Question

I wrote this code:

class Address{

  private:
    std::string street;
    int house;

  public:
    Address(std::string s, int h):
      street(s), house(h) {}

    void setHouse(int h) {house = h;}

    friend std::ostream &operator << (std::ostream& os, Address &a);
};

class Person{

    private:
      std::string name;
      Address A;

    public:
      Person(std::string n, std::string v, int c) :
        name(n), A(v, c) {}

      Address& getAddress(){return A;}

    friend std::ostream &operator << (std::ostream& os, Person &a);

};

std::ostream &operator << (std::ostream& os, Address &a){

  return os << "[" << a.street << ", " << a.house << "]";
}

std::ostream &operator << (std::ostream& os, Person &p){

  return os << p.name << " " << p.A;
}

int main(){

  Person pietro("Pietro", "Champs Elysees", 16);

  std::cout << pietro << std::endl;

  Address ma = pietro.getAddress();
  ma.setHouse(333); 

  std::cout << pietro << std::endl;

  return EXIT_SUCCESS;
}

I have these questions:

1. Why does the line ma.setHouse(333); does not have any side effect on pietro (ie in the second print the house-number is not changed)?
2. (I know that it’s not the right way, but) how can i make ma.setHouse(333); having side effects on pietro ?
3. Why should i write const Address& getAddress() const {return A;} if the above code does not have side effects?

Answer 1

Q1. Because you are acting on a copy of pietro‘s Address:

Address ma = pietro.getAddress(); // ma is a copy of pietro's address
ma.setHouse(333);                 // modify copy, not pietro 

Q2. Take a reference?

Address& ma = pietro.getAddress(); // ma is a reference to pietro's Address.
ma.setHouse(333);                  // modifies pietro's Address

Q3. So you can have const correctness. Without the const method and const return value, you can affect the internals of the object. Furthermore, you cannot call the method on const instances or references.

const Address& ma = pietro.getAddress(); // Calls const method. Cannot modify pietro
Address& ma = pietro.getAddress(); // calls non-const method. Can modify pietro