On Monday, 1 June 2015 at 14:59:38 UTC, Jonathan M Davis wrote:
On Monday, 1 June 2015 at 14:43:22 UTC, Marc Schütz wrote:
On Monday, 1 June 2015 at 14:20:46 UTC, Jonathan M Davis wrote:
On Monday, 1 June 2015 at 12:50:28 UTC, Marc Schütz wrote:
Also, object with destructors need to have more restrictions:
S {
~this();
}
void foo() {
S s;
if(condition)
bar(s);
// <- should we run the destructor here?
}
This can either be solved by making such cases non-eligible,
or by "remembering" whether an object was moved using a
hidden boolean variable. AFAIK the latter is incidentally
the solution Rust chose.
Wouldn't the compiler just do something like
if(condition)
{
bar(s); // Do a move; the destructor is run inside bar
return;
}
else
{
s.__dtor();
return;
}
In which case, the destructor is always run in the right
spot, and the compiler can guarantee that a move is done
rather than a copy?
When this can be done, it's of course a good solution. But
when there is additional code between the move and the
destruction, it usually doesn't work:
void foo() {
S s;
if(condition)
bar(s);
baz();
// <- should we run the destructor here?
}
Then, like you suggested, that would indeed appear to require
either an invisible bool that indicates whether a move was done
or not or disallowing the move. I think that I'd favor the
bool, since moving is likely to save more than the extra bool
is going to cost.
Yes, and I think an optimizer can even eliminate the cost in some
cases, e.g. do the rewrite you suggested.
For reference, here are some discussions the Rust community had
about this topic (didn't read them completely):
https://github.com/rust-lang/rfcs/pull/320
https://github.com/rust-lang/rfcs/blob/master/text/0320-nonzeroing-dynamic-drop.md
https://github.com/pnkfelix/rfcs/blob/a773ba113ba135ddb7f481c4829882733eaa5355/active/0000-remove-drop-flag-and-zeroing.md#require-explicit-drops-rather-than-injecting-them
Some of the suggested solutions are not applicable for us,
because we don't have to deal with explicit moves like they do.
But we can certainly learn from their discussions.
