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boost::signals2::signal_type — Specify a the template type parameters of a boost::signals2::signal using named parameters.
// In header: <boost/signals2/signal_type.hpp> template<typename A0, typename A1 = boost::parameter::void_, typename A2 = boost::parameter::void_, typename A3 = boost::parameter::void_, typename A4 = boost::parameter::void_, typename A5 = boost::parameter::void_, typename A6 = boost::parameter::void_> class signal_type { public: // types typedef implementation-detail signature_type; typedef implementation-detail combiner_type; typedef implementation-detail group_type; typedef implementation-detail group_compare_type; typedef implementation-detail slot_function_type; typedef implementation-detail extended_slot_function_type; typedef implementation-detail mutex_type; typedef typename signal<signature_type, combiner_type, ..., mutex_type> type; };
The signal_type metafunction employs the Boost.Parameter library to
            allow users to specify the template type parameters of a signals2::signal
            using named parameters.  The resulting signal type is provided through the
            signal_type::type typedef.  Named template type parameters
            can enhance readability of code, and provide convenience for specifying classes
            which have a large number of template parameters.
          
The template type parameters may be passed positionally, similarly to passing them
            to the signals2::signal class directly.  Or, they may be passed as named template parameters
            by wrapping them in one of the template keyword classes provided in the
            boost::signals2::keywords namespace.  The supported template keywords are:
            keywords::signature_type, keywords::combiner_type,
            keywords::group_type, keywords::group_compare_type,
            keywords::slot_function_type, keywords::extended_slot_function_type,
            and keywords::mutex_type.
          
The default types for unspecified template type parameters are the same as those for the signal class.
            Named template type parameters are especially convenient when you only wish to change a
            few of a signal's template type parameters from their defaults, and the parameters
            you wish to change are near the end of the signal's template parameter list.
            For example, if you only wish to change the Mutex template type
            parameter of a signal, you might write:
          
namespace bs2 = boost::signals2; using namespace bs2::keywords; bs2::signal_type<void (), mutex_type<bs2::dummy_mutex> >::type sig;
For comparison, to specify the same type using the signal class directly looks like:
namespace bs2 = boost::signals2; bs2::signal < void (), bs2::optional_last_value<void>, int, std::less<int>, boost::function<void ()>, boost::function<void (const connection &)>, bs2::dummy_mutex > sig;