Run-time type information

From Wikipedia, the free encyclopedia
  (Redirected from Dynamic cast)
Jump to: navigation, search

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[edit]

C++ RTTI can be used to do safe typecasts, using the dynamic_cast<> operator, and to manipulate type information at run time, using the typeid operator and std::type_info class.

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 functionality. There may be a resource cost to making RTTI available even if a program does not use it.

typeid[edit]

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.[citation needed] 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[edit]

#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[edit]

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[edit]

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[edit]

References[edit]

  1. ^ Sun Microsystems (2000). "Runtime Type Identification". C++ Programming Guide. Oracle. Retrieved 16 April 2015. 
  2. ^ Bjarne Stroustrup (March 1993). "A History of C++: 1979—1991" (PDF). Bjarne Stroustrup. p. 50. Retrieved 2009-05-18. 
  3. ^ C++ standard (ISO/IEC14882) section 5.2.8 [expr.typeid], 18.5.1 [lib.type.info] -- http://cs.nyu.edu/courses/fall11/CSCI-GA.2110-003/documents/c++2003std.pdf
  4. ^ http://docs.oracle.com/javase/8/docs/api/java/lang/ClassCastException.html

External links[edit]