Re: DIP 1016--ref T accepts r-values--Formal Assessment

[Steven Schveighoffer via Digitalmars-d-announce]

On 1/31/19 11:04 AM, Olivier FAURE wrote:

On Thursday, 31 January 2019 at 02:10:05 UTC, Manu wrote:

I still can't see a truck-sized hole.

I don't know if it's truck-sized, but here's another corner case:

     int doubleMyValue(ref int x) {
         x *= 2;
         return x;
     }

     Point pt;
     pt.x = 5;
     pt.y = foobar();

     doubleMyValue(pt.x);
     assert(pt.x == 10);

Question: in the above code, will the assertion pass?

Answer: it depends on Point's implementation. If x is a member variable, then yes. If it's a getter, then doubleMyValue will take a rvalue and x won't be mutated and the assertion will fail.

I think this is a non-trivial conceptual problem.

Yeah, that's already a thing that ref in D doesn't protect against:

struct Point
{
   private int _x, _y;
   ref int x() { return _x; }
   ref int y() { return _y; }
}

struct Rect
{
   private Point _origin, _lengths;
   Point origin() { return _origin; }
   Point lengths() { return _lengths; }
   void origin(Point p) { _origin = p; }
   void lengths(Point p) { _lengths = p; }
}

Rect r;
r.origin = Point(1, 2);
r.lengths = Point(5, 5);
doubleMyValue(r.lengths.x);
assert(r.lengths.x == 10); // fail

-Steve