CPP Dangerous Reference Handling

Returning Out-of-Scope Object References.

• Reference is always an alias to an object so passing it as argument to a function or method should not be any problem.
• However, what happen if you try to return a reference of an object, which was created inside the method or the function?
• This is a dangerous situation as such an object will no longer exists when the function or method are finish with their tasks.
  Example of a dangerous return of an object reference from a method:

  // Returning a reference to an object which no longer exists
  #include <iostream>
  class Person {
  public:
    Person () {
    }
    Person (int age) {
      this->age = age;
    }
    ~Person() {}
    int getAge() { return age; }
    Person & getNewPerson(int age) {
      Person person(age);
      return person;
    }
  private:
    int age;
  };
  
  int main() {
    Person * pPerson= new Person();
    Person & Ricard = pPerson->getNewPerson(50);
    // the reference of the returned object
    // has no object reference as the object is now out of scope
    int age = Ricard.getAge();
    std::cout << "Ricard is " << age << " years old!" << std::endl;
    return 0;
  }

  When we run this application the result could will be (or may be the compiler fails):

  Ricard is -858993460 years old!

  You can download this example here (needed tools can be found in the right menu on this page).

• You may be thing you could overcome this problem with creating the object on the free store and return the reference of that.
• Well it will work but you got a new problem to solve:
  Returning an object reference from a method, where the object exists on the free store:

  // Returning a reference to an object created on the free store
  #include <iostream>
  class Person {
  public:
    Person () {
    }
    Person (int age) {
      this->age = age;
    }
    ~Person() {}
    int getAge() { return age; }
    Person & getNewPerson(int age) {
      Person * pPerson= new Person(age);
      return *pPerson;
    }
  private:
    int age;
  };
  
  int main() {
    Person * pPerson= new Person();
    Person & Ricard = pPerson->getNewPerson(50);
    // the reference of the returned object
    // has no object reference as the object is now out of scope
    int age = Ricard.getAge();
    std::cout << "Ricard is " << age << " years old!" << std::endl;
    // Now you should delete the location on the free store
    Person * pRicard= &Ricard;
    delete pRicard ;
    // Every thing work find BUT Ricard refer now to what???
    return 0;
  }

  When we run this application, the result will be:

  Ricard is 50 years old!

  You can download this example here (needed tools can be found in the right menu on this page).