Forward reference in struct

[Regan Heath]

Simen Haugen wrote:
> I've hit a problem with forward referencing. Is it possible to manually add 
> a forward reference, or isn't what I'm trying to do here allowed?
> 
> struct A {
>  B b;
> }
> 
> struct B {
>  union {
>   int i;
>   A a;
>  }
> }
> 
> 
> t.d(5): struct test.A unable to resolve forward reference in definition
> t.d(10): anonymous union unable to resolve forward reference in definition
> t.d(9): struct test.B unable to resolve forward reference in definition 

This is not allowed because it's impossible to resolve, I believe.

At the point where you declare A the compiler wants to calculate the 
size of the structure, it can/will delay this till it finds a definition 
for B, but when it finds B it also needs to calculate the size of B,
but to calculate the size of B it needs to know the size of A,
but to calculate the size of A it needs to know the size of B,
but to calculate the size of B it needs to know the size of A,
but to calculate the size of A it needs to know the size of B,
...

Try:

 > struct A {
 >  B* b;
 > }

instead.  I don't have DMD here so I can't test it but this should be 
possible as pointers have a fixed/known size.

Regan