Run-time type information--RTTI

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]

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
  }