Use of reinterpret_cast

Question

Is there anything wrong with the following code? In particular I'm interested in the use of reinterpret_cast.

class Base1
{
public:
    virtual void foo(){}
};

class Base2
{
public:
    virtual void bar(){}
};

class Derived : public Base1, public Base2
{
};

int main()
{
    Base1* instance1 = new Derived();
    instance1->foo();

    Base2* instance2 = reinterpret_cast<Base2*>(instance1);
    instance2->bar();

    return 0;
}

Yes. Otherwise you can't cast between `Base2` and `Derived`: http://stackoverflow.com/q/860339/3425536 — Emil Laine, Nov 04 '15 at 17:34
@ksl just because something compiles does not make it correct. — NathanOliver, Nov 04 '15 at 17:35
Well you're using `reinterpret_cast`, so obviously. Probably undefined behavior though. — Emil Laine, Nov 04 '15 at 17:35

score 3 · Accepted Answer · answered Nov 04 '15 at 17:41

reinterpret_cast doesn't know how to handle casting between siblings (for example in vtable implementations it won't fix up the this pointer) so it will definitely not work. Note that it may appear to do what you expect. You'll want to use dynamic_cast in this case, or alternately static_cast to derived and use the implicit conversion to base2.

Emil Laine · Answer 2 · 2015-11-04T17:43:26.027

1

In this case, a simple dynamic_cast to Derived* would be preferred:

Base2* instance2 = dynamic_cast<Derived*>(instance1);

(Or static_cast if you know that *instance1 is really a Derived, and you don't want the dynamic dispatch overhead. Not recommended though.)

edited Nov 04 '15 at 17:43

answered Nov 04 '15 at 17:29

Emil Laine

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@ksl Cast to `Derived*`. – Emil Laine Nov 04 '15 at 17:42

Use of reinterpret_cast

2 Answers2