Boost.Bind'ing a member function and posting it to io_service

I am trying to wrap an object that represents a job to be done by an io_service.

The job is of arbitrary type, and does not have to be an IO operation. Similar to what is described here.

I have been able to post bound regular functions, but was not able to post member functions.

Why this code does not compile:

#include <iostream>
#include "boost/asio.hpp"
#include "boost/thread.hpp"

using namespace std;
namespace asio = boost::asio;

class class_fun1 {
public:

    void an_expensive_calculation(int num) {
        cout << "an_expensive_calculation: " << num << endl;
    }
};

class class_fun2 {
public:
    void a_long_running_task(int num) {
        for (int x = 0; x < num; ++x)
            cout << "a_long_running_task: " << num << endl;
    }
};

int main(int argc, char** argv) {

    int my_thread_count = 4;

    asio::io_service io_service;
    asio::io_service::work work(io_service);

    boost::thread_group threads;
    for (std::size_t i = 0; i < my_thread_count; ++i)
        threads.create_thread(boost::bind(&asio::io_service::run, &io_service));

    class_fun1 f1();
    class_fun2 f2();
    io_service.post(boost::bind(&class_fun1::an_expensive_calculation, &f1, 42));
    io_service.post(boost::bind(&class_fun2::a_long_running_task, &f2, 123));

    threads.join_all();

    return 0;
}

while this one works:

#include <iostream>
#include "boost/asio.hpp"
#include "boost/thread.hpp"

using namespace std;
namespace asio = boost::asio;

void an_expensive_calculation(int num) {
    cout << "an_expensive_calculation: " << num << endl;
}

void a_long_running_task(int num) {
    for (int x = 0; x < num; ++x)
        cout << "a_long_running_task: " << num << endl;
}


int main(int argc, char** argv) {

    int my_thread_count = 4;

    asio::io_service io_service;
    asio::io_service::work work(io_service);

    boost::thread_group threads;
    for (std::size_t i = 0; i < my_thread_count; ++i)
        threads.create_thread(boost::bind(&asio::io_service::run, &io_service));

    io_service.post(boost::bind(an_expensive_calculation, 42));
    io_service.post(boost::bind(a_long_running_task, 123));

    threads.join_all();

    return 0;
}

I went through some of the online tutorials and documentation, and as far as I know that first should work. I followed the guidelines for binding a member function and posting it to io_service, but it did not work.