Dynamic binding and virtual functions - Vector of base class objects, access the "correct" member function

Question

I'm fairly new to c++, and I'm trying to understand as much as possible of the language and the underlying mechanics.

I am confused about a specific case where I can't seem to access the correct member function when I have two classes (Base and Derived) even though I define them as virtual.

class Base {
  public:
  virtual void print() { std::cout << "Base" << std::endl; };
};

class Derived : public Base {
  public:
  virtual void print() { std::cout << "Derived" << std::endl; };
};

int main() {

  Base b;
  Derived d;

  b.print();
  d.print();

  Base* p1 = &b;
  Base* p2 = &d;

  p1->print();
  p2->print();

  std::vector<Base> v { b, d };

  v[0].print();
  v[1].print(); // Why doesn't this print "Derived?"

  return 0;
}

Output:

Base
Derived
Base
Derived
Base
Base <- ??

virtual seems to "work" with pointers of type Base, but not for objects in vector<Base>.

What's going on?

You can't create vectors of baseclass objects. This is like assigning a derived object to a base object and causes "slicing" or "truncation". This is a well-known issue. — Ulrich Eckhardt
– Ulrich Eckhardt, Commented Oct 27, 2021 at 7:24

Kaldrr · Accepted Answer · 2021-10-27 09:10:44Z

1

Because of object slicing. When you create your vector you copy your objects into it.

std::vector<Base> v{ b, d };

The catch is that inside the std::vector you have only objects of type Base, NOT Base and Derived. If you want polymorphic behavior with containers in C++, you usually do it with pointers, as you can't keep raw references in them.

std::vector<Base*> v{ &b, &d };
for (Base* ptr : v) {
    ptr->print();
}

edited Oct 27, 2021 at 9:10

answered Oct 27, 2021 at 7:22

Kaldrr

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