Reference counting for resource management

[Don]

LMB wrote:
> Hello,
> 
> This should be my first post here, but I just posted the same message on the "standard" D newsgroup by mistake (what a noob! :-P) So, here I go again, on the right forum this time...
> 
> I am trying to create yet another D2 wrapper for SQLite. As usual with many C libraries, SQLite provides us some "objects" and functions to create and destroy them. So, I decided to encapsulate these SQLite objects in a struct, and use reference counting to destroys the objects as soon as I can safely do so.
> 
> The problem is that I can't make it work correctly in all situations. It guess that I am not increasing the reference count on every situation in which my object is copied, because I got "negative" reference counts in some tests. But this is just a guess, I am not sure at all.
> 
> So, is there any complete example on how to implement reference counting in D2?

I don't think so. While trying to do it, Bartosz found some severe bugs 
in D which made it impossible; they were fixed in the last release.
He's since found bug 3516 might still prevent a complete implementation.

> 
> I found some discussions about ref counting in D, like Bartoz's nice post (http://bartoszmilewski.wordpress.com/2009/08/19/the-anatomy-of-reference-counting/), but not a complete working example.
> 
> If there is no example around, I'd be pleased if someone could give me any advice. This is what one of my wrappers roughly looks like (for brevity, I removed all code not related to reference counting):
> 
> struct Database
> {
>    public this(in string fileName)
>    {
>       sqlite3_open(toStringz(fileName), &db_);
> 
>       refCount_ = cast(uint*)(malloc(uint.sizeof));
>       *refCount_ = 1;
>    }
> 
>    this(this)
>    body
>    {
>       ++(*refCount_);
>    }
> 
>    Database opAssign(Database other)
>    {
>       db_ = other.db_;
>       refCount_ = other.refCount_;
>       ++(*refCount_);
>       return this;
>    }
> 
>    public ~this()
>    {
>       if (refCount_ !is null)
>       {
>          --(*refCount_);
> 
>          if (*refCount_ == 0)
>          {
>             free(refCount_);
>             refCount_ = null;
>             immutable status = sqlite3_close(db_);
>          }
>       }
>    }
> 
>    private sqlite3* db_;
>    private uint* refCount_;
> }
> 
> 
> Thanks!
> 
> LMB
>