C++ Programming

Syntax

    typeid( object );

The typeid operator, used to determine the class of an object at runtime. It returns a reference to a std::type_info object, which exists until the end of the program, that describes the "object". If the "object" is a dereferenced null pointer, then the operation will throw a std::bad_typeid exception.

Objects of class std::bad_typeid are derived from std::exception, and thrown by typeid and others.

The use of typeid is often preferred over dynamic_cast<class_type> in situations where just the class information is needed, because typeid, applied on a type or non de-referenced value is a constant-time procedure, whereas dynamic_cast must traverse the class derivation lattice of its argument at runtime. However, you should never rely on the exact content, like for example returned by std::type_info::name(), as this is implementation specific with respect to the compile.

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 member 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