Overriding CAST operator (i think it is called downcasting)

Question

I ma quite new to c++ and i got a small problem that is probably easy for others. I have a class A and a class B that extends class A. I have an object A, lets call it a1. I want to downcast a1 to a type B class using the syntax: "B b1=(B)a1;"

 class IAmortizabil
 {
 public:
     virtual double getAmortizare()=0;
 };



 class Utilaj : public IAmortizabil{
 protected:
     float valInventar;
     char codUtilaj[10];
     char *denUtilaj;
     int anIntrFunct;
     int durataNormata;

 public:

     Utilaj(){
         denUtilaj=NULL;
         valInventar=0;
         anIntrFunct=0;
         durataNormata=0;
     }

     Utilaj(char *codUtilaj, char *denUtilaj, float valInventar, int      anIntrFucnt, int durataNormata){
         strcpy(this->codUtilaj, codUtilaj);
         this->denUtilaj=new char[strlen(denUtilaj)+1];
         strcpy(this->denUtilaj, denUtilaj);
         this->valInventar=valInventar;
         this->anIntrFunct=anIntrFucnt;
         this->durataNormata=durataNormata;
     }

     Utilaj(Utilaj &u)
     {
         strcpy(codUtilaj, u.codUtilaj);
         denUtilaj = new char[strlen(u.denUtilaj) + 1];
         strcpy(denUtilaj, u.denUtilaj);
         valInventar = u.valInventar;
         anIntrFunct = u.anIntrFunct;
         durataNormata = u.durataNormata;
     }


     friend ostream &operator<<(ostream &iesire, Utilaj &u)
     {
         iesire << " cod utilaj " << u.codUtilaj << endl;
         iesire << "denumire utilaj " << u.denUtilaj << endl;
         iesire << "valoare inventar " << u.valInventar << endl;
         iesire << "an intrare in functiune " << u.anIntrFunct << endl;
         iesire << " durata normata " << u.durataNormata << endl;
         return iesire;
     }

     Utilaj operator=(Utilaj &u)
     {
         strcpy(codUtilaj, u.codUtilaj);
         if (denUtilaj != NULL)
             delete[]denUtilaj;
         denUtilaj = new char[strlen(u.denUtilaj) + 1];
         strcpy(denUtilaj, u.denUtilaj);
         valInventar = u.valInventar;
         anIntrFunct = u.anIntrFunct;
         durataNormata = u.durataNormata;
         return *this;
     }



     double getAmortizare()
     {
         cout << '\n';
         if (durataNormata != 0)
             return valInventar / durataNormata;
         else {
             cout << "Durata normata este 0=>eroare";
             return 0;
         }
     }


     ~Utilaj(){
         if(denUtilaj!=NULL){
             delete[]denUtilaj;
         }
     }
 };


 class Strung :public Utilaj
 {
 protected:
     int nrBucati;

 public:
     Strung() :Utilaj()
     {
         nrBucati = 0;
     }
     Strung(char *_codUtilaj, char *_denUtilaj, float _valInventar, int _an, int _durata, int _nrBucati) :Utilaj(_codUtilaj, _denUtilaj, _valInventar, _an, _durata)
     {
         nrBucati = _nrBucati;
     }

     Strung(Strung &s) :Utilaj(static_cast< Utilaj&>(s))
     {
         nrBucati = s.nrBucati;
     }

         };

EDIT: Addeted my classes maybe it helps. I know nothing here is "the way to do it" but my teacher wants them like this and it is no point in arguing with him.

