Register protected callback in base class

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Hi! The compiler complains about me trying to subscribe to messages and register a parent's class protected callback method. Here is the (simplified) code:

class Base{
public:
Base () {}
protected:
void callback(const CameraInfoConstPtr & msg) { ... }
ros::Subscriber sub;
}

class Derived : public Base{
public:
Derived() {
NodeHandle nh;
sub = nh.subscribe("topic", 1,
        &Base::callback,
dynamic_cast<Base*>(this));
}
}

And this is the compiler (g++) error:

error: 'void Base::callback(const CameraInfoConstPtr&)' is protected

The problem dissolves if I make the callback public, but I do not want to do that. Can somebody explain why it does not work and show a workaround? Thank you!