On 07/07/2017 06:26 AM, Ion Gaztañaga wrote:
On 05/07/2017 17:24, Martin Sebor wrote:[*] While the example (copied below) is valid, accessing the object after someFunction() has returned via a reference or pointer to it is not. void somefunction(const Object& object); { void* p = &object; object.~Object(); new(p) Object(); }I think it's problematic as explained in p0532r0.pdf. We construct and destroy objects in the internal buffer of std::vector and we don't update the pointer every time. I don't see myself understanding when std::launder must be used, looks too expert-grade feature.
The problem with vector is new in C++ 17 and caused by lifting the assignability requirement without fully considering the ramifications. But there are a number of intertwined problems here and this is just one of them. The text that makes references and pointers invalid was added in C++ 03 to fix one problem (CWG issue #89). Launder tries to patch over a problem caused by introducing optional in C++ 14, again without considering CWG 89. As a result of its narrow focus (as Niko's paper points out) it doesn't fix other manifestations of it. CWG issue #2182 describes a related defect in arithmetic involving pointers to individual objects constructed at consecutive locations in the same block of memory that launder doesn't do anything for. As Niko's paper also highlights, launder isn't a complete or, IMO, even a very good solution to the problems. It adds even more complexity without solving all the underlying problems. Adding more, complex features that even experts have trouble understanding is what caused these problems to begin with. Martin
