Re: Improving DIP74: functions borrow by default, retain only if needed

[Andrei Alexandrescu via Digitalmars-d]

On 2/27/15 12:34 PM, Steven Schveighoffer wrote:
On 2/27/15 3:30 PM, Steven Schveighoffer wrote:
void main()
{
C c = new C; // ref counted class
C2 c2 = new C2; // another ref counted class
c2.c = c;
foo(c, c2);
}

Bleh, that was dumb.

void main()
{
    C2 c2 = new C2;
    c2.c = new C;
    foo(c2.c, c2);
}

Still same question. The issue here is how do you know that the
reference that you are sure is keeping the thing alive is not going to
release it through some back door.

Thanks! In ARC, there are autorelease pools that keep at least one 
reference to the objects they own. I think that's what they are for.

So let me add a complete example:

class C {
   void someFunc();
   void opAddRef();
   void opRelease();
}

class C2 {
   C c;
   void opAddRef();
   void opRelease();
}

void foo(C c, C2 c2) {
   c2.c = null;
   c.someFunc(); // crash
}

void main() {
   C2 c2 = new C2;
   c2.c = new C;
   foo(c2.c, c2);
}

Distinguishing these is an interesting problem. In fact we can reduce 
the matter to one class only:

class C {
   C c;
   void someFunc();
   void opAddRef();
   void opRelease();
}

void foo(C c1, C c2) {
   c2.c = null;
   c1.someFunc(); // crash
}

void main() {
   C obj = new C;
   obj.c = new C;
   foo(obj.c, obj);
}


Andrei