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

[Andrei Alexandrescu via Digitalmars-d]

On 2/27/15 1:02 PM, Michel Fortin wrote:
> On 2015-02-27 20:34:08 +0000, Steven Schveighoffer said:
>
>> 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.
>
> You have to retain 'c' for the duration of the call unless you can prove
> somehow that calling the function will not cause it to be released. You
> can prove it in certain situations:
>
> - you are passing a local variable as a parameter and nobody has taken a
> mutable reference (or pointer) to that variable, or to the stack frame
> (be wary of nested functions accessing the stack frame)
> - you are passing a global variable as a parameter to a pure function
> and aren't giving to that pure function a mutable reference to that
> variable.
> - you are passing a member variable as a parameter to a pure function
> and aren't giving to that pure function a mutable reference to that
> variable or its class.
>
> There are surely other cases, but you get the idea. These three
> situations are probably the most common, especially the first one. For
> instance, inside a member function, 'this' is a local variable and you
> will never pass it to another function by ref, so it's safe to call
> 'this.otherFunction()' without retaining 'this' first.

Thanks. So it seems we continue as we were with DIP74 and leave the rest 
to the implementation.

Andrei