Protobuf version conflicts with Qt

Question

I\'m trying to use protobufs v 3.3.2 with Qt 5.9.1. This works with some Qt applications, but only if they are command line programs. Once I create a GUI application with Qt and

Answer 1

As Blabdouze said, this error is caused by the libqgtk3 plugin which is used to set the GUI style. libqgtk3 uses the libmir system library, which uses protobuf 2.6.1. This leads to conflicts when the application starts.

I found a workaround that allows you to avoid editing of Qt files:

1. You need to copy the "plugins" folder from ".../Qt/5.хх.хх/gcc_64/" to some other location (for example, next to the project build folder).
2. Then you must remove "platformthemes/libqgtk3.so" and "platformthemes/libqgtk3.so.debug" from the copied folder.
3. In main(), before creating a QApplication instance, call the static function "QApplication::setLibraryPaths("path/to/copied/plugins/folder")".
4. Finally, you must add variable "LD_LIBRARY_PATH" with the value ".../Qt/5.хх.хх/gcc_64/lib" (correct path will depend on your Qt version) in a project's "environment settings" in Qt Creator. You also may add a "QT_DEBUG_PLUGINS" variable with a value of "1". It will allow you to check which plugins are used by your project and remove unnecessary plugins from the release version.

In conclusion I would like to note that this error occurred when running the project in Ubuntu 16.04, but it disappeared when I switched to version 18.04. It seems that in version 18.04 app uses the default Qt style instead of the GTK style.