CODEWITHSUNDEEP
c++polymorphism
Martin Boryczko
Martin Boryczko

Reputation: 1

C++ - polymorphism: cannot use function

I cannot find the answer, so maybe someone will be able to help me. I don't understand why I can't call ptr->other(). THX

class A
{
public:
  A(){}
  virtual ~A(){}
  virtual void fun(){ cout<<"fun A"<<endl;}
};

class B: public A
{
public:
  B(){}
  ~B(){}
  virtual void fun(){ cout<<"fun B"<<endl;}
  void other(){cout<<"other fuction"<<endl;}
};

int main()
{
  A *ptr = new B();
  ptr->fun();
  ptr->other(); //it doesn't work :(
}

Upvotes: 0

Views: 139

Answers (4)

ObliteratedJillo
ObliteratedJillo

Reputation: 5166

Use have to implicitly tell that the ptr belongs to an object B since ptr used a B constructor :

((B*)(ptr))->other();

Upvotes: 0

Serge Ballesta
Serge Ballesta

Reputation: 148910

That is not the way polymorphism is supposed to be used.

You can indeed assign a B* to a A*, but all the time that you use a pointer to A, you can only use members belonging to A.

If you have a pointer to A and need to use a member of B, you must first downcast the pointer. You can use a static_cast is you know that the pointer does point to a B object, or a dynamic_cast if you want to have a chance to control that:

B* pb = static_cast<B *>(ptr); // unconditional cast
pb->other();                   // undefined behaviour if ptr does not point to a B object

but

B* pb = dynamic_cast<B *>(ptr);  // pb will be null if ptr does not point to a B object
if (pb != nullptr) pb->other();     // perfectly defined behaviour

Upvotes: 3

zar
zar

Reputation: 12227

It's not suppose to work because other() is not defined in A.

In your example the function fun() is correct way of using polymorphism, it needs to be defined in base class and overridden in derived class. It is not the case with other() function.

Upvotes: 1

luther
luther

Reputation: 5554

C++ is statically typed, and ptr is an A *. When you call ptr->fun, the compiler sees that A has a virtual fun and uses what's called a virtual function table to find B::fun at run time. When you call ptr->other, the compiler sees that A has no other at all, so that's an error. That's why you have to use a static_cast to tell the compiler to look in B first.

Upvotes: 0

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