RTTI

Oh, do not confuse this word with RTI - right to information. This is not a political blog :).

RTTI - run time type identification is a concept in C++, which gives the type of the object during run time. But it can be used only with polymorphic classes - i.e. the classes which have at least one virtual function.

Typeid

typeid is an operator in C++ which gives the actual type of the object. So what is actual type of object? Is it different from the one we have defined it with?

It may be.

class A
{
public:
   virtual void print(){}
};
class B:public A
{
};
int main()
{
   int a;
   A obj1;
   B obj2;
   A *ptr1 = new B;
}
   

 In the code above, type of a is integer. Type of obj1 is A. Type of obj2 is B.

But what about *ptr1? Is the type of *ptr1 A or B?

typeid operator correctly gives its type as B.

Note : We have added a dummy print function in base class, to make A as polymorphic.

Now let us use typeid operator - which needs the inclusion of header file typeinfo

#include<iostream>
#include<typeinfo>
using namespace std;
class A
{
public:
   virtual void print(){}
};
class B:public A
{
};
int main()
{
   int v1;
   A obj1;
   B obj2;
   A *ptr1 = new B;
   cout<<"Type of v1 is "<<typeid(a).name()<<endl;
   cout<<"Type of obj1 is "<<typeid(obj1).name()<<endl;
   cout<<"Type of *ptr1 is "<<typeid(*ptr1).name()<<endl; 
}
   

 But when you run this program, you may see some strange names instead of class names.

The reason is typeid().name returns mangled name in case of gcc compilers. To get unmangled name, straight forward method is to use

./a.out |c++filt -t
Type of v1 is int
Type of obj1 is A
Type of *ptr1 is B


Next let us see how to use dynamic_cast

Dynamic cast

dynamic_cast is used for safe conversion of variables. It can be used for upcast or even downcast also. If we use correct type for dynamic_cast, we get the required value. If not, the program aborts with type error.

#include<iostream> 
using namespace std;
class A
{
 int na;  
public:
virtual void print()
{
   cout<<"print function of class A"<<endl;
}
};
class B:public A
{
  int nb;
};
class C:public A
{
  int nc;
};
void printDyn(A&obja)
{
  cout<<"We are in printDyn function"<<endl; 
  C &refc = dynamic_cast<C&>(obja);
  cout<<"we call print function";
  refc.print();
}
int main()
{ 
    C objc;
    printDyn(objc);
    B objb;
    printDyn(objb);  
}

In this program in printDyn() function, we are converting the reference parameter to C reference. In the first call printDyn(objc), the code works fine because, objc is a C type object.

But when we call the function with object of B class and try to convert it into C reference, the program gives a bad_cast exception and aborts.

Output of the program will be
./a.out
We are in printDyn function
we call print functionprint function of class A
We are in printDyn function
terminate called after throwing an instance of 'std::bad_cast'
  what():  std::bad_cast
Aborted


Please remember that target of dynamic_cast must be either a pointer or a reference to a class.

You can get these notes and also programs, quiz on C++ by downloading the app Simplified C++ by Hegdeapps