こういうこと？

#include <iostream>
#include <vector>
#include <functional>
class AClass {
public:
void operator+=(std::function<void()> f) { handlers.push_back(f); }
void notify() { for (auto &f : handlers) f(); }
std::vector<std::function<void()>> handlers;
};
class BClass {
public:
void Test(){std::cout << __func__ << std::endl;}
};
int main () {
AClass a;
BClass b;
a += std::bind(&BClass::Test, &b);
a.notify();
}