typeid

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In C++, the typeid keyword is used to determine the class of an object at runtime. It returns a reference to type_info[1]. The use of typeid 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[2].

Objects of class bad_typeid are thrown when typeid is called on a dereferenced null pointer. The class is derived from exception, and thrown by typeid and others.

It is generally only useful to use typeid on the dereference of a pointer or reference (i.e. typeid(*ptr) or typeid(ref)) to an object of polymorphic class type (a class with at least one virtual function). This is because these are the only expressions that are associated with run-time type information. The type of any other expression is statically known at compile time.

Example

#include <iostream>
#include <typeinfo>  //for 'typeid' to work
 
class Person {
public:
   // ... Person members ...
   virtual ~Person() {}
};
 
class Employee : public Person {
   // ... Employee members ...
};
 
int main () {
   Person person;
   Employee employee;
   Person *ptr = &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)
}

Output (exact output varies by system):

Person
Employee
Person*
Employee

See also

References

  1. C++ standard (ISO/IEC14882) section 5.2.8 [expr.typeid]
  2. Kalev, Danny. Performance of typeid vs. dynamic_cast<> , 1999-02-25. Retrieved on 2009-02-22.
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