Re: References

#include <iostream>
#include <cmath>
#include <vector>
// use a smartpointer for automatic lifetime management.
// a raw pointer would work, but requires more care
#include <boost/smart_ptr.hpp>

class Packet;

class PackerBase {
public:
    virtual void NotifyPacketChanged(Packet& p)=0;
    virtual ~PackerBase() {}
};
typedef boost::shared_ptr<PackerBase> PackerPtr;

class Packet {
private:
    int x_;
    std::vector<PackerPtr> packers_;
private:
    void NotifyPackers() {
        for (size_t i=0; i<packers_.size(); ++i) {
            packers_[i]->NotifyPacketChanged(*this);
        }
    }
public:
     void setX(int x) {
     x_ = x;
     NotifyPackers();
     }
     Packet(int x) : x_(x) {}
     void AddPacker(PackerPtr p) {
     packers_.push_back(p);
     p->NotifyPacketChanged(*this);
     }
     int GetX() const {return x_;}
};

class Packer1: public PackerBase {
private:
    int y_;
private:
    virtual void NotifyPacketChanged(Packet& p) {
        y_ = 2*p.GetX();
        std::cout << "Packer1 set to: " << y_ << "\n";
    }
public:
    Packer1(): y_(0) {}
};

class Packer2: public PackerBase {
private:
    double z_;
private:
    virtual void NotifyPacketChanged(Packet& p) {
        z_ = std::sqrt(double(p.GetX()));
        std::cout << "Packer2 set to: " << z_ << "\n";
    }
public:
    Packer2(): z_(0) {}
};

int main() {
    Packet p1(10);
    p1.AddPacker(PackerPtr(new Packer1()));
    p1.AddPacker(PackerPtr(new Packer2()));
    std::cout << p1.GetX() << std::endl;
    p1.setX(3);
    std::cout << p1.GetX() << std::endl;
    return 0;
}

Output:

Packer1 set to: 20
Packer2 set to: 3.16228
10
Packer1 set to: 6
Packer2 set to: 1.73205
3