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<section last-revision="$Date: 2008-05-21 16:57:05 -0400 (Wed, 21 May 2008) $">
  <title>Frequently Asked Questions</title>
  
  <qandaset>
    <qandaentry>
      <question>
        <para>Don't noncopyable signal semantics mean that a class
        with a signal member will be noncopyable as well?</para>
      </question>
      <answer>
        <para>No. The compiler will not be able to generate a copy
        constructor or copy assignment operator for your class if it
        has a signal as a member, but you are free to write your own
        copy constructor and/or copy assignment operator. Just don't
        try to copy the signal.</para>
      </answer>
    </qandaentry>
    <qandaentry>
      <question>
        <para>Is Boost.Signals thread-safe?</para>
      </question>
      <answer>
        <para>No. Using Boost.Signals in a multithreaded concept is
        very dangerous, and it is very likely that the results will be
        less than satisfying. Boost.Signals will support thread safety
        in the future.</para>
      </answer>
    </qandaentry>
    <qandaentry>
      <question>
        <para>How do I get Boost.Signals to work with Qt?</para>
      </question>
      <answer>
        <para>When building with Qt, the Moc keywords
        <code>signals</code> and <code>slots</code> are defined using
        preprocessor macros, causing programs using Boost.Signals and
        Qt together to fail to compile.</para>

        <para><emphasis>For Qt 4.1 and later</emphasis>, This behavior
        can be turned off in Qt on a per-project or per-file basis
        with the <code>no_keywords</code> option.  This works with
        out-of-the-box builds of Boost and Qt. You do not need to
        re-configure, re-build, or duplicate existing libraries. For a
        project where you want to use both Boost.Signals and Qt
        Signals and Slots, the relevant part of your .pro file might
        look like this:</para>

        <programlisting>
CONFIG      += no_keywords # so Qt won't #define any non-all-caps `keywords'
INCLUDEPATH += . /usr/local/include/boost-1_33_1/
macx:LIBS   += /usr/local/lib/libboost_signals-1_33_1.a  # ...your exact paths may vary
</programlisting>

        <para>Now you can mix Boost.Signals and Qt Signals and Slots
        in the same files, and even within the same class or function.
        You will have to use the upper-case versions of Qt macros in
        your own code.  See the article <ulink
        url="http://scottcollins.net/articles/a-deeper-look-at-signals-and-slots.html">A
        Deeper Look at Signals and Slots</ulink> [off-site] for more
        complete examples and a survey of the strengths of the two
        systems.</para>

        <para><emphasis>Older versions of Qt</emphasis> did not
        provide a reliable mechanism for avoiding these unfriendly,
        all lower-case `keyword'-like macros.  Although this is a
        problem with Qt and not Boost.Signals, a user can use the two
        systems together with a little extra effort. There are two
        ways to do this:</para>

        <para>The first way involves defining
        the <code>BOOST_SIGNALS_NAMESPACE</code> 
        macro to some other identifier (e.g., <code>signalslib</code>)
        when building and using the Boost.Signals library. Then the
        namespace of the Boost.Signals library will be
        <code>boost::BOOST_SIGNALS_NAMESPACE</code> instead of
        <code>boost::signals</code>. To retain the original namespace
        name in translation units that do not interact with Qt, you
        can use a namespace alias:</para>

        <programlisting>
  namespace boost {
    namespace signals = BOOST_SIGNALS_NAMESPACE;
  }
</programlisting>

        <para>The second way, provided by Frank Hess and improved by
        Niels Dekker, involves 
        creating a header <code>signalslib.hpp</code> that contains
          the following code:</para>

        <programlisting>#ifndef SIGNALSLIB_HPP_INCLUDED
#define SIGNALSLIB_HPP_INCLUDED 

#if defined(signals) &amp;&amp; defined(QOBJECTDEFS_H) &amp;&amp; \
  !defined(QT_MOC_CPP)
#  undef signals
#  define signals signals
#endif

#include &lt;boost/signal.hpp&gt;
namespace boost
{
  namespace signalslib = signals;
}

#if defined(signals) &amp;&amp; defined(QOBJECTDEFS_H) &amp;&amp; \
  !defined(QT_MOC_CPP)
#  undef signals
// Restore the macro definition of "signals", as it was
// defined by Qt's &lt;qobjectdefs.h&gt;.
#  define signals protected
#endif

#endif</programlisting>

        <para>Use this header to include the Boost library, then refer
          to it in the namespace <code>boost::signalslib</code>. This
          option is often
        preferable to the first option because it can be used without
        recompiling the Signals library binary. </para>
      </answer>
    </qandaentry>
  </qandaset>
</section>