[Development] QTCS2019 Notes from the Qt 6 Changes / Migration session

[André Pönitz]

On Fri, Nov 22, 2019 at 03:01:25PM +0100, Uwe Rathmann wrote:
> On 11/22/19 11:01 AM, Friedemann Kleint wrote:
> 
> > https://wiki.qt.io/Qt_Contributors_Summit_2019_Qt_6_Changes_/_Migration
> 
> Maybe one aspect I would like to add: there is a lot of software, that has
> to be compilable with Qt 5 and Qt 6. Especially when your product is library
> you can't decide which version of Qt is going to be used in the application.
> 
> It would be good if this does not end up in a total mess of ifdefs.

Since Qt Creator intents to stay compilable "for a while" with probably
5.14 (extrapolating from the usual "latest LTS minus one minor", or in
this special situation possibly 5.15) and 6.0 I guess there will be some
noise if this starts requiring _a lot_ of #ifdefs.

Right now the situation is that Qt dev (a.k.a. Qt 6 in spe) "works"
with a Creator codebase that's not far from being 5.11 compatible.

This has required quite some work already to port away from "near death"
constructs (there are few helper functions toSet/toList, and there's a
handful of unavoidable #ifdefs due to the collapsing of QList and QVector, 
some changes around the Qt namespace, QDataStream, etc) but overall I am
fairly optmistic that the envisioned scenario that porting a codebase
piece-by-piece to 5.15 that's then 6.0 compatible is somewhat realistic.
 
> My experience from the past is, that it takes years until the majority of
> the applications has upgraded to the next major Qt version - and probably
> only few applications will seriously try out Qt x.0 versions.

That matches my experience from the past.

Andre'