Constructing a class in-place

[Steven Schveighoffer]

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