Boost C++ Libraries of the most highly regarded and expertly designed C++ library projects in the world. Herb Sutter and Andrei Alexandrescu, C++ Coding Standards

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Frequently Asked Questions

1. Don't noncopyable signal semantics mean that a class with a signal member will be noncopyable as well?
2. Is Boost.Signals thread-safe?
3. How do I get Boost.Signals to work with Qt?


Don't noncopyable signal semantics mean that a class with a signal member will be noncopyable as well?

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.


Is Boost.Signals thread-safe?

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.


How do I get Boost.Signals to work with Qt?

When building with Qt, the Moc keywords signals and slots are defined using preprocessor macros, causing programs using Boost.Signals and Qt together to fail to compile.

For Qt 4.1 and later, This behavior can be turned off in Qt on a per-project or per-file basis with the no_keywords 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:

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

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 Deeper Look at Signals and Slots [off-site] for more complete examples and a survey of the strengths of the two systems.

Older versions of Qt 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:

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

  namespace boost {
    namespace signals = BOOST_SIGNALS_NAMESPACE;

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


#if defined(signals) && defined(QOBJECTDEFS_H) && \
#  undef signals
#  define signals signals

#include <boost/signal.hpp>
namespace boost
  namespace signalslib = signals;

#if defined(signals) && defined(QOBJECTDEFS_H) && \
#  undef signals
// Restore the macro definition of "signals", as it was
// defined by Qt's <qobjectdefs.h>.
#  define signals protected


Use this header to include the Boost library, then refer to it in the namespace boost::signalslib. This option is often preferable to the first option because it can be used without recompiling the Signals library binary.

Last revised: May 21, 2008 at 21:57:05 +0100