Calling virtual functions inside constructors

Question

Suppose I have two C++ classes:

class A
{
public:
  A() { fn(); }

  virtual void fn() { _n = 1; }
  int getn() { return _n; }

protected:
  int _n;
};

clas         


        

Answer 1

The reason is that C++ objects are constructed like onions, from the inside out. Base classes are constructed before derived classes. So, before a B can be made, an A must be made. When A's constructor is called, it's not a B yet, so the virtual function table still has the entry for A's copy of fn().