test.cpp

struct Foo {
  int getA() const { return a; }
  private:
  int a = 10;
  friend class Bar;
};

class Bar {
  Bar(Foo* instance):instance(instance){}
  Foo* operator ->() {
    return instance;
  }
  void setA(int a) {
    instance->a = a;
  }
  private:
  Foo* instance = nullptr;
};