I am trying to understand the concept of virtual function in C++ and I read it online but I am not able to understand why the below program output is 2 instead of 1? Can anyone explain?
Class A
{
int a;
public:
A()
{
a = 1;
}
virtual void show()
{
cout <<a;
}
};
Class B: public A
{
int b;
public:
B()
{
b = 2;
}
virtual void show()
{
cout <<b;
}
};
int main()
{
A *pA;
B oB;
pA = &oB;
pA->show();
return 0;
}
you achieve polymorphism with overriding virtual functions and pointers:
in your example you used pA polymorphically so it is a pointer to a base class (A) but you but you assign to it a class B's object which is child of A.
and you you declared Show() as virtual.
this is the main goal of polymorphis; which means we don't know the type of object the base pointer points to until runtime eg in main:
int main()
{
int choice;
cout << "1: A object 2: B object:\n\n";
cin >> choice;
if(1 == choice)
pA = new A; // result will be 1
else
if(2 == choice)
pA = new B; // result will be 2
if(pA)
pA->show();
delete pA;
pA = NULL;
// B* pB = new A; // error: cannot convert from class A* to class B* because C++ is not contravariant
B* pB = new B;
pB->show(); // result: 2
delete pB;
pB = NULL;
return 0;
}
From cppreference
Virtual functions are member functions whose behavior can be overridden in derived classes. As opposed to non-virtual functions, the overridden behavior is preserved even if there is no compile-time information about the actual type of the class. If a derived class is handled using pointer or reference to the base class, a call to an overridden virtual function would invoke the behavior defined in the derived class. This behavior is suppressed if the function is selected using qualified name lookup (that is, if the function's name appears to the right of the scope resolution operator ::)
Since, you are overriding show() in class B, pA->show() will call show() in class B.Hope this helps
Related
I have the following code:
struct A
{
virtual void foo() {std::cout << "A\n";}
};
struct B : public A
{
virtual void foo() {std::cout << "B\n";}
};
void bar(A * a)
{
a->foo();
}
Without changing this code, is it possible to cast bp pointer to B, so calling bar would print "A"?
int main()
{
B * bp = new B();
bar(/* do somethig*/ bp);
return 0;
}
Tried every cast I remebered:
int main()
{
B * bp = new B();
bar((A*)bp);
bar(static_cast<A*>(bp));
bar(reinterpret_cast<A*>(bp));
bar(dynamic_cast<A*>(bp));
return 0;
}
You could make a shim wrapper around B, and have the shim's virtual function dispatched to BWrap::foo() call directly to A::foo();.
There's not really any point in the example to carrying along the B& member variable reference, but for more interesting examples there may be a use case.
struct BWrap : public A
{
B& b;
BWrap(B& bb) : b{bb} {}
virtual void foo() { b.A::foo(); }
};
int main()
{
B* bp = new B();
BWrap bw{*bp};
bar(&bw);
}
If you insist on the A object being a base class subobject of a B object and on not modifying the first code snippet at all, then the only solution is to add an even more derived class that can override the virtual call as explained in the answer by #Eljay (which I completely forgot to think about when first writing this answer).
Other options are to create a complete A object, not a B object, or to modify bar to do a call without virtual dispatch by using a qualified name:
a->A::foo();
All of the casts you are showing have the same effect as the implicit conversion, except for reinterpret_cast which will cause undefined behavior when used this way.
When is dynamic_cast from parent to child useful in practice?
Is this always a bad practice?
This is NOT a duplicate of:
C++ Polymorphism: from parent class to child
Run-time type information in C++
First, let's start with a code example and then let's draw some conclusions.
#include<iostream>
using namespace std;
class B { public: virtual void fun() {cout<<"!class B!\n";} };
class D: public B { public: void fun() {cout<<"!class D!\n";} };
int main()
{
///////// 1. Dynamic initialization parent from child
B *bAux = new D;
D *d = dynamic_cast<D*>(bAux);
cout<<"1.1. for d is "; d->fun(); //class D
B *b = dynamic_cast<B*>(d);
cout<<"1.2. for b is "; b->fun(); //class D
///////// 2. Dynamic initialization child from parent
//D *dAux4 = new B; //invalid conversion from ‘B*’ to ‘D*’
///////// 3. Casting the parent to child
B *bAux2 = new B;
cout<<"3.1. for bAux2 is "; bAux2->fun(); //class B
D *d2 = dynamic_cast<D*>(bAux2);
if (d2 != NULL){ cout<<"3.2. for d2 is "; d2->fun(); } //cannot cast parent to child
else cout<<"3.2. cannot cast B* to D* \n";
///////// 4. Casting the child to parent
D *dAux3 = new D;
cout<<"4.1. for dAux3 is "; dAux3->fun(); //class D
B *b3 = dynamic_cast<B*>(dAux3);
cout<<"4.2. for b3 is "; b3->fun(); //class D
getchar();
return 0;
}
Second, let's draw the conclusions.
