Re: static_cast and dynamic_cast

From:

Goran <goran.pusic@gmail.com>

Newsgroups:

comp.lang.c++.moderated

Date:

Tue, 12 Oct 2010 17:31:53 CST

Message-ID:

<53b369f8-29d8-46aa-a344-b35d754be0ed@j25g2000yqa.googlegroups.com>

On Oct 11, 5:51 pm, MC <manan.cho...@gmail.com> wrote:

#include <iostream>
using namespace std;

class X{
     public:
     virtual void f(){}

};

class Y {
     public:
     virtual void g() {}

};

int main()
{
     X * x = new X();
     Y* y = dynamic_cast<Y*>(x); //A
     // Y* y = static_cast<Y*>(x); //B
     cout << y << endl;

}

When I try to typecast an unrelated class(not in its inheritance
hierarchy) into another as in the above example,
using a static_cast (line B) I get a compile time error. Whereas with
dynamic_cast the code compiles just fine.
I am curious why doesn't the compiler give a compile type error also
in the case of dynamic_cast?

static_cast requires that compiler can find base (or derived) class in
the inheritance chain. dynamic_cast does not, but it will return 0 if
cast isn't possible. static_cast is static in a sense that it happens
at compile time, dynamic_cast happens at run-time. It will work e.g.
with this:

class base { virtual ~base(); };
class side_interface{ virtual void f(); };
class implementation : public base, public side_interface {};

void work(base& param)
{
   side_interface* pi = dynamic_cast<side_interface*>(&param);
   if (pi)
     pi->f();
}
implementation object;
work(object);

Goran.

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