New API in libsigc++ 2.4

Member sigc::signal_base::block (bool should_block=true) noexcept

Member sigc::signal_base::blocked () const noexcept

Member sigc::signal_base::unblock () noexcept

Member sigc::track_obj (const T_functor & func, const T_obj &... obj)

Class sigc::track_obj_functor< T_functor, T_obj >