The output is:
1.1. for d is !class D!
1.2. for b is !class D!
3.1. for bAux2 is !class B!
3.2. cannot cast B* to D*
4.1. for dAux3 is !class D!
4.2. for b3 is !class D!
But for case 3, we actually try to dynamic cast from B* to D* (as mentioned, not possible). I wonder in which cases is this useful? You can break the cast typing (as a parallel, in the case of static type checking, there is the well-known problem: Since a Bear is an Animal, shouldn't a set of Bears be a set of Animals? No, because then you can insert a Wolf into the set and have a Wolf among the Bears).
So, the question is, in which case is the dynamic cast from parent to child useful? Is this a bad practice?
EDIT:
Following the discussion, a particular case:
///////// 5. Try to cast the parent initialized from a child to another child
B *bAux = new D;
F *f = dynamic_cast<F*>(bAux);
if(f == NULL)
cout<<"5.1. Cannot cast the parent initialized from a child to another child \n";// f->fun(); //segmentation fault
B *b = dynamic_cast<B*>(f);
if(b == NULL)
cout<<"5.2. Cannot cast the NULL pointer back to parent \n"; //b->fun(); //segmentation fault
dynamic_cast can be useful for the case you want to get derived objects interface:
if there is f() in derived and no f() function in base than you can't call f() via B*. but
you can dynamic_cast to D* and call f() (if B* points to D object). (Remembering that dynamic_cast is for polymorphic types)
Consider the following code:
class B {
public:
virtual void g()
{
std::cout << "Base::g()\n";
}
};
class D : public B
{
public:
void f()
{
std::cout << "f()\n";
}
void g()
{
std::cout << "Derived::g()\n";
}
};
int main()
{
B* p = new D;
//p->f(); // class B has no member f
D* pd = dynamic_cast<D*>(p);
pd->f(); // ok f() will be called
return 0;
}
One example is when there are multiple base classes of the same type. Only dynamic_cast can downcast from a non-unique base class to the most derived:
struct A { virtual ~A() = 0; };
struct B1 : A {};
struct B2 : A {};
struct C : B1, B2 {};
C& f(A& a) { return dynamic_cast<C&>(a); } // Only dynamic_cast can cast A& to C&.
Another example is that only dynamic_cast<void*> can return a pointer to the address of the complete object, no other cast can do.
What's the difference between redefining and using a virtual function? Don't they serve the same purpose? You are allowing objects of a derived class to call their own versions of a function by the same name in both cases..so where is the distinction?
An example says it best:
#include <iostream>
using namespace std;
class A {
public:
virtual void f1() { cout << "Class A" << endl; }
void f2() { cout << "Class A" << endl; }
virtual ~A(){}
};
class B : public A {
public:
virtual void f1() { cout << "Class B" << endl; }
void f2() { cout << "Class B" << endl; }
virtual ~B(){}
};
int main()
{
A *a = new B;
a->f1();
a->f2();
}
...
$ ./override
Class B
Class A
You can see that when we reference the instance of B, f1() still calls B's version, but f2() calls A's.
When you declare a function virtual, you are saying that when we call it we should use the vtable to look up the right version of the function to call, and so you will always get the most derived version of the function, even if you reference it as an ancestor type. Without virtual, it will simply use the definition in the type you are referencing it as.
The distinction is when you have a reference or pointer to the base class. A call to a virtual function will call the most-derived version while a call to a normal function will call the base-class version.
If you're using a variable directly or a reference or pointer to the most-derived class, there is no practical difference.
TL;DR
The only way to take advantage of the polymorphism in C++ is via virtual functions and pointers (and references). The virtual keyword will tell the compiler to allocate a virtual function table where to look when deciding what version of the virtual function to call when dealing with pointer to the base class with a dynamic type that is of a child class.
How does polymorphism work in C++
Let's take a simple example:
class A { public: virtual void eat() { std::cout << "Class A" << std::endl; }
class B : public A {};
class C : public B { virtual void eat() { std::cout << "Class C" << std::endl; }
Note: the virtual keyword could be omitted after the first function/method definition.
The following:
A a; B b; C c;
A* ptrA = &a; A* ptrB = &b; A* ptrC = &c;
ptrA->eat();
ptrB->eat();
ptrC->eat();
will print:
Class A
Class A
Class C
If we wouldn't have declared the function eat virtual, the output would have simply been:
Class A
Class A
Class A
But why
A* ptrA = &a; A* ptrB = &b; A* ptrC = &c;
Instead of
A* ptrA = &a; B* ptrB = &b; C* ptrC = &c;
With the second version, the eat() method can simply be redefined for each derived class, without the need for virtual, right?
