Re: How dynamic_cast works internally?
On Oct 22, 2:20 am, Lance Diduck <lancedid...@nyc.rr.com> wrote:
On Oct 22, 4:51 am, Bo Yang <struggl...@gmail.com> wrote:> Hi,
I can understand static_cast, reinterpret_cast and const_cast, they
all work at compile time. But I can figure out how the C++'s dynamic-
cast works? Could you please explain how for me?
Thanks in advance!
Regards!
Bo
Consider this case:
class A{virtual ~A(){};};
class B:public A{};
A*a=new B;
B*b =dynamic_cast<B*>(a);. .
B* Dynamic_Cast(A*a ){
if(*((a->vptr)+1)()==B_RTTI())
return a;
return 0;
}
This implementation might work for typeid - because typeid has to
confirm only that type of the object being tested - matches the
specified type exactly.
A dynamic_cast operator however has to perform a much more complicated
test: whether an object (of unknown thype) is related to another type.
And to make do with a lot less information about the classes involved
than existed in the example above. For example:
struct A
{
virtual ~A() {}
};
bool TestA(void *p)
{
assert( p != NULL);
return dynamic_cast<A*>(p) != NULL;
}
Note that no derived classes of A (if any exist) are visible when this
source file is compiled. So no matter how many large or complex this
program's class hierarchy might be, dynamic_cast<> has only the "A"
class declaration with which to work here.
So how is dynamic_cast<> supposed to figure out whether p points to a
type that is somehow related to A? Well, given these constraints,
there is only one way to solve this problem. dynamic_cast must search
p's class hierarchy (essentially, discovering its layout as it goes
along) and to continue the search until either a A class is located
with the hierarchy - or there are no other classes left in the
hierarchy that need to be checked.
Greg