Dynamic casting between array of objects(base case) and a derived class

Question

I have an array of objects whose class is a base class, and the elements of the array are derived classes.

    Object *object [kNumPlayers] =
   {
        new Human ("Ms. Jones", 50, 1.0f, 2.0f),
        new Alien ("Mx. Charm", 70),
        new Human,
        new Alien,
   };

So here, Object is the base class, and Human and Alien are derived classes. Now my problem is that I need to access each of the objects and add extra functionality. As in, I need to access object[0](which is Human) and add whatever needs to be added. So I tried,

   Human human0 = (Human)object[0]; // ERROR:'no matching function to call                  Human::Human(Object*&)'
                        OR
   Human *human0;
   human0 = dynamic_cast<Human*>(object[0]); //ERROR: cannot dynamic_cast 'object[0]' (of type 'class Oject*') to type 'class Human*' (source is not polymorphic)'

with raw pointers you should write Human* human0 = (Human*)object[0]; — vsoftco
– vsoftco, Commented Apr 13, 2015 at 16:03
How do I then access object[0] in Human class? If i have 2 objects say, Human* human0 = (Human*)object[0]; and Human* human1 = (Human*)object[1], then how do I say "if (object[0]) then - "This is want I want to do" — user2948246
– user2948246, Commented Apr 13, 2015 at 19:54

Sarfaraz Nawaz · Accepted Answer · 2015-04-13 15:58:02Z

For the dynamic_cast to compile, the rule is that the source class must be polymorphic class — which means it must have at least one virtual function, even be it the destructor only. In your case, the source class is the Object class which must be polymorphic:

class Object
{
   public:
     virtual ~Object() = default; //at least, it should be virtual
     //etc
};

Once you fix that the code should at least compile. However, there are many things you should avoiding using in C++:

Avoid using C-style arrays (dynamic or otherwise)
- Prefer std::vector or std::array.
Avoid managing memory yourself — that is, avoid using naked new.
- Prefer RAII — in your case use std::unique_ptr or std::shared_ptr whatever suits your need.

That is you should something like this:

 std::array<std::shared_ptr<Object>, 10> objects {
        std::make_shared<Human>("Ms. Jones", 50, 1.0f, 2.0f),
        std::make_shared<Alien>("Mx. Charm", 70)
        //etc
 };

And cast as:

 std::shared_ptr<Human> human = std::dynamic_pointer_cast<Human>(objects[0]);

Hope that helps.

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