class TestClass{
  private:
    Stream* mySerial = nullptr;
    int (*func)(int, int);
  public:
    TestClass(Stream &ser){
      mySerial = &ser;
    }
    void setExtern(void ((*f))){
      func = f;
    }
    void print(char* s){
      if (mySerial != nullptr){
        mySerial->print(s);
      }
    }  
    void print(String &s){
      if (mySerial != nullptr){
        mySerial->print(s);
      }
    }
    void println(char *s){
      if (mySerial != nullptr){
        mySerial->println(s);
      }
    }  
   void println(String &s){
      if (mySerial != nullptr){
        mySerial->println(s);
      }
    } 
    void doSomething(){
      mySerial->println(func(3,4));
     };
};

int multiply(int a, int b){
  return a*b;
}

TestClass test(Serial);
String aString = "String object!";

void setup() {
  Serial.begin(115200);
  Serial.println("Start");
  test.println("Hello World");
  test.println(aString);
  test.setExtern(&multiply);
  test.doSomething();
}

void loop() {
  

}