[Dlang-study] [lifetime] Initial thoughts on lifetime management

[Marc Schütz]

On Wednesday, 28 October 2015 at 15:24:21 UTC, Andrei 
Alexandrescu wrote:
> On 10/28/2015 11:15 AM, Marc Schütz wrote:
>> On Wednesday, 28 October 2015 at 00:46:34 UTC, Andrei 
>> Alexandrescu wrote:
>>> I should add here another pattern that turned problematic for 
>>> our
>>> older attempts in DIP74:
>>>
>>> C c = new C();
>>> foo(c);
>>>
>>> int foo(scope C d) {
>>>      c = new C();    // c's old instance gets deleted
>>>      return d.i;        // oops! d is invalid
>>> }
>>>
>>
>> That's because global variables have potentially shared 
>> ownership. They
>> need special treatment (for example, in Rust many things you 
>> can do with
>> them require unsafe blocks). If `c` were an argument to `foo`, 
>> the
>> compiler could catch the problem at the call site.
>
> Interesting. Would it be sensible to add a restriction that no 
> @rc object can be reachable from a global? (At least in @safe 
> code.)
>
> Without the restriction, it seems we always need to assume that 
> any object is reachable via a global so we need to add a bunch 
> of incs/decs.

Globals are just a special case, though. Rewriting your example 
without globals:

void bar() {
     C c = new C();
     foo(c, c);
}

int foo(ref C c, scope C d) {
     c = new C();    // c's old instance gets deleted
     return d.i;        // oops! d is invalid
}

RC elision may only be done if the call doesn't create new 
mutable aliases to the owner.