Re: RCArray is unsafe

[Andrei Alexandrescu via Digitalmars-d]

On 3/1/15 7:44 AM, "Marc =?UTF-8?B?U2Now7x0eiI=?= <schue...@gmx.net>" wrote:

Walter posted an example implementation of a reference counted array
[1], that utilizes the features introduced in DIP25 [2]. Then, in the
threads about reference counted objects, several people posted examples
[3, 4] that broke the suggested optimization of eliding
`opAddRef()`/`opRelease()` calls in certain situations.

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
     }

Thanks for pointing this out - it's a real problem. -- Andrei