You can't cast an object to another, you can only do it to pointer or reference. — llllllllll, Feb 01 '18 at 22:42
The compiler should flag that as an error. Getting an answer to just this question won't help you in the long run. Please go through [a good textbook](https://stackoverflow.com/questions/388242/the-definitive-c-book-guide-and-list) to build a solid foundation. — R Sahu, Feb 01 '18 at 22:42
What, like object slicing in reverse? How would that make sense? — melpomene, Feb 01 '18 at 22:44
@liliscent And even then, it's only valid if the base class pointer actually points to an instance of the derived class. — Barmar, Feb 01 '18 at 22:47
You can't convert a base class into a derived class because not all the bits are there. But if you have a derived class you can convert it to a base class (via slicing). With objects, casting is done with the pointer. For example if you have a base class pointer pointing to a derived object you can cast the base class pointer into a derived class pointer using dynamic_cast() — Jerry Jeremiah, Feb 01 '18 at 22:47
Oh wait. I know what he is trying to do. He is trying to construct a derived class from a base class. You need a constructor that takes the base class object as a parameter for that to work. — Jerry Jeremiah, Feb 01 '18 at 22:49
@Barmar Yes, just too many fundamental errors in OP's description. — llllllllll, Feb 01 '18 at 22:51
Show us the definition of the classes. If the derived class has a constructor that takes a base class object you just need to `B b(a1);` And if it doesn't you will have to add one. — Jerry Jeremiah, Feb 01 '18 at 22:52
@Barmar You can tell from his example that he isn't using pointers - I think he is trying to construct a B from an A. — Jerry Jeremiah, Feb 01 '18 at 22:54
I think this will help: https://stackoverflow.com/questions/120876/what-are-the-rules-for-calling-the-superclass-constructor — Jerry Jeremiah, Feb 01 '18 at 22:55
_" I want to downcast a1 to a type B"_ I suspect that you **don't** want to do that. What problem are you solving that makes you want to do that? — Drew Dormann, Feb 01 '18 at 22:56
I have a main function from my teacher that needs to run certain lines of code. One of them is "B b1=(B)a1;". I am not new to programming, i programed java for almost 2 yreas, but i am new to C++... — Alexandru Sandu, Feb 02 '18 at 07:59

score 0 · Accepted Answer · answered Feb 01 '18 at 23:44

0

In C++ you can cast a pointer to a base type into a pointer to a derived type using dynamic_cast, assuming it pointed to the derived type in the first place.

class A
{
};

class B : public A
{
};

int main()
{
    A * a = new B();
    B * b = dynamic_cast<B*>(a);

    if( b == nullptr )
    {
        // Error - "a" was not a "B" object
    }

    return 0;
}

When you see (sometype)Object, in C++, that is a C-style cast, and is not the way to enlightenment. This is equivalent to a C++ reinterpret cast and can be quite dangerous. It certainly should not be used where a dynamic_cast is sufficient.

Also, in C++, when you have a polymorphic type, you must use a pointer. You would not cast an instance of an object, but rather, you would cast a pointer to the object.

Please Google both "dynamic_cast" and "reinterpret_cast" for a better understanding of both.

answered Feb 01 '18 at 23:44

Christopher Pisz

3,757
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I already managed to do it like u said, with a dynamic_cast but i was told by my teacher that it should be only with a C-style cast and got 0 points until i manage to do that... – Alexandru Sandu Feb 02 '18 at 08:08
@Alexandru If you are taking a class where you are learning C++, and your teacher told you to use a c-style cast, then your teacher is an idiot. Drop the class and go to the dean for a refund. Unfortunately, many college professors don't have the first clue about the differences between C and C++. Your choices in C++ are static_cast, dynamic_cast, reinterpret_cast, and const_cast. All with their own purpose. C-style cast is there for backwards compatibility with C, but you should always opt for one of the others in C++ as they are more safe. – Christopher Pisz Feb 02 '18 at 13:32
Thx, i read more about what you said, static_cast, dynamic and reinterpret and i had a little chitchat with m teacher giving him facts and relevant information and i mannage to convince him that what he was asking was wrong. – Alexandru Sandu Feb 05 '18 at 12:18
@AlexandruSandu Excellent! We've helped hundreds of future students in the process! Glad to hear! – Christopher Pisz Feb 09 '18 at 16:06

Overriding CAST operator (i think it is called downcasting)

1 Answers1