On Freitag, 4. April 2025 17:44:49 Mitteleuropäische Sommerzeit Thiago Macieira wrote: > On Friday, 4 April 2025 08:07:27 Pacific Daylight Time Milian Wolff wrote: > > Because if you do so, and the type is public (i.e. all-inline), then you > > would not be able to safely put that into a `std::any` in one DSO and > > extract it in another DSO. The worst thing is that this is only detectable > > at runtime. In the context of Qt, maybe not a lot of people use std::any > > (yet?), but being compatible with std types should be paramount I hope you > > all agree. > > This is not news. It's the same old dynamic_cast<> and typeid() and > exception- throwing failure that we've known for over 20 years. If the > class in question is not exported — and most simple, non-polymorphic > classes will not be! — and you compile with -fvisibility=hidden, then the > typeid will be emitted per shared library / DLL and will be hidden too. > This implies the pointers to the same std::type_info obtained in two > different DLLs will be different. > > See: https://gcc.godbolt.org/z/es97foYa4. Note how the typeinfo object is > marked .hidden. > > We have the same problem with QMetaType: https://qt.godbolt.org/z/YaKzEMcoT. > Note here how the QMetaTypeInterface object is also marked .hidden. > > The conclusion from this is that, from the point of view of the C++ > language, they *are* different types, even if they have the same mangling > and can (and are) passed across library boundaries in by value, by pointer, > in containers, etc. The big problem here is that the C++ standard does not > recognise shared libraries & DLLs exist, so this is an area that is > underdeveloped. Despite being the mainstream. > > IMNSHO, this needs to be addressed in the Standard, ahead of modules even.
Agreed to all of this. But for me at least, the impact of visibility on non- polymorphic types, which you don't use dynamic cast nor throw as exceptions, was, so far, not clear at all. And this is the "new" part. Also note again the clear rule that I also learned from Marc that one should not wholesale export all-inline types, which goes against this :( -- Milian Wolff m...@milianw.de http://milianw.de
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