[Development] QProperty and library coding guide

[Volker Hilsheimer]

> On 17 Jul 2020, at 09:34, Jean-Michaël Celerier <jeanmichael.celerier at gmail.com> wrote:
> 
> 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
> 


We are not casting these structs to or from anything though, do we? The moc implementation looks like this:

case 12: _t->QAction::d_func()->text.setValue(_t->QAction::d_func(), *reinterpret_cast< QString*>(_v)); break;

instead of

case 12: _t->setText(*reinterpret_cast< QString*>(_v)); break;


Volker




> 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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