Andrei Alexandrescu wrote:
Jarrett Billingsley wrote:
On Sun, Apr 19, 2009 at 8:41 AM, Denis Koroskin <2kor...@gmail.com>
wrote:
What's a rationale behind an issue described bug 2621?
http://d.puremagic.com/issues/show_bug.cgi?id=2621
Why isn't it allowed anymore?
It broke quite a lot of my code. And while it is fixable by doing
auto tmp = someFunctionThatRetunsStruct();
someMethodThatAcceptsStructByReference(tmp);
it looks ugly and unnecessary.
I just thought of something. Why the hell should we keep C++'s "const
ref" anyway? When you use "const ref" it means you want it to be
read-only and fast to pass large structures. But why should the onus
of passing value types byref be on the programmer? Why not make it so
"const valuetype" will pass byval for smaller values and byref for
larger, completely freeing the programmer from this tedious crap?
It's not something that I care about, and the threshold of byval vs.
byref differs from platform to platform.
Let's nip this in the bud right now. A const value type parameter
should automatically decide whether to pass by reference or not.
I suggested that change. First let me clarify that the suggestion above
cannot work. Due to aliasing, leaving it to the compiler to choose
between by-value and by-reference leads to functions breaking when the
size of the object changes, which is unacceptable.
struct S { int x; ... }
void foo(const S s1, ref S s2)
{
s1.x = 5;
s2.x = 6;
}
S s;
foo(s, s); // messed up
Now onto why ref was disallowed to bind to an rvalue. This is because
some functions take things by ref intending to change them. Passing an
rvalue is in such cases a bug.
I agree there are functions that only want to use ref for speed
purposes. I suggested Walter to use ref? for those. ref? means it could
be an rvalue or an lvalue.
Aside from the real issue at hand, ref? nees some other notation. Using
a question mark here makes it look appalling, and kludgy. One also feels
it's an interrogative of some kind.
