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<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="id2546638"></a>Frequently Asked Questions</h2></div></div></div>
<div class="qandaset">
<a name="id2546646"></a><dl>
<dt>1. <a href="s04.html#id2546648">Don't noncopyable signal semantics mean that a class
with a signal member will be noncopyable as well?</a>
</dt>
<dt>2. <a href="s04.html#id2546671">Is Boost.Signals thread-safe?</a>
</dt>
<dt>3. <a href="s04.html#id2546691">How do I get Boost.Signals to work with Qt?</a>
</dt>
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<a name="id2546648"></a><a name="id2546650"></a><p><b>1.</b></p>
</td>
<td align="left" valign="top"><p>Don't noncopyable signal semantics mean that a class
with a signal member will be noncopyable as well?</p></td>
</tr>
<tr class="answer">
<td align="left" valign="top"></td>
<td align="left" valign="top"><p>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.</p></td>
</tr>
<tr class="question">
<td align="left" valign="top">
<a name="id2546671"></a><a name="id2546673"></a><p><b>2.</b></p>
</td>
<td align="left" valign="top"><p>Is Boost.Signals thread-safe?</p></td>
</tr>
<tr class="answer">
<td align="left" valign="top"></td>
<td align="left" valign="top"><p>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.</p></td>
</tr>
<tr class="question">
<td align="left" valign="top">
<a name="id2546691"></a><a name="id2546693"></a><p><b>3.</b></p>
</td>
<td align="left" valign="top"><p>How do I get Boost.Signals to work with Qt?</p></td>
</tr>
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<td align="left" valign="top">
<p>When building with Qt, the Moc keywords
<code class="computeroutput">signals</code> and <code class="computeroutput">slots</code> are defined using
preprocessor macros, causing programs using Boost.Signals and
Qt together to fail to compile.</p>
<p><span class="emphasis"><em>For Qt 4.1 and later</em></span>, This behavior
can be turned off in Qt on a per-project or per-file basis
with the <code class="computeroutput">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:</p>
<pre class="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
</pre>
<p>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 <a href="http://scottcollins.net/articles/a-deeper-look-at-signals-and-slots.html" target="_top">A
Deeper Look at Signals and Slots</a> [off-site] for more
complete examples and a survey of the strengths of the two
systems.</p>
<p><span class="emphasis"><em>Older versions of Qt</em></span> 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:</p>
<p>The first way involves defining
the <code class="computeroutput">BOOST_SIGNALS_NAMESPACE</code>
macro to some other identifier (e.g., <code class="computeroutput">signalslib</code>)
when building and using the Boost.Signals library. Then the
namespace of the Boost.Signals library will be
<code class="computeroutput">boost::BOOST_SIGNALS_NAMESPACE</code> instead of
<code class="computeroutput">boost::signals</code>. To retain the original namespace
name in translation units that do not interact with Qt, you
can use a namespace alias:</p>
<pre class="programlisting">
namespace boost {
namespace signals = BOOST_SIGNALS_NAMESPACE;
}
</pre>
<p>The second way, provided by Frank Hess and improved by
Niels Dekker, involves
creating a header <code class="computeroutput">signalslib.hpp</code> that contains
the following code:</p>
<pre class="programlisting">#ifndef SIGNALSLIB_HPP_INCLUDED
#define SIGNALSLIB_HPP_INCLUDED
#if defined(signals) && defined(QOBJECTDEFS_H) && \
!defined(QT_MOC_CPP)
# undef signals
# define signals signals
#endif
#include <boost/signal.hpp>
namespace boost
{
namespace signalslib = signals;
}
#if defined(signals) && defined(QOBJECTDEFS_H) && \
!defined(QT_MOC_CPP)
# undef signals
// Restore the macro definition of "signals", as it was
// defined by Qt's <qobjectdefs.h>.
# define signals protected
#endif
#endif</pre>
<p>Use this header to include the Boost library, then refer
to it in the namespace <code class="computeroutput">boost::signalslib</code>. This
option is often
preferable to the first option because it can be used without
recompiling the Signals library binary. </p>
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<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
<td align="left"><p><small>Last revised: May 21, 2008 at 21:57:05 +0100</small></p></td>
<td align="right"><div class="copyright-footer">Copyright © 2001-2004 Douglas Gregor<p>Use, modification and distribution is subject to the Boost
Software License, Version 1.0. (See accompanying file
<code class="filename">LICENSE_1_0.txt</code> or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)</p>
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