On 7/26/18 8:45 AM, Johan Engelen wrote:
On Wednesday, 25 July 2018 at 08:11:59 UTC, rikki cattermole wrote:
Standard solution[0].
[0] https://dlang.org/phobos/std_conv.html#.emplace.4
Thanks for pointing to D's placement new. This is bad news for my
devirtualization work; before, I thought D is in a better situation than
C++, but now it seems we may be worse off.
Before I continue the work, I'll have to look closer at this (perhaps
write an article about the situation in D, so more ppl can help and see
what is going on). In short:
C++'s placement new can change the dynamic type of an object, which is
problematic for devirtualization. However, in C++ the pointer passed to
placement new may not be used afterwards (it'd be UB). This means that
the code `A* a = new A(); a->foo(); a->foo();` is guaranteed to call the
same function `A::foo` twice, because if the first call to `foo` would
do a placement new on `a` (e.g. through `this`), the second call would
be UB.
In D, we don't have placement new, great! And now, I learn that the
_standard library_ _does_ have something that looks like placement new,
but without extra guarantees of the spec that C++ has.
For some more info:
https://stackoverflow.com/a/49569305
https://stackoverflow.com/a/48164192
Reading those items, though, doesn't emplace effectively do what
std::launder does in C++, since it's crossing a function boundary? Is
std::launder a special part of the spec, or does it just return its
parameter to remove the potential for UB?
-Steve
