(-: Geekerie, logiciel libre et vilain gauchisme

Investigating build failure with both Qt4 and Qt5 installed on Mac OS X

I’m getting these errors while building Qt5 programs on my Mac:

In file included from /usr/local/opt/qt5/lib/QtWidgets.framework/Headers/QApplication:1:
In file included from /usr/local/opt/qt5/lib/QtWidgets.framework/Headers/qapplication.h:48:
In file included from /usr/local/opt/qt5/lib/QtGui.framework/Headers/qguiapplication.h:39:
/usr/local/opt/qt5/lib/QtGui.framework/Headers/qinputmethod.h:82:5: error: unknown type name 'QLocale'
    QLocale locale() const;
/usr/local/opt/qt5/lib/QtGui.framework/Headers/qinputmethod.h:91:21: error: no type named 'InputMethodQueries' in namespace 'Qt'; did you mean 'InputMethodQuery'?
    void update(Qt::InputMethodQueries queries);
/usr/local/include/QtCore/qnamespace.h:1541:10: note: 'InputMethodQuery' declared here
    enum InputMethodQuery {
In file included from /usr/local/opt/qt5/lib/QtWidgets.framework/Headers/QApplication:1:
In file included from /usr/local/opt/qt5/lib/QtWidgets.framework/Headers/qapplication.h:48:
/usr/local/opt/qt5/lib/QtGui.framework/Headers/qguiapplication.h:85:12: error: unknown type name 'QWindowList'
    static QWindowList allWindows();
/usr/local/opt/qt5/lib/QtGui.framework/Headers/qguiapplication.h:86:12: error: unknown type name 'QWindowList'
    static QWindowList topLevelWindows();
/usr/local/opt/qt5/lib/QtGui.framework/Headers/qguiapplication.h:138:12: error: unknown type name 'QFunctionPointer'
    static QFunctionPointer platformFunction(const QByteArray &function);
/usr/local/opt/qt5/lib/QtGui.framework/Headers/qguiapplication.h:143:16: error: no type named 'ApplicationState' in namespace 'Qt'
    static Qt::ApplicationState applicationState();
/usr/local/opt/qt5/lib/QtGui.framework/Headers/qguiapplication.h:164:38: error: no type named 'ApplicationState' in namespace 'Qt'
    void applicationStateChanged(Qt::ApplicationState state);

A little search on the web shows this is a problem of conflict with Qt4, and that uninstalling Qt4 should help. Indeed, even just unlinking the qt formula with homebrew makes the build pass:

brew unlink qt

Unfortunately, other programs are now failing:

$ qcachegrind 
dyld: Library not loaded: /usr/local/lib/QtGui.framework/Versions/4/QtGui
  Referenced from: /usr/local/bin/qcachegrind
  Reason: image not found
Trace/BPT trap: 5

So I had to find a better alternative!

clang — and probably all the compilers in their own way — has a flags that shows the headers actually included during the parsing: -H.

In my case, for the Qt5 headers, it shows something like:

.. /usr/local/opt/qt5/lib/QtWidgets.framework/Headers/QApplication
... /usr/local/opt/qt5/lib/QtWidgets.framework/Headers/qapplication.h
.... /usr/local/include/QtCore/qcoreapplication.h
..... /usr/local/include/QtCore/qobject.h

The command line looks like that (I’ve suppressed everything not needed to make it a bit easier to read):

c++ -isystem /usr/local/opt/qt5/lib/QtCore.framework/Headers \
  -iframework /usr/local/opt/qt5/lib \
  -isystem /usr/local/opt/qt5/lib/QtWidgets.framework/Headers \
  -isystem /usr/local/include \ 

The first two lines of includes are ok, but then, at the third line, it switched to Qt4 includes (note the lack of qt5 in the path).

In qapplication.h, the header QtCore/qcoreapplication.h is included. According to Apple’s documentation on frameworks, this means that the qcoreapplication.h header could be located in the Qt5 framework QtCore.framework/Headers (or QtCore/PrivateHeaders), if QtCore.framework is in /System/Library/Frameworks or /Library/Frameworks, or if the base directory of QtCore.framework is passed to the compiler with the flag -framework or -iframework.

QtCore.framework is in /usr/local/opt/qt5/lib, which is not one of the default framework directories, but the flag -iframework is properly used:

-iframework /usr/local/opt/qt5/lib

so this is why it works when there is Qt5 alone.

However, when Qt4 is also installed, the compiler seems to first look at the normal includes before looking at the frameworks, and so the first QtCore/qcoreapplication.h found is the one from Qt4, which is in the include dir for other libraries: /usr/local/include.

Qt5 also has its own include directory that contain the QtCore directory (and others) as Qt4 does, and including that directory before the Qt4 directory is enough to fix the build.

With CMake unfortunately, it is not included when using the standard find_package(Qt5...) commands. It has to be done by hand, with

include_directories(BEFORE /usr/local/opt/qt5/include)

or if you are not the developer, you can set the CMAKE_CXX_FLAGS option to -I/usr/local/opt/qt5/include:

cmake -DCMAKE_CXX_FLAGS:STRING=-I/usr/local/opt/qt5/include ..