Re: DIP74: Reference Counted Class Objects

[bitwise via Digitalmars-d]

On Wednesday, 4 March 2015 at 18:59:54 UTC, Andrei Alexandrescu 
wrote:
On 3/4/15 10:48 AM, bitwise wrote:
After looking at DIP74, it seems that there is no way to 
create a weak
reference to a ref counted object, but is not explicitly 
stated. Has
this idea been considered at all?

DIP74 aims at not disallowing weak references whilst leaving it 
to libraries to effect implementations, but we'd need a proof 
of concept implementation to make sure DIP74 is enough as is. 
-- Andrei

I suppose you could add "getWeak()" and "releaseWeak()" to a ref 
counted class, and implement a "struct Weak(T)" that calls them 
for you, but I doesn't seem like much of a leap to just add 
something like this to the DIP.

couldn't "opAddWeak" and "opReleaseWeak" be added and follow 
identical rules to "opAddRef" and "opRelease"? The compiler could 
simply insert "opAddWeak() instead of opAddRef() when it found 
the "weak" keyword.

For example:

class Widget {
    private uint _refs = 1;
    private uint _weaks = 0;

    private int[] _payload; // owned
        
    void opAddRef() {
        ++_refs;
    }
    void opRelease() {
        if (--_refs == 0) {
            GC.free(_payload.ptr);
            _payload = null;

            // I'm assuming this would leave "_weaks" and "_refs" 
untouched
            this.destroy();

            if(_weaks == 0) {
                GC.free(cast(void*)this);
            }
        }
    }

    void opAddWeak() {
        ++_weaks;
    }
    void opReleaseWeak() {
        if(--_weaks == 0 && _refs == 0) {
                GC.free(cast(void*)this);
        }
    }

    // optional/recommended
    bool expired() {
                return _refs == 0;
    }
}

Widget a = new Widget; // refs == 1, weaks == 0

Widget b = weak a; // refs == 1, weaks == 1
//a.opAddWeak();

Widget c = b;
// b.opAddRef(); // refs == 2, weaks == 1

a = null; // refs == 1, weaks == 1
//a.opRelease();

assert(b.expired() == false);

c = null; // refs == 0, weaks == 1
// c.opRelease();

assert(b.expired() == true);

b = null; // refs == 0, weaks == 0
// b.opReleaseWeak();

////////// or for function calls:

void func(weak Widget a) {
        //...
}

Widget a = new Widget;
func(a);