On Monday, 4 November 2013 at 09:42:53 UTC, Jakob Ovrum wrote:
On Monday, 4 November 2013 at 07:02:26 UTC, Lars T. Kyllingstad
wrote:
I was quite surprised to see that the following program
compiles just fine with DMD:
struct S
{
@disable this(this);
int n;
}
S createS(int i)
{
S s;
s.n = i;
return s;
}
void main(string[] args)
{
auto foo = createS(1);
foo = createS(2);
}
I already knew that the compiler was allowed to elide copies
on return from functions, but I thought this was an
optimisation, and not part of the language proper. I would
have expected the compiler to complain that createS() can't
return an S since S's postblit constructor is disabled.
My question is therefore, is this by design? Can I rely on
this to work in the future, and on all compilers? If this is
the case, it really should be added to the spec. (Or maybe
it's there already, but I couldn't find it.)
Lars
My understanding is that your example illustrates a *move*, not
a *copy*. AFAICT, non-copyable structs would be next to useless
if we couldn't move them.
I know, and I agree. The question is whether this is a move *by
specification*, i.e. whether the language makes a guarantee that
return values are always moved under certain circumstances. If
so, this should be mentioned in the spec, along with a detailed
description of said circumstances.
I am using this "feature" in a program I'm working on right now.
It would be a shame if this is a mere DMD artifact, as opposed to
a language feature, because then I can't depend on it working in
other compilers or in future DMD versions. I really don't know
any other way to solve my problem either, so I'm keeping my
fingers crossed that this can become part of the official spec.
For anyone interested, the actual use case is a no-arguments
constructor for a non-copyable struct, emulated with static
opCall():
struct Foo
{
// "Constructor"
static Foo opCall()
{
Foo f;
// Initialize f.
return f;
}
// Foo should not be copyable.
@disable this(this);
}
// Construct a new Foo
auto foo = Foo();
