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# Common Problems

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

This indicates that you include the Q_OBJECT macro in a class declaration and probably also run the moc, but forget to link the moc-generated object code into your executable. Read 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 the Qt distribution:

Header files - Compile time

  Programmers need to include the Qt header files. Those with a command-line compiler will typically use options such as `/I%QTDIR%\include` or `-I${QTDIR}/include.` They will need the header files of the version of Qt they wish to build programs with. The header files are in the `include` subdirectory of Qt distributions.

Meta Object Compiler - Compile time

  Programmers need to run the Meta Object Compiler - moc. The moc is found in the `bin` subdirectory of Qt distributions.

Static or shared libraries - Link time

  Programmers need to link with static or shared libraries. Those with a command-line compiler will typically use options such as `/L%QTDIR%\lib\qt.lib` or `-L${QTDIR}/lib` `-lqt.` They will need the libraries of the version of Qt they wish to build programs with. The libraries are in the `lib` subdirectory of Qt distributions.

Shared libraries - Run time

  Users of programs built with the shared Qt libraries need these same shared libraries to run the programs. The libraries are 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 the directories listed in the environment variable `${LD_LIBRARY_PATH}` on Unix/Linux.

Binary packages usually consist of two parts:

- run time libraries in the run time package, usually called `qt2.`
- header files, the moc and static libraries in the developers' kit, usually called `qt2-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 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 2.x on a KDE desktop based on Qt 1.4x. 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 moc and the libraries of Qt 2.x. They are included in the developer package of Qt 2.x (`qt2-dev` or the like).
  - To run programs you will need the shared libraries of Qt 2.x and Qt 1.4x. They are included in the regular packages of Qt 2.x (`qt2` or the like) and Qt 1.4x (`qt1` or the like).

  Just install those 3 packages. You may need to set the environment variable `QTDIR.`

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

  You build and run programs for Qt v1.4x and Qt 2.x. You will need:
  - the header files, the moc and the libraries of Qt 2.x and Qt 1.4x to build programs,
  - the shared libraries of Qt 2.x and Qt 1.4x to run programs (if you use shared linking).

  Get source distributions of both Qt 1.4x and Qt 2.x.

  1. Install and build Qt 1.4x and Qt 2.x, usually in `/opt` or `/usr/local.` In the case of `/opt:`

         $ cd /opt
         $ gunzip -c qt-1.44.tar.gz | tar xf -
         $ cd qt-1.44
         $ setenv QTDIR /opt/qt-1.44
         $ configure [options]
         $ make

         $ cd /opt
         $ gunzip -c qt-2.1.0.tar.gz | tar xf -
         $ cd qt-2.1.0
         $ setenv QTDIR /opt/qt-2.1.0
         $ configure [options]
         $ make

  2. Make shared libraries available to programs at run time. Either add both `/opt/qt-1.44` and `/opt/qt-2.0.1` to your environment variable `LD_LIBRARY_PATH` or make links to the libraries in a standard directory like `/usr/local/lib:`

         cd /usr/local/lib
         ln -s /opt/qt-1.44/lib/libqt.so.1 .
         ln -s /opt/qt-2.1.0/lib/libqt.so.2 .

  To develop with Qt 1.4x use:

      setenv QTDIR /opt/qt-1.44
      setenv PATH ${QTDIR}/bin:${PATH}

  To develop with Qt 2.x use:

      setenv QTDIR /opt/qt-2.1.0
      setenv PATH ${QTDIR}/bin:${PATH}

  Setting the `PATH` ensures that the proper version of moc is being used. Your `Makefile` should refer to `${QTDIR}/include` and `${QTDIR}/lib` to include the proper header files and link with the proper libraries.

## Using Qt on X11 without a window manager

When using Qt without a window manager on Unix/X11, you will very 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 toplevel window.

## Other common problems

Other common problems are covered by the online Technical FAQ.