What is undefined?

[Seebs]

Poking around, I ran into an interesting thing; nil.object_id == 0x4, but
there's another magical object_id, 0x6, which shows up in the Ruby source
as "Qundef".

.... But what does it *do*? How have I managed to use Ruby for a few things
here and there and never once encounter this value? It it used purely
internally, and never exposed to Ruby code, or what?

(And if you could tell me where I should have gone to look this up, that'd
be cool too; I tried a couple of times to find stuff but eventually gave
up on finding anything other than code snippets and patches which were
focused on explaining some other aspect of the interpreter code.)

-s

 

[Robert Klemme]

Poking around, I ran into an interesting thing; nil.object_id == 0x4, but
there's another magical object_id, 0x6, which shows up in the Ruby source
as "Qundef".

... But what does it *do*? How have I managed to use Ruby for a few things
here and there and never once encounter this value? It it used purely
internally, and never exposed to Ruby code, or what?

You'll find this in error.c:

if (x == Qundef) {
rb_bug("undef leaked to the Ruby space");
}

http://www.google.com/codesearch#kO...kage:ruby lang:^c$ file:error.c&type=cs&l=284

(And if you could tell me where I should have gone to look this up, that'd
be cool too; I tried a couple of times to find stuff but eventually gave
up on finding anything other than code snippets and patches which were
focused on explaining some other aspect of the interpreter code.)

Try this:

http://www.google.com/codesearch#search/&q=Qundef package:ruby lang:^c$&type=cs

Or search through sources with something like

$ find . -type f -name '*.c' -print0 \
| xargs -r0 fgrep -n Qundef

Kind regards

robert