I am quite confused with the dynamic_cast
keyword in C++.
struct A {
virtual void f() { }
};
struct B : public A { };
struct C { };
void f () {
A a;
B b;
A* ap = &b;
B* b1 = dynamic_cast<B*> (&a); // NULL, because 'a' is not a 'B'
B* b2 = dynamic_cast<B*> (ap); // 'b'
C* c = dynamic_cast<C*> (ap); // NULL.
A& ar = dynamic_cast<A&> (*ap); // Ok.
B& br = dynamic_cast<B&> (*ap); // Ok.
C& cr = dynamic_cast<C&> (*ap); // std::bad_cast
}
the definition says:
The
dynamic_cast
keyword casts a datum from one pointer or reference type to another, performing a runtime check to ensure the validity of the cast
Can we write an equivalent of dynamic_cast
of C++ in C so that I could better understand things?
Here's a rundown on static_cast<>
and dynamic_cast<>
specifically as they pertain to pointers. This is just a 101-level rundown, it does not cover all the intricacies.
This takes the pointer in ptr
and tries to safely cast it to a pointer of type Type*
. This cast is done at compile time. It will only perform the cast if the types are related. If the types are not related, you will get a compiler error. For example:
class B {};
class D : public B {};
class X {};
int main()
{
D* d = new D;
B* b = static_cast<B*>(d); // this works
X* x = static_cast<X*>(d); // ERROR - Won't compile
return 0;
}
This again tries to take the pointer in ptr
and safely cast it to a pointer of type Type*
. But this cast is executed at runtime, not compile time. Because this is a run-time cast, it is useful especially when combined with polymorphic classes. In fact, in certian cases the classes must be polymorphic in order for the cast to be legal.
Casts can go in one of two directions: from base to derived (B2D) or from derived to base (D2B). It's simple enough to see how D2B casts would work at runtime. Either ptr
was derived from Type
or it wasn't. In the case of D2B dynamic_cast<>s, the rules are simple. You can try to cast anything to anything else, and if ptr
was in fact derived from Type
, you'll get a Type*
pointer back from dynamic_cast
. Otherwise, you'll get a NULL pointer.
But B2D casts are a little more complicated. Consider the following code:
#include <iostream>
using namespace std;
class Base
{
public:
virtual void DoIt() = 0; // pure virtual
virtual ~Base() {};
};
class Foo : public Base
{
public:
virtual void DoIt() { cout << "Foo"; };
void FooIt() { cout << "Fooing It..."; }
};
class Bar : public Base
{
public :
virtual void DoIt() { cout << "Bar"; }
void BarIt() { cout << "baring It..."; }
};
Base* CreateRandom()
{
if( (rand()%2) == 0 )
return new Foo;
else
return new Bar;
}
int main()
{
for( int n = 0; n < 10; ++n )
{
Base* base = CreateRandom();
base->DoIt();
Bar* bar = (Bar*)base;
bar->BarIt();
}
return 0;
}
main()
can't tell what kind of object CreateRandom()
will return, so the C-style cast Bar* bar = (Bar*)base;
is decidedly not type-safe. How could you fix this? One way would be to add a function like bool AreYouABar() const = 0;
to the base class and return true
from Bar
and false
from Foo
. But there is another way: use dynamic_cast<>
:
int main()
{
for( int n = 0; n < 10; ++n )
{
Base* base = CreateRandom();
base->DoIt();
Bar* bar = dynamic_cast<Bar*>(base);
Foo* foo = dynamic_cast<Foo*>(base);
if( bar )
bar->BarIt();
if( foo )
foo->FooIt();
}
return 0;
}
The casts execute at runtime, and work by querying the object (no need to worry about how for now), asking it if it the type we're looking for. If it is, dynamic_cast<Type*>
returns a pointer; otherwise it returns NULL.
In order for this base-to-derived casting to work using dynamic_cast<>
, Base, Foo and Bar must be what the Standard calls polymorphic types. In order to be a polymorphic type, your class must have at least one virtual
function. If your classes are not polymorphic types, the base-to-derived use of dynamic_cast
will not compile. Example:
class Base {};
class Der : public Base {};
int main()
{
Base* base = new Der;
Der* der = dynamic_cast<Der*>(base); // ERROR - Won't compile
return 0;
}
Adding a virtual function to base, such as a virtual dtor, will make both Base and Der polymorphic types:
class Base
{
public:
virtual ~Base(){};
};
class Der : public Base {};
int main()
{
Base* base = new Der;
Der* der = dynamic_cast<Der*>(base); // OK
return 0;
}