I have a particular scenario below. The code below should print 'say()' function of B and C class and print 'B says..' and 'C says...' but it doesn't .Any ideas..
I am learning polymorphism so also have commented few questions related to it on the lines of code below.
class A
{
public:
// A() {}
virtual void say() { std::cout << "Said IT ! " << std::endl; }
virtual ~A(); //why virtual destructor ?
};
void methodCall() // does it matters if the inherited class from A is in this method
{
class B : public A{
public:
// virtual ~B(); //significance of virtual destructor in 'child' class
virtual void say () { // does the overrided method also has to be have the keyword 'virtual'
cout << "B Sayssss.... " << endl;
}
};
class C : public A {
public:
//virtual ~C();
virtual void say () { cout << "C Says " << endl; }
};
list<A> listOfAs;
list<A>::iterator it;
# 1st scenario
B bObj;
C cObj;
A *aB = &bObj;
A *aC = &cObj;
# 2nd scenario
// A aA;
// B *Ba = &aA;
// C *Ca = &aA; // I am declaring the objects as in 1st scenario but how about 2nd scenario, is this suppose to work too?
listOfAs.insert(it,*aB);
listOfAs.insert(it,*aC);
for (it=listOfAs.begin(); it!=listOfAs.end(); it++)
{
cout << *it.say() << endl;
}
}
int main()
{
methodCall();
return 0;
}
Your problem is called slicing and you should check this question: Learning C++: polymorphism and slicing
You should declare this list as a list of pointers to As:
list<A*> listOfAs;
and then insert these aB and aC pointers to it instead of creating copies of objects they are pointing to. The way you insert elements into list is wrong, you should rather use push_back function for inserting:
B bObj;
C cObj;
A *aB = &bObj;
A *aC = &cObj;
listOfAs.push_back(aB);
listOfAs.push_back(aC);
Then your loop could look like this:
list<A*>::iterator it;
for (it = listOfAs.begin(); it != listOfAs.end(); it++)
{
(*it)->say();
}
Output:
B Sayssss....
C Says
Hope this helps.
Polymorphism of virtual class hierarchies only works through references or pointers to a base subobject:
struct Der : Base { /* ... */ };
Der x;
Base & a = x;
a.foo(); // calls Der::foo() from x
The function foo is dispatched polymorphically if it is a virtual function in Base; the polymorphism refers to the fact that while you are calling a member function of an object of type Base, the function that actually gets called may be implemented in the class Der.
Containers can only store elements of a fixed type. In order to store a polymorphic collection, you could instead have a container of pointers to the base class. Since you need to store the actual objects elsewhere, lifetime management is non-trivial and best left to a dedicated wrapper such as unique_ptr:
#include <list>
#include <memory>
int main()
{
std::list<std::unique_ptr<Base>> mylist;
mylist.emplace_back(new Der1);
mylist.emplace_back(new Der2);
// ...
for (p : mylist) { p->foo(); /* dispatched dynamically */ }
}
list::iterator it;
B bObj;
C cObj;
A *aB = &bObj;
A *aC = &cObj;
listOfAs.insert(it,*aB);
Do you not need to initialize "it" ?
I believe you should do it = listOfAs.begin(); before starting to insert.
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.
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 certain 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;
}
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.
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.
First, to describe dynamic cast in C terms, we have to represent classes in C.
Classes with virtual functions use a "VTABLE" of pointers to the virtual functions.
Comments are C++. Feel free to reformat and fix compile errors...
// class A { public: int data; virtual int GetData(){return data;} };
typedef struct A { void**vtable; int data;} A;
int AGetData(A*this){ return this->data; }
void * Avtable[] = { (void*)AGetData };
A * newA() { A*res = malloc(sizeof(A)); res->vtable = Avtable; return res; }
// class B : public class A { public: int moredata; virtual int GetData(){return data+1;} }
typedef struct B { void**vtable; int data; int moredata; } B;
int BGetData(B*this){ return this->data + 1; }
void * Bvtable[] = { (void*)BGetData };
B * newB() { B*res = malloc(sizeof(B)); res->vtable = Bvtable; return res; }
// int temp = ptr->GetData();
int temp = ((int(*)())ptr->vtable[0])();
Then a dynamic cast is something like:
// A * ptr = new B();
A * ptr = (A*) newB();
// B * aB = dynamic_cast<B>(ptr);
B * aB = ( ptr->vtable == Bvtable ? (B*) aB : (B*) 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;
}
A dynamic_cast performs a type checking using RTTI. If it fails it'll throw you an exception (if you gave it a reference) or NULL if you gave it a pointer.
There are no classes in C, so it's impossible to to write dynamic_cast in that language. C structures don't have methods (as a result, they don't have virtual methods), so there is nothing "dynamic" in it.
