On 5/6/13 12:10 PM, Steven Schveighoffer wrote:
The counter argument:
foo(makeRvalue()); // error: cannot pass rvalues to ref
// programmer: WTF? This is stupid, but ok:
auto x = makeRvalue();
foo(x);
In other words, explicit nops aren't any better than implicit nops. Even
if we *require* the user to be explicit (and it's not at all clear from
a code-review perspective that the auto x line is to circumvent the
requirements), the fact that this is trivially circumvented makes it a
useless feature. It's like having const you can cast away.
I think the larger issue with binding rvalues to refs is this:
int foo(int i);
int foo(ref int i);
what does foo(1) bind to? It MUST bind to the non-ref, or there is no
point for it.
If this can be solved, binding rvalues to refs is fine.
I think we can technically make the overloading work while also allowing
binding rvalues to ref. But that wouldn't help any. Consider:
ref int min(ref int a, ref int b) { return b < a ? b : a; }
...
int x;
fun(min(x, 100));
Here the result of min may be bound to an lvalue or an rvalue depending
on a condition. In the latter case, combined with D's propensity to
destroy temporaries too early (immediately after function calls), the
behavior is silently undefined; the code may pass unittests.
This is a known issue in C++. Allowing loose binding of rvalues to ref
not only inherits C++'s mistake, but also adds a fresh one.
Andrei
