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FAQs

Other frequently asked questions can be found in the FAQ index and in the Technical FAQ.

• Link error, complaining about a lack of \c vtbl, \c _vtbl, \c __vtbl or similar
• Using different versions of Qt on the same machine
  • Developers building for a single version of Qt on Unix - Qt binary packages
  • Developers building for two versions of Qt on Unix - Qt sources
• Using Qt on X11 without a window manager

Link error, complaining about a lack of vtbl, _vtbl, __vtbl or similar

This indicates that you've included the Q_OBJECT macro in a class declaration and probably also run the moc, but forgot to link the moc-generated object code into your executable. See Using the Meta Object Compiler for details on how to use moc.

Using different versions of Qt on the same machine

Qt programs need the following components of a Qt distribution:

Header files - Compile time

Programmers need to include the Qt header files. The Qt header files are usually located in the include subdirectory of Qt distributions. Care must be taken to include the header files of the relevant release of Qt. Those with a command-line compiler will typically use options such as /I%QTDIR%\include the relevant release of Qt.

Meta Object Compiler and other tools - Compile time

Programmers need to run moc and other tools such as uic. These tools are usually located in the bin subdirectory of Qt distributions. Either run "$QTDIR"/bin/moc and "$QTDIR"/bin/uic or add "$QTDIR"/bin to your PATH and run moc and uic. If you use qmake the appropriate lines will be added to your Makefiles so that uic and moc will be executed as required.

Static or shared libraries - Link time

Programmers need to link with the Qt static or shared libraries. The Qt libraries are usually located in the lib subdirectory of Qt distributions. Care must be taken to link with the libraries of the relevant release of Qt. Those with a command-line compiler will typically use options such as /L%QTDIR%\lib\qt.lib or -L"$QTDIR"/lib -lqt provided QTDIR specifies the relevant release of Qt.

Shared libraries - Run time

Users of programs linked with shared Qt libraries need these same shared libraries to run these programs. The Qt libraries are usually located in the lib subdirectory of Qt distributions. Shared libraries are made available to programs in places such as C:\windows\system on Windows platforms, directories listed in file /etc/ld.so.conf on Linux, standard lib directories on Unix, or directories listed in environment variables LD_LIBRARY_PATH, SHLIB_PATH, or LIBPATH on various Unix flavours. Make the relevant Qt libraries available using one of these mechanisms.

Qt distributions consist of different files needed at compile time, link time, or run time. Trolltech distributes Qt in the form of a source package that contain all these files once they have been built.

Other vendors distribute Qt in the form of binary packages. Binary packages usually consist of two parts:

• shared libraries in the run time package, usually called qt3.

• header files, static libraries, the moc and other tools in the developers' kit, usually called qt3-dev.

Depending on how you are using Qt, you need to make specific parts of the Qt distribution available to your programs. Typical situations are described below.

Developers building for a single version of Qt on Unix - Qt binary packages

You build programs with a single version of Qt, but you still need to run programs linked with another version of Qt. You are typically a Linux developer who builds programs for Qt 3.x on a KDE desktop based on Qt 2.x. Qt packages are usually split into a shared library package with a name like qt and a developer package with a name like qt-dev. You will need the appropriate packages:

• To build programs you will need the header files, the libraries, the moc and other tools from Qt 3.x. They are included in the developer package of Qt 3.x (qt3-dev or similar).

• To run programs you will need the shared libraries of Qt 3.x and Qt 2.x. They are included in the regular packages of Qt 3.x (qt3 or similar) and Qt 2.x (qt2 or similar).

Just install the packages, qt2, qt3, and qt3-dev. You may need to set the environment variable QTDIR to point to Qt 3.x.

Developers building for two versions of Qt on Unix - Qt sources

You build and run programs for Qt 2.x and Qt 3.x. You will need:

• the header files, the libraries, the moc and other tools from Qt 3.x and Qt 2.x to build programs,

• the shared libraries of Qt 3.x and Qt 2.x to run programs.

Get the source distributions of both Qt 2.x and Qt 3.x.

1. Install and build Qt 2.x and Qt 3.x, usually in /opt or /usr/local. In the case of /opt:

   $ cd /opt
   $ gunzip -c \c qt-x11-2.3.1.tar.gz | tar xf -
   $ cd qt-2.3.1
   $ setenv QTDIR /opt/qt-2.3.1
   $ configure [options]
   $ make
   
   $ cd /opt
   $ gunzip -c qt-x11-free-3.0.0.tar.gz | tar xf -
   $ cd qt-3.0.0
   $ setenv QTDIR /opt/qt-3.0.0
   $ configure [options]
   $ make
   

2. Make shared libraries available to programs at run time. Either add both /opt/qt-2.3.1/lib and /opt/qt-3.0.0/lib to your environment variable LD_LIBRARY_PATH or file /etc/ld.so.conf or whataver mechanism you're using, or make links to the libraries in a standard directory like /usr/local/lib:

   cd /usr/local/lib
   ln -s /opt/qt-2.3.1/lib/libqt.so.2 .
   ln -s /opt/qt-2.3.1/lib/libqt-mt.so.2 .
   ln -s /opt/qt-2.3.1/lib/libqutil.so.1 .
   ln -s /opt/qt-3.0.0/lib/libqt.so.3 .
   ln -s /opt/qt-3.0.0/lib/libqui.so.1 .
   

To develop with Qt 2.x use:

setenv QTDIR /opt/qt-2.3.1
setenv PATH ${QTDIR}/bin:${PATH}

To develop with Qt 3.x use:

setenv QTDIR /opt/qt-3.0.0
setenv PATH ${QTDIR}/bin:${PATH}

Setting QTDIR ensures that the proper resources are used, such as the documentation appropriate to the version of Qt you're using. Also your Makfiles may refer to "$QTDIR"/include and "$QTDIR"/lib to include the proper header files and link with the proper libraries. Setting the PATH ensures that the proper version of moc and other tools is being used.

Using Qt on X11 without a window manager

When using Qt without a window manager on Unix/X11, you will most likely experience focus problems. Without a window manager, there is no focus handling on X11, and no concept of an active window either. If you want your application to work in such an environment, you have to explicitly mark a window as active after showing it:

 yourWindow->show();
 yourWindow->setActiveWindow();

Note that setActiveWindow() won't work if the widget does not become physically visible during this event cycle. However, without a window manager running, this is guaranteed to happen. For the curious reader: setActiveWindow() emulates a window manager by explicitly setting the X Input Focus to a widget's top level window.