dynamic_cast and static_cast in C++
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.
static_cast< Type* >(ptr)
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;
}
dynamic_cast< Type* >(ptr)
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;
}
The following is not really close to what you get from C++'s dynamic_cast in terms of type checking but maybe it will help you understand its purpose a little bit better:
struct Animal // Would be a base class in C++
{
enum Type { Dog, Cat };
Type type;
};
Animal * make_dog()
{
Animal * dog = new Animal;
dog->type = Animal::Dog;
return dog;
}
Animal * make_cat()
{
Animal * cat = new Animal;
cat->type = Animal::Cat;
return cat;
}
Animal * dyn_cast(AnimalType type, Animal * animal)
{
if(animal->type == type)
return animal;
return 0;
}
void bark(Animal * dog)
{
assert(dog->type == Animal::Dog);
// make "dog" bark
}
int main()
{
Animal * animal;
if(rand() % 2)
animal = make_dog();
else
animal = make_cat();
// At this point we have no idea what kind of animal we have
// so we use dyn_cast to see if it's a dog
if(dyn_cast(Animal::Dog, animal))
{
bark(animal); // we are sure the call is safe
}
delete animal;
}
More than code in C, I think that an english definition could be enough:
Given a class Base of which there is a derived class Derived, dynamic_cast will convert a Base pointer to a Derived pointer if and only if the actual object pointed at is in fact a Derived object.
class Base { virtual ~Base() {} };
class Derived : public Base {};
class Derived2 : public Base {};
class ReDerived : public Derived {};
void test( Base & base )
{
dynamic_cast<Derived&>(base);
}
int main() {
Base b;
Derived d;
Derived2 d2;
ReDerived rd;
test( b ); // throw: b is not a Derived object
test( d ); // ok
test( d2 ); // throw: d2 is not a Derived object
test( rd ); // ok: rd is a ReDerived, and thus a derived object
}
In the example, the call to test binds different objects to a reference to Base. Internally the reference is downcasted to a reference to Derived in a typesafe way: the downcast will succeed only for those cases where the referenced object is indeed an instance of Derived.
Unless you're implementing your own hand-rolled RTTI (and bypassing the system one), it's not possible to implement dynamic_cast directly in C++ user-level code. dynamic_cast is very much tied into the C++ implementation's RTTI system.
But, to help you understand RTTI (and thus dynamic_cast) more, you should read up on the <typeinfo> header, and the typeid operator. This returns the type info corresponding to the object you have at hand, and you can inquire various (limited) things from these type info objects.