Run-time type information

In computer programming, run-time type information or run-time type identification (RTTI)^[1] refers to a C++ mechanism that exposes information about an object's data type at runtime. Run-time type information can apply to simple data types, such as integers and characters, or to generic types. This is a C++ specialization of a more general concept called type introspection. Similar mechanisms are also known in other programming languages, such as Object Pascal (Delphi).

In the original C++ design, Bjarne Stroustrup did not include run-time type information, because he thought this mechanism was often misused.^[2]

Overview

The dynamic_cast<> operation and typeid operator in C++ are part of RTTI.

The C++ run-time type information permits performing safe typecasts and manipulating type information at run time.

RTTI is available only for classes which are polymorphic, which means they have at least one virtual method. In practice, this is not a limitation because base classes must have a virtual destructor to allow objects of derived classes to perform proper cleanup if they are deleted from a base pointer.

RTTI is optional with some compilers; the programmer can choose at compile time whether to include the function. There may be a resource cost to making RTTI available even if the program does not use it.

typeid

The typeid keyword is used to determine the class of an object at run time. It returns a reference to std::type_info object, which exists until the end of the program.^[3] The use of typeid, in a non-polymorphic context, is often preferred over dynamic_cast<class_type> in situations where just the class information is needed, because typeid is a constant-time procedure, whereas dynamic_cast must traverse the class derivation lattice of its argument at runtime. Some aspects of the returned object are implementation-defined, such as std::type_info::name(), and cannot be relied on across compilers to be consistent.

Objects of class std::bad_typeid are thrown when the expression for typeid is the result of applying the unary * operator on a null pointer. Whether an exception is thrown for other null reference arguments is implementation-dependent. In other words, for the exception to be guaranteed, the expression must take the form typeid(*p) where p is any expression resulting in a null pointer.

Example

#include <iostream>    // cout
#include <typeinfo>    // for 'typeid'

class Person {
public:
   virtual ~Person() {}
};

class Employee : public Person {
};

int main() 
{
   Person person;
   Employee employee;
   Person* ptr = &employee;
   Person& ref = employee;
   // The string returned by typeid::name is implementation-defined
   std::cout << typeid(person).name() << std::endl;   // Person (statically known at compile-time)
   std::cout << typeid(employee).name() << std::endl; // Employee (statically known at compile-time)
   std::cout << typeid(ptr).name() << std::endl;      // Person* (statically known at compile-time)
   std::cout << typeid(*ptr).name() << std::endl;     // Employee (looked up dynamically at run-time
                                                      //           because it is the dereference of a
                                                      //           pointer to a polymorphic class)
   std::cout << typeid(ref).name() << std::endl;      // Employee (references can also be polymorphic)

   Person* p = nullptr;
   try {
      typeid(*p); // not undefined behavior; throws std::bad_typeid
   }
   catch (...) {
   }

   Person& pRef = *p; // Undefined behavior: dereferencing null
   typeid(pRef);      // does not meet requirements to throw std::bad_typeid
                      // because the expression for typeid is not the result
                      // of applying the unary * operator
}

Output (exact output varies by system):

Person
Employee
Person*
Employee
Employee

dynamic_cast and Java cast

The dynamic_cast operator in C++ is used for downcasting a reference or pointer to a more specific type in the class hierarchy. Unlike the static_cast, the target of the dynamic_cast must be a pointer or reference to class. Unlike static_cast and C-style typecast (where type check is made during compilation), a type safety check is performed at runtime. If the types are not compatible, an exception will be thrown (when dealing with references) or a null pointer will be returned (when dealing with pointers).

A Java typecast behaves similarly; if the object being cast is not actually an instance of the target type, and cannot be converted to one by a language-defined method, an instance of java.lang.ClassCastException will be thrown.^[4]

Example

Suppose some function takes an object of type A as its argument, and wishes to perform some additional operation if the object passed is an instance of B, a subclass of A. This can be accomplished using dynamic_cast as follows.

#include <typeinfo> // For std::bad_cast
#include <iostream> // For std::cout, std::err, std::endl etc.

class A {
public:
    // Since RTTI is included in the virtual method table there should be at least one virtual function.
    virtual ~A() { };
    void methodSpecificToA() { std::cout << "Method specific for A was invoked" << std::endl; };
};
 
class B : public A {
public:
    void methodSpecificToB() { std::cout << "Method specific for B was invoked" << std::endl; };
    virtual ~B() { };
};
 
void my_function(A& my_a)
{
    try {
        B& my_b = dynamic_cast<B&>(my_a); // cast will be successful only for B type objects.
        my_b.methodSpecificToB();
    }
    catch (const std::bad_cast& e) {
        std::cerr << "  Exception " << e.what() << " thrown." << std::endl;
        std::cerr << "  Object is not of type B" << std::endl;
    }
}

int main()
{
    A *arrayOfA[3];          // Array of pointers to base class (A)
    arrayOfA[0] = new B();   // Pointer to B object
    arrayOfA[1] = new B();   // Pointer to B object
    arrayOfA[2] = new A();   // Pointer to A object
    for (int i = 0; i < 3; i++) {
        my_function(*arrayOfA[i]);
        delete arrayOfA[i];  // delete object to prevent memory leak
    }
}

Console output:

Method specific for B was invoked
Method specific for B was invoked
Exception std::bad_cast thrown.
Object is not of type B

A similar version of my_function can be written with pointers instead of references:

void my_function(A* my_a)
{
    B* my_b = dynamic_cast<B*>(my_a);

    if (my_b != nullptr)
        my_b->methodSpecificToB();
    else
        std::cerr << "  Object is not B type" << std::endl;
}

See also

• Template (C++)
• Reflection (computer science)

References

External links

• dynamic_cast operator at IBM Mac OS X Compilers
• dynamic_cast operator at MSDN