C++ RTTI

What is RTTI?

• RTTI stands for Run-time Type Identification.
• RTTI is useful in applications in which the type of objects is known only at run-time.
• Use of RTTI should be minimized in programs and wherever possible static type system should be used.
• RTTI allows programs that manipulate objects or references to base classes to retrieve the actual derived types to which they point to at run-time.
• Two operators are provided in C++ for RTTI.
• dynamic_cast operator. The dynamic_cast operator can be used to convert a pointer that refers to an object of class type to a pointer to a class in the same hierarchy. On failure to cast the dynamic_cast operator returns 0.
• typeid operator. The typeid operator allows the program to check what type an expression is. When a program manipulates an object through a pointer or a reference to a base class, the program needs to find out the actual type of the object manipulated.
• The operand for both dynamic_cast and typeid should be a class with one or more virtual functions.

EXAMPLE: Demonstrate the RTTI mechanisms in C++

#include <iostream>
#include <typeinfo> // Header for typeid operator
using namespace std;

// Base class
class MyBase {
  public:
    virtual void Print() {
        cout << "Base class" << endl;
    };
};

// Derived class
class MyDerived : public MyBase {
  public:
    void Print() {
        cout << "Derived class" << endl;
    };
};

int main()
{
    // Using typeid on built-in types types for RTTI
    cout << typeid(100).name() << endl;    
    cout << typeid(100.1).name() << endl;

    // Using typeid on custom types for RTTI
    MyBase* b1 = new MyBase();
    MyBase* d1 = new MyDerived();

    MyBase* ptr1;
    ptr1 = d1;

    cout << typeid(*b1).name() << endl;
    cout << typeid(*d1).name() << endl;
    cout << typeid(*ptr1).name() << endl;

    if ( typeid(*ptr1) == typeid(MyDerived) ) {
    cout << "Ptr has MyDerived object" << endl;
    }

    // Using dynamic_cast for RTTI
    MyDerived* ptr2 = dynamic_cast<MyDerived*> ( d1 );
    if ( ptr2 ) {
    cout << "Ptr has MyDerived object" << endl;
    }
}

OUTPUT:
i
d
6MyBase
9MyDerived
9MyDerived
Ptr has MyDerived object
Ptr has MyDerived object

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