[Development] Proposing CMake as build tool for Qt 6

[Bogdan Vatra]

Hi,

În ziua de joi, 13 iunie 2019, la 18:22:14 EEST, Thiago Macieira a scris:
> On Thursday, 13 June 2019 01:06:06 PDT Bogdan Vatra via Development wrote:
> > Hi,
> > 
> >   There is one more missing feature to add to your list: build & debug
> > 
> > builds for msvc in one go (same as qmake does now, a single "make" command
> > will build both targets).
> 
> I wouldn't call that a hard requirement. Separate builds for debug and for
> release are acceptable, the same way that we've ostensibly had support for
> shared-and-release builds but in practice they're always built separately.
> 
> The only difficult part I see is enforcing the suffix. On Mac we've had
> problems with this, since a debug-only build is un-suffixed. Maybe it's time
> to rethink the strategy and make the suffix optional altogether (using the
> lib infix feature). On Windows, we should stop enforcing that unoptimised
> builds be -MDd and make it easy for people to debug into -MD libraries.
> Hardly anyone ever debugs into Microsoft's libraries...
> 

I think libs built with -MDd are a must to enable debug builds for your 
application. AFAIK (if I'm wrong I apologize) you can't mix debug apps with 
release libs.
So, as long as we need to provide libs, (qml) plugins  for debug & release I 
think building them in one go is a must have feature.

> >   Also I wonder when do you plan to release Qt6? Because according to
> > 
> > Thiago's "what a buildsystem must to have list" he mentioned that the
> > buildsystem should be at least 2 years in the wild. If the needed cmake
> > support is not implemented yet, are you going to delay Qt 6 release (I
> > won't mid to use C++20 in Qt 6 :) ) just to give time to cmake to mature?
> > Or that (part of the) list was just an excuse for TQC to ditch QBS ;-).
> 
> 2 years in the wild was not for all features, it was for the buildsystem
> itself and the major features. But we need to build with the version that
> comes with the host systems that we are targetting (if they ship tools like
> this in the first place; Windows doesn't).
> 
> That means that the iOS support only needs to be present in whatever version
> of CMake that Homebrew installs. Since Homebrew always has the latest, this
> is not an impediment.
> 
> But if we want to build on Ubuntu 16.04, then that three year old version's
> CMake needs to be sufficient for a native build of Qt and whatever cross-
> builds get targeted from Linux (Android and QNX, I'd guess). For Qt 6, I'd
> just recommend raising the minimum to 18.04...

  What makes you think that you can build Qt for any platform with cmake found 
in Ubuntu 16.04 (or any later version) ?
  According to https://code.qt.io/cgit/qt/qtbase.git/tree/CMakeLists.txt?
h=wip/cmake#n1 Qt needs CMake *3.15.0* which is not even released. This means 
you'll need *NEXT* Ubuntu LTS (20.04?) to build Qt, this means we should put 
Qt6 on pause until 2022 :).

  Just to be clear, I don't want to change TQC mind again, but I don't like 
people hiding or twisting the true. The true is that in this moment CMake is 
not the best option but, arguably, the worst one. It has the least features 
and it has the ugliest syntax than the alternatives qmake and QBS.

Cheers,
BogDan.