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

[Timon Gehr]

On 10/28/2015 06:21 PM, Andrei Alexandrescu wrote:
> On 10/28/2015 12:24 PM, Marc Schütz wrote:
>> 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
>> }
>
> Yah, the thing is direct passing is easily under the control of the
> compiler. With some made-up notation, the compiler could do something
> like this:
>
> void bar() {
>      C c = new C();
>      [[auto __t = c; __t.refs += 2;]]
>      foo(c, c);
>      [[__t.refs -= 2;]]
> }

+=2 seems arbitrary. Only one new reference is generated.

Anyway, the solution is indeed to make it the responsibility of the 
caller to keep the borrowed object alive. I.e. when passing a non-scope 
reference to a scope argument, the reference count must be increased for 
the duration of the call. The benefit is that scope references can be 
duplicated arbitrarily without updating the reference count.
This applies to globals just as well as to other cases.

(I haven't read DIP74 though, so I might be misinterpreting your 
intentions.)

(Note that here, it would be unproblematic to allow assigning scope 
references to non-scope references given that the reference count is 
increased, but that's not the meaning of scope.)