[Development] Proposing CMake as build tool for Qt 6
kevin.kofler at chello.at
Tue Jun 11 10:27:24 CEST 2019
Lars Knoll wrote:
> That thread doesn’t explain why qtchooser isn’t a solution. It does work
> without problems for all my use cases, and I certainly do have quite a few
> Qt versions installed in parallel.
There are 4 crucial design flaws that make qtchooser unsuitable for
1. The choice of preferred Qt version is a user configuration. It should be
a configuration set by the build system of the program being built. The
program should know what Qt version(s) it supports. I should not have to
configure the version to use before I compile something. This can be
worked around by explicitly passing the -qt option in the build system,
but that leads us to:
2. The build system has no portable way to know whether the binaries it is
about to run are wrapped by qtchooser or raw binaries. If it tries
passing qtchooser options to the raw binaries, it will likely get an
error (and catching that error is the only way to know for sure). Even if
the raw binaries get "fixed" to ignore qtchooser options, that will not
solve the issue for all the old raw binaries around. (If you call
unsuffixed binaries on Fedora, you will likely get Qt 3 ones.)
3. The names or numbers that can be used with -qt are not standardized.
Something like -qt5 will likely work in almost all qtchooser setups.
Something like -qt5.12 or -qt5_64bit may or may not work. This also
limits the usefulness for automatic use in build systems or scripts.
4. The build system has to actually run the qtchooser wrappers to know what
Qt version(s) is/are installed (and then try to actually understand the
returned list, see point 3 above), not just check for file existence.
In contrast, with suffixed binaries (e.g., qmake-qt5):
1. The build system decides whether to run qmake-qt5 or qmake-qt6 (and those
should always be tried BEFORE unsuffixed qmake, which is likely the
oldest version that the distro still supports and can be as old as Qt 3!
Ideally, Qt upstream would finally standardize on suffixing for Qt 6 and
so unsuffixed qmake wouldn't have to be tried at all anymore).
2. The build system always gets to run the real binaries (and does not have
to rely on wrapper options to get the correct binary).
3. The suffix should be standardized by the upstream installation (i.e.,
make install should install the tools with that suffix). The de facto
standard suffixes of the form "-qt5" (used in several distributions)
should be adopted. If ABI breaks in minor releases are needed (which I
still think is a horrible idea), then the suffix needs to be changed to
something like "-qt6.3", e.g., "qmake-qt6.3".
4. The build system only has to check whether qmake-qt6 exists to know
whether Qt 6 is installed, it does not have to actually run anything.
So the suffixed binaries are the far superior approach.
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