#include <iostream>
using namespace std;

struct T {
  int a;
  float b;
  // many other types...
};

union T_union {
 T t1;
 struct T_owner {
  T t;
 } t2;
};

int main() {
    T_union un;
    un.t1.a = 10;
    cout << un.t2.t.a << endl; // I can always do that?

    return 0;
}

#include <iostream>
using namespace std;

struct T {
  int a;
  float b;
  // many other types...
};

struct T_owner_base {
  T t;
 };

struct T_owner_child : T_owner_base {
  double c;
 };

union T_union {
 T t1;
 T_owner_child t2;
};

int main() {
    T_union un;
    un.t1.a = 10;
    cout << un.t2.t.a << endl; // I can always do that?

    return 0;
}