borrowed pointers vs ref

[Dicebot via Digitalmars-d]

On Tuesday, 13 May 2014 at 17:41:23 UTC, Walter Bright wrote:
> On 5/13/2014 6:36 AM, Dicebot wrote:
>> Main problem about making `ref` borrowed pointer is that you 
>> will need to
>> prohibit storing it in function transitively. This will need 
>> to become invalid
>> code:
>>
>> struct A
>> {
>>     int* ptr;
>> }
>>
>> int* gptr;
>>
>> void foo(ref A a)
>> {
>>     gptr = a.ptr; // error, can't leak borrowed a.ptr into 
>> global context
>> }
>
> The lifetime of &a is not at all the same as the lifetime of 
> a.ptr, those are independent pointers. I.e. ref is not 
> transitive (unlike const which is transitive).

It has to be transitive to be useful as borrowed pointer. 
Consider this example:

{
     scope A a; // has some internally managed resources
     foo(a);
}

It is not safe to destruct a in the end of the scope here because 
foo may have stored references to a owned resources. But if foo 
signature is `foo(scope ref A a)` then compiler can statically 
verify that it is safe which is the very point of borrowing 
guarantees. It must be transitive to guarantee anything of course.