On Thu, 14 Oct 2010 05:53:51 +0000, Lars T. Kyllingstad wrote:
> On Wed, 13 Oct 2010 21:25:15 -0700, Jonathan M Davis wrote:
>> Okay. in the code that I'm working on at the moment, I get an
exception
>> saying that a cyclic dependency was detected but no information
>> whatsoever as to where it is or what it means. I haven't been able to
>> find much information on them other than some discussions of making
it
>> so that the compiler detects them at compile time. I have yet to
figure
>> out _what_ it is that is causes such errors. Could someone clue me
in?
>> It's really hard to track down a bug when the error only tells you
that
>> there's a bug and doesn't say anything about where it happens, and I
>> have no clue what sort of things can be cyclically dependent.
>
> Say you have two modules, a and b. Both have static constructors,
and a
>
> imports b:
> // a.d
> module a;
> import b;
> static this() { ... }
>
> // b.d
> module b;
> static this() { ... }
>
> The language is then defined so that b's constructor is run before
a's,
> since a depends on b. But what happens if b imports a as well? There
> is no way to determine which constructor to run first, so the runtime
> throws a "cyclic dependency" exception.
I should mention that this can happen with larger cycles as well, i.e.
"a
imports b, b imports c, c imports ..., ... imports a". This can make
some cyclic dependencies very hard to track down.
There was a discussion about this on the Phobos mailing list a few
months
ago:
http://lists.puremagic.com/pipermail/phobos/2010-June/thread.html#949
-Lars
