What sorts of things cause cyclic dependencies?

Lars T. Kyllingstad public at kyllingen.NOSPAMnet
Wed Oct 13 22:53:51 PDT 2010

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.

The way to fix it is to move one of the module constructors into a 
separate module:

  module a;
  import a_init;
  import b;

  module a_init;
  static this() { ... }
  module b;
  import a;
  static this() { ... }

For a "real" example of how this is done, check out std.stdio and 
std.stdiobase in Phobos.


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