Re: Don't want to define derived class?

From:

=?UTF-8?B?RXJpayBXaWtzdHLDtm0=?= <Erik-wikstrom@telia.com>

Newsgroups:

comp.lang.c++

Date:

Wed, 03 Sep 2008 16:06:49 GMT

Message-ID:

<tCyvk.2027$U5.1097@newsb.telia.net>

On 2008-09-03 00:35, Immortal Nephi wrote:

     The rule of inheritance states that you define from top to bottom.
Sometimes, you want to define base class and set reference from
dervied class to base class, but you violate the rule.
    Here is an example of my code below.

class A {};
class B : public A {};

int main(void)
{
// A a;
    B &b = a; // Compiler cannot compile -- error
// B &b = reinterpret_cast<B&> ( a ); // OK

    B b;
    A &a = b; // OK

    return 0;
}

    Please state your opinion. Is it ok to violate the rule? is this
code portable to all different machines?

Absolutely not, consider the following code:

class Base
{
public:
  virtual ~Base() {}
};

class Derived
{
  int i;
public:
  void foo() { i = 1; }
};

int main()
{
  Base a;
  Derived& b = reinterpret_cast<Derived&>(a);

  b.foo();
}

When I run foo(), exactly what memory is it that I set to 1? When I run
this in VS2008 I overwrite the vtable pointer but.

You can never cast a class of base type to a class to derived type since
there does not exist a is-a relation in that direction (i.e. which a
derived class is a base class, a base class is not a vehicle. That would
be the same thing as saying that a vehicle is a car just because a car
is a vehicle).

--
Erik Wikstr??m