How to use pointer to member inside a class object (right syntax)?

In this example i’m trying to return x, y or z from get_by_ptr function, but can’t unedrastand rigth(legal) syntax to do it. I understand that this pointer holds base address of object and pointer to member holds "address shift" and I can calculate address. But is there legal syntax way?

class foo {
public:
    size_t x{ 33 };
    size_t y{ 77 };
    size_t z{ 99 };
    size_t get_by_ptr(size_t foo::*ptr) {
        //how to return x, y or z using ptr
    }
};

int main() {
    foo bar;
    size_t foo::*ptr{ &foo::y };
    size_t z{ bar.get_by_ptr(ptr) };
}

Thank You.

>Solution :

You likely want to write:

size_t get_by_ptr(size_t foo::*ptr) {
    return this->*ptr;
}