type_info


class
<typeinfo> 
class type_info;

Type information type

Stores information of a type.

This is the type returned by the typeid operator. It can be used to compare the type of objects or to retrieve the name of a type.

typeid can be applied directly to types, in which case it returns its information; Or to objects, in which case it returns information on the type of the object.

When typeid is applied to a dereferenced pointer to an object of a polymorphic class type (a class declaring or inheriting a virtual function), it considers its dynamic type (i.e., the type of the most derived object). This requires the RTTI (Run-time type information) to be available.

When typeid is applied to a dereferenced null pointer a bad_typeid exception is thrown.

It is declared as:
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class type_info {
public:
  virtual ~type_info();
  bool operator== (const type_info& rhs) const;
  bool operator!= (const type_info& rhs) const;
  bool before (const type_info& rhs) const;
  const char* name() const;
private:
  type_info (const type_info& rhs);
  type_info& operator= (const type_info& rhs);
};


Members

operator==
operator!=
Comparison operators. They return whether the two types describe the same type.
A derived type is not considered the same type as any of its base classes.
before
Returns true if the type precedes the type of rhs in the collation order.
The collation order is just an internal order kept by a particular implementation and is not necessarily related to inheritance relations or declaring order.
name
Returns a null-terminated character sequence with a human-readable name for the type.
copy constructor and copy operator
These are private, preventing type_info values from being copiable.


Example

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// type_info example
#include <iostream>
#include <typeinfo>
using namespace std;

struct Base {};
struct Derived : Base {};
struct Poly_Base {virtual void Member(){}};
struct Poly_Derived: Poly_Base {};

int main() {
  // built-in types:
  int i;
  int * pi;
  cout << "int is: " << typeid(int).name() << endl;
  cout << "  i is: " << typeid(i).name() << endl;
  cout << " pi is: " << typeid(pi).name() << endl;
  cout << "*pi is: " << typeid(*pi).name() << endl << endl;

  // non-polymorphic types:
  Derived derived;
  Base* pbase = &derived;
  cout << "derived is: " << typeid(derived).name() << endl;
  cout << " *pbase is: " << typeid(*pbase).name() << endl;
  cout << boolalpha << "same type? "; 
  cout << ( typeid(derived)==typeid(*pbase) ) << endl << endl;

  // polymorphic types:
  Poly_Derived polyderived;
  Poly_Base* ppolybase = &polyderived;
  cout << "polyderived is: " << typeid(polyderived).name() << endl;
  cout << " *ppolybase is: " << typeid(*ppolybase).name() << endl;
  cout << boolalpha << "same type? "; 
  cout << ( typeid(polyderived)==typeid(*ppolybase) ) << endl << endl;
}


Possible output:

int is: int
  i is: int
 pi is: int *
*pi is: int


derived is: struct Derived
 *pbase is: struct Base
same type? false


polyderived is: struct Poly_Derived
 *ppolybase is: struct Poly_Derived
same type? true


See also

bad_typeid Exception thrown on typeid of null pointer (class)