如何制作可复制的boost :: signal？

#include<boost/signal.hpp>
#include<boost/bind.hpp>

boost::signal<void ()> signal;

void my_slot() {
    std::cout << "Slot function called!"<< std::endl;
}

signal.connect(boost::bind(&my_slot));

signal();

#include<iostream>
#include<boost/signal.hpp>
#include<boost/bind.hpp>

void my_slot() {
    std::cout << "Slot function called!"<< std::endl;
}

int main() {
    boost::signal<void ()> signal;
    signal.connect(boost::bind(&my_slot));
    signal();
    return 0;
}

Slot function called!