How to dump a derived class object into file then load it back?

I want to dump some derived class into file, then dump it back.

here the the class definition:

// base.h
#include <string>
class Base {
 public:
  virtual std::string show_string() = 0;
  int b;
};

class A : public Base {
 public:
  std::string show_string() override { return "this is A"; }
  int a;
};

the dumper code is:

#include <bits/stdc++.h>
#include "./base.h"
using namespace std;

int main() {
  std::fstream f;
  f.open("a.bin", std::ios::out);
  for (int i = 0; i < 10; ++i) {
    A a;
    a.a = i;
    a.b = i - 1;
    f.write((char*)&a, sizeof(a));
  }
  f.close();
}

the loader code is:

#include <bits/stdc++.h>
#include "./base.h"
using namespace std;

int main() {
  std::fstream f;
  f.open("a.bin", std::ios::in);
  char buff[65536];
  f.write(buff, sizeof(buff));
  A* a = (A*)buff;
  cout << a->show_string() << endl;
  f.close();
}

the loader crashed, I think the problem happened because there is a virtual pointer in A, which lost the address when it is loaded back.

so, how can i make it correct?