[Development] Suggested addition to wiki.qt.io/Coding_Conventions

[Marc Mutz]

On Thursday 14 May 2015 02:04:44 Thiago Macieira wrote:
> On Thursday 14 May 2015 02:13:59 Marc Mutz wrote:
> > On Wednesday 13 May 2015 09:30:05 Thiago Macieira wrote:
> > > The drawbacks only appear in debug builds, so is this worth the
> > > uglification?
> > 
> > No, the drawbacks are *due to* MSVC debug builds. They *appear* in
> > release builds and all platforms, too. Exporting the whole class
> > restricts what kind
> > 
> > of changes we can make in a BC way. See e.g. QStringRef:
> >     // ### Qt 6: make this constructor constexpr, after the destructor is
> > 
> > made trivial
> > 
> >     inline QStringRef():m_string(0), m_position(0), m_size(0){}
> >     // ### Qt 6: remove this copy constructor, the implicit one is fine
> >     inline QStringRef(const QStringRef &other)
> >     
> >         :m_string(other.m_string), m_position(other.m_position),
> > 
> > m_size(other.m_size)
> > 
> >         {}
> >     
> >     // ### Qt 6: remove this destructor, the implicit one is fine
> >     inline ~QStringRef(){}
> > 
> > I remember many more situations where this prevented useful changes,
> > though I'd have to dig deeper to provide details.
> 
> None of the three in your example are due to DLL exporting, outside of MSVC
> debug builds.
> 
> The restriction on removing them right now is because doing so would make
> the class trivially copyable or even trivial, which in turn means it's
> passed differently (in registers) when passed by value. That has nothing
> to do with DLLs and we cannot make trivial what wasn't trivial or
> vice-versa.

Interesting. I didn't know that. Please document it on 
https://techbase.kde.org/Policies/Binary_Compatibility_Issues_With_C++

I guess that means we should revert ab8366b5923ec0feb730df98040885669f7bbe38, 
too?

Then again, C++98 doesn't have the concept of trivial types. And QStringRef, 
after the proposed changes for Qt 6, still isn't a C++98 POD. Does that mean 
that C++11/C++98 builds become binary incompatible?

Thanks,
Marc

-- 
Marc Mutz <marc.mutz at kdab.com> | Senior Software Engineer
KDAB (Deutschland) GmbH & Co.KG, a KDAB Group Company
Tel: +49-30-521325470
KDAB - The Qt Experts