Conan, working with several Qt versions

Sometimes you have to deal with C++ projects that are using Qt as a main framework, and Conan, as dependency manager. It is quite usual that Qt is installed as a system library (or integrated into your IDE), and your dependencies also use Qt.

In this environment, if you want to upgrade (or just testing) a new version of Qt, you will also need to rebuild each dependency which is using Qt. This situation is going to be worst in case you need to keep or work with different Qt versions and the same dependencies.

To remove this tedious work, we will use Conan recipe’s ‘options’, because the package ABI compatibility is defined by three thins in Conan: - Package settings, - Package options, and - Package requires

Let’s see the following conan’s recipe:

class QEEntityConan(ConanFile):
    name = "QEEntity"
    version = "1.0.0"
    requires = "QEAnnotation/1.0.0@fmiguelgarcia/stable"
    settings = "os", "compiler", "build_type", "arch"
    license = "https://www.gnu.org/licenses/lgpl-3.0-standalone.html"
    description = "QE Annotated Entity library"

    # STEP 1: qt_version is a new option.
    options = { "qt_version": "ANY"}

    # STEP 2: Load qt version using 'qmake' and store into our option.
    def configure(self):
        self.options.qt_version = os.popen("qmake -query QT_VERSION").read().strip()
        self.output.info("Configure Qt Version: %s" % self.options.qt_version)

	 # ...

In the above code, we have defined an option qt_version, that can contain any value. Then, we use configure() function to load Qt version from system, using qmake –query. In this way, our package ABI compatibility will be linked with the current Qt version. This means that, two packages will be available at same time in our local Conan’s cache:

• Package QEEntity 1.0.0 on Linux, x64, GCC-5.0, Release, with qt_version=5.6.2, and
• Package QEEntity 1.0.0 on Linux, x64, GCC-5.0, Release, with qt_version=5.9

And done, using this method for each depencency, it allows us to change easily the Qt in our solution, and Conan will use the corresponding package version for each dependency.