signals and slots in c++

signal.h

#include <vector>
#include <functional>

namespace qp {

template <typename ...Args>
using delegate = std::function<void(Args...)>;

template <typename ...Args>
class signal {
private:
    using fn_t = delegate<Args...>;
    std::vector<fn_t> observers;

public:
    void connect(fn_t f) {
        observers.push_back(f);
    }

    void operator ()(Args... a) {
        for (auto i: observers)
            i(a...);
    }
};

}

test.cpp

#include "signal.h"
#include <string>
#include <iostream>
#include <functional>

struct button {
    qp::signal<std::string> update;

    void click() {
        update("clicked!");
    }
};

struct label {
    std::string text;
    void changeText(std::string s) {
        text = s;
        std::cout << s << std::endl;
    }
};

int main() {
    using namespace std::placeholders;

    label label1;
    label label2;

    button button1;
    button1
        .update
        .connect(std::bind(&label::changeText, std::ref(label1), _1));
    // or
    // button1.update.connect([&](std::string s) { label1.changeText(s); });
    button1
        .update
        .connect(std::bind(&label::changeText, std::ref(label2), _1));

    button1.click();
    return 0;
}