Is casting an instance of a class to its subclass legal?

Question

Is casting a class to a subclass of itself without additional data fields well defined?

For example if I wanted to hide certain internal only methods from an end user like in the following.

#include <string>
#include <iostream>

// public.hpp
class Talker {
public:
  Talker(std::string text) {
    this->text = text;
  }
  virtual void talk() {
    std::cout << this->text << std::endl;
  }
protected:
  std::string text;
};

// private.cpp
class InnerTalker : public Talker {
public:
  void setText(std::string text) {
    this->text = text;
  }
};

int main(int argc, char **argv) {
  Talker t("Hello World!");
  t.talk();

  InnerTalker *ht = reinterpret_cast<InnerTalker *>(&t);
  ht->setText("Farewell World!");

  t.talk();

  return 0;
}

I have compiled this example and it works as I expect. Printing:

Hello World!
Farewell World!

However that does not make it well defined behavior.

Thank you in advance for any information you can provide.

Answer 1

From cppreference:

Performing a class member access that designates a non-static data member or a non-static member function on a glvalue that does not actually designate an object of the appropriate type - such as one obtained through a reinterpret_cast - results in undefined behavior:

You can perform the reinterpret_cast, but doing nearly anything that uses the resulting pointer will result in UB.