Libsigc++ [SEP] license
checking required the addition of template typecasting adapters which convert the functor callback profile to match the required signal pattern. 
libsigc++ was a natural expansion of the C++ standard library functors to the tracking of objects necessary to implement the observer pattern. It inspired multiple C++ template based signal and slot implementations including the signal implementation used in the boost C++ libraries. 
libsigc++ is released as free software under the GNU Lesser General Public License (LGPL).
Libsigc++
libsigc++ is a C++ library for typesafe callbacks.
libsigc++ implements a callback system for use in abstract interfaces and general programming. libsigc++ is one of the earliest implementations of the signals and slots concept implemented using C++ template metaprogramming. It was created as an alternative to the use of a meta compiler such as found in the signals and slots implementation in Qt. libsigc++ originated as part of the gtkmm project in 1997 and later was rewritten to be a standalone library. Each signal has a particular function profile which designates the