On Thursday, 24 January 2019 at 07:18:58 UTC, Mike Parker wrote:
Walter and Andrei have declined to accept DIP 1016, "ref T
accepts r-values", on the grounds that it has two fundamental
flaws that would open holes in the language. They are not
opposed to the feature in principle and suggested that a
proposal that closes those holes and covers all the bases will
have a higher chance of getting accepted.
You can read a summary of the Formal Assessment at the bottom
of the document:
https://github.com/dlang/DIPs/blob/master/DIPs/rejected/DIP1016.md
The second problem is the use of := (which the DIP Author
defines as representing "the initial construction, and not a
copy operation as would be expected if this code were written
with an = expression"). This approach shows its deficiencies in
the multiple arguments case; if the first constructor throws an
exception, all remaining values will be destroyed in the void
state as they never have the chance to become initialized.
Although not specified by the DIP, I think this could be easily
remedied by saying that the order of construction is the same as
if the temporaries were not bound to ref, i.e.
---
struct A {~this();} struct B{ ~this();}
A a();
B b();
void f(A a, B b);
void g(ref A a, ref B b);
f(a(),b()); //(1)
g(a(),b()); //(2)
---
and a() or b() may throw (and are pure), that (1) and (2) exhibit
the same exception/destructor semantics.
Which is what I would expect to happen under no given
specification (although specifying it is a good idea!). To put it
another way, they should be constructed in place in the order
they are used, and given they are a temporary, they can only be
used once.
