Re: Missed optimization with const member

[Geza Herman <geza dot herman at gmail dot com>]

On 07/05/2017 01:26 PM, Geza Herman wrote:
> On 07/05/2017 01:14 PM, Jonathan Wakely wrote:
> > I think the reason it's not optimized away is for this case:
> >
> > void somefunction(const Object& object);
> > {
> >    void* p = &object;
> >    object.~Object();
> >    new(p) Object();
> > }
> >
> > This means that after calling someFunction there could be a different
> > object at the same location (with a possibly different value for that
> > member).
>
> https://stackoverflow.com/questions/39382501/what-is-the-purpose-of-stdlaunder 
>
>
> I don't know whether the anwser at stackoverflow is right, but: the 
> compiler can assume that m_initType doesn't change, if I don't launder 
> it. (in this case, though, I don't know how would I tell the compiler 
> that it must call the destructor "laundered".)

Have a look at this doc: 
http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2017/p0532r0.pdf

It explitly says that after "new(p) Object();", it is UB to access 
object (even the mutable members!) via p. If you want to access it, you 
have to use the pointer which "new" returned. Or, std::launder must be 
used (c++17 proposal).