c++中的成员选择符

#include <iostream>
using namespace std;
struct C
{
    int x;
    float y;
    float z;
};
int main()
{
    float f;
    int* i_ptr;

    C c1, c2;

    float C::*f_ptr; // a pointer to a float member of C
    f_ptr = &C::y;  //point to C member y
    c1.*f_ptr = 3.14; // set c1.y = 3.14
    c2.*f_ptr = 2.01; // set c2.y = 2.01

    f_ptr = &C::z;  // point to C member z
    c1.*f_ptr = -9999.99; //set c1.z = -9999.99

    f_ptr = &C::y; //point to C member y
    c1.*f_ptr = -777.77; // set c1.y

    f_ptr = &C::x; // ****Error: x is not float****

    f_ptr = &f; // ****Error: f is not a float member of C****

    i_ptr = &c1.x; //c1.x is an int

    //...

    return 0;
}