No, not easily. The compiler assigns a unique identity to every class, that information is referenced by every object instance, and that is what gets inspected at runtime to determine if a dynamic cast is legal. You could create a standard base class with this information and operators to do the runtime inspection on that base class, then any derived class would inform the base class of its place in the class hierarchy and any instances of those classes would be runtime-castable via your operations.
edit
Here's an implementation that demonstrates one technique. I'm not claiming the compiler uses anything like this, but I think it demonstrates the concepts:
class SafeCastableBase
{
public:
typedef long TypeID;
static TypeID s_nextTypeID;
static TypeID GetNextTypeID()
{
return s_nextTypeID++;
}
static TypeID GetTypeID()
{
return 0;
}
virtual bool CanCastTo(TypeID id)
{
if (GetTypeID() != id) { return false; }
return true;
}
template <class Target>
static Target *SafeCast(SafeCastableBase *pSource)
{
if (pSource->CanCastTo(Target::GetTypeID()))
{
return (Target*)pSource;
}
return NULL;
}
};
SafeCastableBase::TypeID SafeCastableBase::s_nextTypeID = 1;
class TypeIDInitializer
{
public:
TypeIDInitializer(SafeCastableBase::TypeID *pTypeID)
{
*pTypeID = SafeCastableBase::GetNextTypeID();
}
};
class ChildCastable : public SafeCastableBase
{
public:
static TypeID s_typeID;
static TypeID GetTypeID()
{
return s_typeID;
}
virtual bool CanCastTo(TypeID id)
{
if (GetTypeID() != id) { return SafeCastableBase::CanCastTo(id); }
return true;
}
};
SafeCastableBase::TypeID ChildCastable::s_typeID;
TypeIDInitializer ChildCastableInitializer(&ChildCastable::s_typeID);
class PeerChildCastable : public SafeCastableBase
{
public:
static TypeID s_typeID;
static TypeID GetTypeID()
{
return s_typeID;
}
virtual bool CanCastTo(TypeID id)
{
if (GetTypeID() != id) { return SafeCastableBase::CanCastTo(id); }
return true;
}
};
SafeCastableBase::TypeID PeerChildCastable::s_typeID;
TypeIDInitializer PeerChildCastableInitializer(&PeerChildCastable::s_typeID);
int _tmain(int argc, _TCHAR* argv[])
{
ChildCastable *pChild = new ChildCastable();
SafeCastableBase *pBase = new SafeCastableBase();
PeerChildCastable *pPeerChild = new PeerChildCastable();
ChildCastable *pSameChild = SafeCastableBase::SafeCast<ChildCastable>(pChild);
SafeCastableBase *pBaseToChild = SafeCastableBase::SafeCast<SafeCastableBase>(pChild);
ChildCastable *pNullDownCast = SafeCastableBase::SafeCast<ChildCastable>(pBase);
SafeCastableBase *pBaseToPeerChild = SafeCastableBase::SafeCast<SafeCastableBase>(pPeerChild);
ChildCastable *pNullCrossCast = SafeCastableBase::SafeCast<ChildCastable>(pPeerChild);
return 0;
}
static_cast< Type* >(ptr)
static_cast in C++ can be used in scenarios where all type casting can be verified at compile time.
dynamic_cast< Type* >(ptr)
dynamic_cast in C++ can be used to perform type safe down casting. dynamic_cast is run time polymorphism. The dynamic_cast operator, which safely converts from a pointer (or reference) to a base type to a pointer (or reference) to a derived type.
eg 1:
#include <iostream>
using namespace std;
class A
{
public:
virtual void f(){cout << "A::f()" << endl;}
};
class B : public A
{
public:
void f(){cout << "B::f()" << endl;}
};
int main()
{
A a;
B b;
a.f(); // A::f()
b.f(); // B::f()
A *pA = &a;
B *pB = &b;
pA->f(); // A::f()
pB->f(); // B::f()
pA = &b;
// pB = &a; // not allowed
pB = dynamic_cast<B*>(&a); // allowed but it returns NULL
return 0;
}
For more information click here
eg 2:
#include <iostream>
using namespace std;
class A {
public:
virtual void print()const {cout << " A\n";}
};
class B {
public:
virtual void print()const {cout << " B\n";}
};
class C: public A, public B {
public:
void print()const {cout << " C\n";}
};
int main()
{
A* a = new A;
B* b = new B;
C* c = new C;
a -> print(); b -> print(); c -> print();
b = dynamic_cast< B*>(a); //fails
if (b)
b -> print();
else
cout << "no B\n";
a = c;
a -> print(); //C prints
b = dynamic_cast< B*>(a); //succeeds
if (b)
b -> print();
else
cout << "no B\n";
}
dynamic_cast uses RTTI. It can slow down your application, you can use modification of the visitor design pattern to achieve downcasting without RTTI http://arturx64.github.io/programming-world/2016/02/06/lazy-visitor.html