RCArray is unsafe

[Steven Schveighoffer via Digitalmars-d]

On 3/1/15 2:21 PM, Walter Bright wrote:
> On 3/1/2015 7:44 AM, "Marc =?UTF-8?B?U2Now7x0eiI=?= <schuetzm at gmx.net>"
> wrote:
>> A weakness of the same kind affects DIP25, too. The core of the
>> problem is
>> borrowing (ref return as in DIP25), combined with manual (albeit
>> hidden) memory
>> management. An example to illustrate:
>>
>>      struct T {
>>          void doSomething();
>>      }
>>      struct S {
>>          RCArray!T array;
>>      }
>>      void main() {
>>          auto s = S(RCArray!T([T()])); // s.array's refcount is now 1
>>          foo(s, s.array[0]);           // pass by ref
>>      }
>>      void foo(ref S s, ref T T) {
>>          s.array = RCArray!T([]);      // drop the old s.array
>>          t.doSomething();              // oops, t is gone
>>      }

This is an odd example, how does one take a ref to an RCArray element 
without the machinery to retain the array? I would think that RCArray[x] 
would return something that isn't passable to a function. Or am I 
missing something?

>
> The trouble seems to happen when there are two references to the same
> object passed to a function. I.e. there can be only one "borrowed" ref
> at a time.

Not exactly. Note that we are taking by reference S, which is NOT 
reference counted. So you are passing indirectly a reference to an RC 
object. You aren't "borrowing" that reference.

-Steve