Forward references and more

[Steven Schveighoffer]

On Mon, 12 Oct 2009 05:26:58 -0400, bearophile <bearophileHUGS at lycos.com>  
wrote:

> What's wrong with this code?
>
> struct MemoryPool(T) {
>     alias T[100_000 / T.sizeof] Chunk;
>     Chunk*[] chunks;
> }
> struct Foo {
>     int x;
>     MemoryPool!(Foo) pool;
> }
> void main() {}
>
> It prints "Error: struct problem.Foo no size yet for forward reference".
> T.sizeof must be 8 in all cases.
>
>
> So I have tried to pull pool out:
>
> struct MemoryPool(T) {
>     alias T[100_000 / T.sizeof] Chunk;
>     Chunk*[] chunks;
> }
> MemoryPool!(Foo) pool;
> struct Foo {
>     int x;
>     // here uses pool
> }
> void main() {}
>
> But there's a problem still:
> Error: struct problem2.Foo no size yet for forward reference
>
> To compile the code I have to move pool forward still:
>
> struct MemoryPool(T) {
>     alias T[100_000 / T.sizeof] Chunk;
>     Chunk*[] chunks;
> }
> struct Foo {
>     int x;
>     // here uses pool
> }
> MemoryPool!(Foo) pool;
> void main() {}
>
> When possible it's better to avoid global variables. To avoid the global  
> variable I may pass the instance pool to Foo. But to do this Foo has to  
> become a struct template. But I am not sure how I can do this.
> Do you have any comments or suggestions?

It looks strange what you are doing.  A Foo can have a memory pool of a  
lot of Foo's?  Do you mean to make the memory pool static?  I think that  
might work.

I think the main problem is you are defining MemoryPool!(Foo).Chunk which  
specifically needs to know the size of Foo before Foo is completely  
declared.

It's like you are doing this:

struct X
{
   X x;
}

Which clearly is incorrect.

-Steve