On Saturday, 7 February 2015 at 15:02:43 UTC, Andrei Alexandrescu
wrote:
On 2/7/15 6:35 AM, Daniel Murphy wrote:
"Peter Alexander" wrote in message
news:[email protected]...
I'm writing a blog post about why we don't need rvalue
references in
D. It seems that we rely on NRVO being performed, not just as
an
optimization, but for correct semantics (e.g. for objects
without
destructors or postblits). This doesn't appear to be
documented anywhere.
Is it meant to be part of the spec?
NRVO isn't required for correct semantics, as structs can be
moved with
bitcopy.
It is required for structs that disable postblit. -- Andrei
NRVO specifically means that a pointer to the destination object
is passed to the function, and the returned object is constructed
in place. The in place construction isn't required. What is
required is that the local is moved.
e.g.
S foo() {
S s;
return s;
}
S s = foo();
With NRVO becomes:
void foo(ref S dst) {
dst = S();
}
S s = void;
foo(s);
But this isn't necessary. Would also be valid to just do:
void foo(ref S dst) {
S s;
move(dst, s); // do the memcpys
}
S s;
foo(s);
This distinction matters because NRVO cannot be performed when
foo may return two different objects, but we can still move and
avoid postblit.
