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# Common Issues and Special Use Cases

This document describes how to use more than one Qt version on one machine and how to use Qt on X11 without a window manager. In addition it explains the most common source of link errors with Qt.

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

## 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.

## Other common problems

Other common problems are covered by the online Technical FAQ.