[Development] QProperty and library coding guide

[Jean-Michaël Celerier]

It does work "in practice", but from the answer I got on SO, it's still
technically UB.
If anyone wants to add another, more positive answer sourced from the
standard though it'd be very welcome :)

https://stackoverflow.com/questions/57823192/does-casting-an-empty-base-class-optimized-object-to-another-type-break-strict-a/57823574

All the best,
Jean-Michaël

On Fri, Jul 17, 2020 at 9:26 AM Lars Knoll <lars.knoll at qt.io> wrote:

>
>
> > On 16 Jul 2020, at 23:35, Thiago Macieira <thiago.macieira at intel.com>
> wrote:
> >
> > On Thursday, 16 July 2020 13:16:41 PDT Giuseppe D'Angelo via
> Development
> > wrote:
> >> Il 16/07/20 12:43, Volker Hilsheimer ha scritto:
> >>> For pre-C++20 (where it’s possible to have zero-size structs), and for
> >>> compilers that don’t respect the [[no_unqiue_address]] attribute, all
> >>> these struct-instances are put into a union. In that case, a class
> using
> >>> QProperty will be larger (by the same amount no matter the number of
> >>> properties) than the same class in Qt 5. With C+++ 20 and compilers
> that
> >>> do respect [[no_unique_address]], the size and layout of these classes
> >>> will be the same.
> >> I'm not fully understanding this last part -- does changing compiler
> >> break ABI, because the implementation of properties change (from an
> >> union to [[no_unique_address]] members)?
> >
> > Yes.
> >
> > Already sent a -2 to the commit that added this support, because it
> can't
> > work. The commit will need to be reverted.
>
> Yes, it can and it does work. And it’s what we should be doing on
> compilers that support [[no_unique_address]].
>
> It does mean that a pure C++17 and a C++20 build aren’t binary compatible,
> but I think we should pay that price.
>
> If I remember correctly, the only compiler that doesn’t yet support it is
> MSVC. BC mainly matters on desktop Linux and those all support the feature,
> so we can enable it from the get go.
>
> Cheers,
> Lars
> >
> > --
> > Thiago Macieira - thiago.macieira (AT) intel.com
> >  Software Architect - Intel System Software Products
> >
> >
> >
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