C++ virtual function from constructor [duplicate]

Question

Why the following example prints "0" and what must change for it to print "1" as I expected ?

#include <iostream> struct base {    virtual const int value() const {       return 0;    }    base() {       std::cout << value() << std::endl;    }    virtual ~base() {} };  struct derived : public base {    virtual const int value() const {       return 1;    } };  int main(void) {    derived example; } 

Answer 1

Because base is constructed first and hasn't "matured" into a derived yet. It can't call methods on an object when it can't guarantee that the object is already properly initialized.

Answer 2

When a derived object is being constructed, before the body of the derived class constructor is called the base class constructor must complete. Before the derived class constructor is called the dynamic type of the object under construction is a base class instance and not a derived class instance. For this reason, when you call a virtual function from a constructor, only the base class virtual function overrides can be called.