t1.cpp

class Quux;

class Foo {
public:
    void (Quux::*ptr)();
};

int main() {
    Foo x;
    printf("The pointer to member function is %d bytes\n", sizeof(x.ptr));
}
//output: The pointer to member function is 16 bytes

t2.cpp

class Quux {};

class Foo {
public:
    void (Quux::*ptr)();
};

int main() {
    Foo x;
    printf("The pointer to member function is %d bytes\n", sizeof(x.ptr));
}
//output: The pointer to member function is 4 bytes

t3.cpp

class Quux;

class Foo {
public:
    void (Quux::*ptr)();
};

class Quux{};

int main() {
    Foo x;
    printf("The pointer to member function is %d bytes\n", sizeof(x.ptr));
}
//output: The pointer to member function is 16 bytes