Re: Cast to derived class?

From:
"Alf P. Steinbach" <alfps@start.no>
Newsgroups:
comp.lang.c++
Date:
Mon, 12 Nov 2007 09:05:58 +0100
Message-ID:
<13jg2b8s75a2d78@corp.supernews.com>
* christian.pontesegger@googlemail.com:

Hi all,

lately I had a problem where I wanted to cast a basic class to a
derived type, which did not introduce any new members and did not
change any stuff with dynamically allocated memory. I just wanted to
add some methods to the class.

So my question is:
Is it valid C++ to create a basic class and to cast it to a derived
one the way I did in my example below?


No, it's Undefined Behavior.

Example code:

<code>

#include <stdio.h>

class Basic {

public:
    Basic(int a, int b) {
        a_ = a;
        b_ = b;
    };

    int getA() {
        return a_;
    };


The prefix "get" serves no useful purpose. It just clutters the code
and incorrectly indicates some dynamic action to obtain the value.

    int getB() {
        return b_;
    };

private:
    int a_, b_;
};

class Derived : Basic {

public:
    Derived(int a, int b) : Basic(a, b) {}
    ;

    int multiply() {
        return getA() * getB();
    };
};

int main() {
    Basic *basic = new Basic(2, 4);

    Derived *derived = (Derived *)basic;
    printf("%d * %d = %d\n", basic->getA(), basic->getB(), derived-

multiply());


    delete basic;

    return 0;
}

</code>


All you're achieving is the member function notation, and that's far too
little to justify using implementation specific behavior.

Instead, in this particular case, define

   int productOver( Base const& x ) { return x.a()*x.b(); }

Cheers, & hth.,

- Alf

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