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

[Michel Fortin via Digitalmars-d]

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.

-- 
Michel Fortin
michel.fortin at michelf.com
http://michelf.com/