025 at 20:29:22 UTC, Brother Bill wrote:

From page 233 of "Programming in D".

import std.stdio;
import std.exception;

void main() {
	MyClass variable;
	use(variable);
}

class MyClass {
	int member;
}

void use(MyClass variable) {
	writeln("variable: ", variable);
	
	try {
		writeln(variable.member); // ← BUG
	} catch (Exception ex) {
		writeln("Exception: ", ex);
	}
}

Why does this run, but not display expected null reference exception?

Not all errors are exceptions, Expection is just a class in the std, and then theres Error which is "more important" and catch(Error) gets you some extra cases, you can define your own. etc.

For most cases of try to do anything someone had to write a literal Throw

but I think the os kills you before you even passed something to writeln. Fundmentally youd have to have every access of a class be null checked, which I bet people want, but must not be part of the 30 year old c compiler that d comes from.

It does die from the segfault. Program terminated with signal: SIGSEGV Note: by default D does not throw an exception (it would be an Error not Exception). There is some code to do this for linux, and we've approved if someone is willing to implement it, a read barrier to check for null deref behind a CLI switch. Unfortunately right now, segfaults in D may or may not generate a stack trace, it depends upon the platform and if you've got debug info compiled in. For posix systems you have to inspect the core dump of the process after it dies. What to do for this isn't D specific, and you can find tutorials for it by platform.

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