Typeid

In C++, the typeid keyword is used to determine the class of an object at runtime. According to the C++ specification, it returns a reference to type_info. 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.

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.

Note that it is only useful to use typeid on the dereference of a pointer or reference (i.e. typeid(*ptr) or typeid(ref)) to a polymorphic class (a class with at least one virtual method). 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, and so is not very interesting.

Caution: the actual text returned by typeid is platform specific so don't rely on it if attempting to produce platform independent code.

Example

#include
#include //for 'typeid' to workusing namespace std;

class Person {public: // ... Person members ... virtual ~Person() {;

class Employee : public Person { // ... Employee members ...};

int main (){ Person person; Employee employee; Person *ptr = &employee;

cout << typeid(person).name() << endl; // Person (statically known at compile-time) cout << typeid(employee).name() << endl; // Employee (statically known at compile-time) cout << typeid(ptr).name() << endl; // Person * (statically known at compile-time) cout << typeid(*ptr).name() << endl; // Employee (looked up dynamically at run-time // because it is the dereference of a pointer to a polymorphic class) return 0;}

Output (exact output varies by system):

Person Employee *Person Employee

See also

* Run-time type information