sub_pub.cpp

#include <iostream>
#include <string>

class A
{
private:
  std::string s;

protected:
  std::string& to_string() {return s;}

public:
  A() : s{"default"}{}

  A(const std::string& s) :s{s} {}

};


class B : public A
{

  public:
  using  A::A;
  using  A::to_string;

};


int main()
{
  B b{"doggie"};

  A& a = b;

  std::cout <<  b.to_string() << "\n";


}