As the post has close to no content, it might be of some interest to link to an actual working implementation in modern C++ of signals and slots with the observer pattern: libsigc++

https://github.com/libsigcplusplus/libsigcplusplus

There's also an article on wikipedia on libsigc++.

https://en.m.wikipedia.org/wiki/Libsigc%2B%2B