[chuck-users] Bad return signature in pre-constructor leads to null instance

Robert Poor rdpoor at gmail.com
Thu Mar 12 00:26:56 EDT 2009

Got a bug for the team.  Don't ask me how long it took me to track it  
down.  Consider this code and the effect of running it:
	public class Foo {

		fun float bad_pre_constructor() {
			<<< "bad_pre_constructor, this =", this >>>;

	Foo bad_foo;
	<<< bad_foo =", bad_foo>>>
	% chuck foo.ck
	bad_pre_constructor, this = 0x634720
	bad_foo = 0x0
Note that the instance of bad_foo is null.  Now consider this code:
	public class Foo {

		fun void good_pre_constructor() {
			<<< "good_pre_constructor, this =", this >>>;

	Foo good_foo;
	<<< good_foo =", good_foo>>>
	% chuck foo.ck
	good_pre_constructor, this = 0x634720
	good_foo = 0x634720
Note that the instance of good_foo is a reasonable non-null value.

Did you spot the difference?  "bad_pre_constructor" is declared  
(improperly) to return a float, but it doesn't, and an instantiation  
of Foo silently returns a null.  "good_pre_constructor" is properly  
declared with a void return, and an instantiation of Foo returns a  
good value.

So yes, this was operator error -- it's an error not to return a value  
when you claim you're going to.  But ChucK fails silently, at least  
until you get hit with a NullPointerException some time later in the  
execution.  This is not friendly behavior.

- Rob

P.S.: Uh, is this the right forum for reporting bugs like this?

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