auto ref - again

[Andrei Alexandrescu]

On 1/26/13 9:28 PM, Era Scarecrow wrote:
> Having ref accept rValues seems like a bad idea. If the source is
> const/immutable then it might be allowed, but more likely temporaries
> will be made for convertion from rValues to lValues, then passing off
> the temporaries. The temporaries to work right likely need to be at the
> top-most scope of whatever function you happen to be in; Beyond that I'm
> not sure how else the auto ref could be implemented safely.

One simple matter that has been discussed here, which I've hit only 
yesterday in a script was:

string lines;
...
auto r = regex(...);
auto m = lines.match(r);
// Get rid of the whole match
m.captures.popFront();
// This should be the first actual capture
auto s = m.captures.front;

The problem here is m.captures returns an rvalue. In turn that rvalue is 
bound (arguably incorrectly) to the reference "this" in the call to the 
member function popFront(). The method pops the front off the temporary, 
leaving the original intact. The subsequent invocation m.capture returns 
the value anew.

This issue is a combination of properties that return rvalues with the 
current (too liberal IMHO) binding of rvalues to temporaries. The latter 
has been a design tidbit inherited from C++, and it's been in D for a 
long time.


Andrei