关于对virtual函数开新thread

[失散糖]

用的std::thread开新线程，打算用类c的实例调用b::f，如下：

#include <stdio.h>
#include <thread>

class a
{
public:
    virtual int f()
    {
        std::this_thread::sleep_for (std::chrono::seconds (1));
        printf ("a\n");
        return 1;
    }
};

class b : public a
{
public:
    virtual int f()
    {
        std::this_thread::sleep_for (std::chrono::seconds (1));
        printf ("b\n"); 
        return 2;
    }
};

class c : public b
{
public:
    virtual int f()
    {
        std::this_thread::sleep_for (std::chrono::seconds (1));
        printf ("c\n");
        return 3;
    }
};


int main()
{
    a *x = new c;
    std::thread t(&b::f, static_cast<b*> (x));
    printf ("m\n");
    t.join ();

    return 0;
}

结果输出的是：
m
c

该怎么写std::thread t( )括号里面的东西，才能达到调用x->b::f()的效果呢？或者说，这只能单独写一个函数呢