How can boost::bind call private methods?

boost::bind is extremely handy in a number of situations. One of them is to dispatch/post a method call so that an io_service will make the call later, when it can.

In such situations, boost::bind behaves as one might candidly expect:

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

boost::asio::io_service ioService;

class A {
public:     A() {
                // ioService will call B, which is private, how?
                ioService.post(boost::bind(&A::B, this));
            } 
private:    void B() {}
};

void main()
{
    A::A();
    boost::asio::io_service::work work(ioService);
    ioService.run();
}

However, as far as I know boost creates a functor (a class with an operator()()) able to call the given method on the given object. Should that class have access to the private B? I guess not.

What am I missing here ?