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 C++ Standard (ISO/IEC 14882-2014) say's:

Member functions, including virtual functions (10.3), can be called during construction or destruction (12.6.2). When a virtual function is called directly or indirectly from a constructor or from a destructor, including during the construction or destruction of the class’s non-static data members, and the object to which the call applies is the object (call it x) under construction or destruction, the function called is the final overrider in the constructor’s or destructor’s class and not one overriding it in a more-derived class. If the virtual function call uses an explicit class member access (5.2.5) and the object expression refers to the complete object of x or one of that object’s base class subobjects but not x or one of its base class subobjects, the behavior is undefined.

So, Don't invoke virtual functions from constructors or destructors that attempts to call into the object under construction or destruction, Because the order of construction starts from base to derived and the order of destructors starts from derived to base class.

So, attempting to call a derived class function from a base class under construction is dangerous.Similarly, an object is destroyed in reverse order from construction, so attempting to call a function in a more derived class from a destructor may access resources that have already been released.