ref parameters: there is no escape

[dsimcha]

I think this is an absolutely terrible idea, unless it has an "I know 
what I'm doing, let me cast away the safety" loophole.  Consider the 
case of designing a D wrapper for C functionality.

// C, we know it doesn't escape its parameters but the compiler doesn't.
void cFun(int* a, int* b);

// D:
void dWrapper(ref int a, ref int b) {
     cFun(&a, &b);
}

If you want the compiler to put extra restrictions on you in the name of 
safety, that's what SafeD is for.  If you're writing an @system 
function, then the compiler should stay out of your way and let you do 
what you want, unless it can **prove** that it's wrong.

On 8/14/2011 10:20 AM, Andrei Alexandrescu wrote:
> Walter and I have had a long discussion and we thought we'd bring an
> idea for community review.
>
> We believe it would be useful for safety purposes to disallow escaping
> addresses of ref parameters. Consider:
>
> class C {
> int * p;
> this(ref int x) {
> p = &x; // escapes the address of a ref parameter
> }
> }
>
> Such code is accepted today. We believe it is error-prone and dangerous,
> particularly because the caller has no syntactic cue that the address of
> the parameter is passed into the function (in this case constructor).
> Worse, such a function cannot be characterized as @safe.
>
> So we want to make the above an error. The workaround is obvious - just
> take int* as a parameter instead of ref int. What a function can do with
> a ref parameter in general is:
>
> * use it directly just like a local;
>
> * pass it down to other functions (which may take it by value or
> reference);
>
> * pass its address down to pure functions because a pure function cannot
> escape the address anyway (cool insight by Walter);
>
> * take its address as long as the address doesn't outlive the frame of
> the function.
>
> The third bullet is not easy to implement as it requires flow analysis,
> but we may start with a conservative version first. Probably there won't
> be a lot of broken code anyway.
>
> Please chime in with any comments you might have!
>
>
> Thanks,
>
> Andrei