13.48 <typeinfo>

The <typeinfo> header declares the type_info class (for the typeid operator) and two exception classes related to type information and casting.

class bad_cast : public exception {
public:
  bad_cast(  ) throw(  );
  bad_cast(const bad_cast&) throw(  );
  bad_cast& operator=(const bad_cast&) throw(  );
  virtual ~bad_cast(  ) throw(  );
  virtual const char* what(  ) const throw(  );
};

The dynamic_cast<> operator throws bad_cast when the cast of a reference fails. See dynamic_cast in Chapter 3 for more information.

See Also

dynamic_cast operator

class bad_typeid : public exception {
public:
  bad_typeid(  ) throw(  );
  bad_typeid(const bad_typeid&) throw(  );
  bad_typeid& operator=(const bad_typeid&) throw(  );
  virtual ~bad_typeid(  ) throw(  );
  virtual const char* what(  ) const throw(  );
};

The typeid operator throws bad_typeid when it is applied to an expression of the form *p, in which p is a null pointer. See typeid in Chapter 3 for more information.

See Also

typeid operator

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);
};

The typeid operator (described in Chapter 3) returns a static type_info object. The type information includes the type's name and a collation order, both of which are implementation-defined. An implementation might derive classes from type_info to provide additional information.

Note that the copy constructor and assignment operators are inaccessible, so you must store pointers if you want to use a standard container. Example 13-38 shows how to store type_info pointers in a set, where the order is determined by the before member function.

Example

Example 13-38. Storing type information

#include <algorithm>
#include <functional>
#include <iostream>
#include <ostream>
#include <set>
#include <typeinfo>
   
typedef bool (*type_info_compare) (const std::type_info*, const std::type_info*);
   
typedef std::set<const std::type_info*, type_info_compare>
  typeset;
   
// Return true if *a comes before *b (comparison function to store type_info
// pointers in an associative container).
bool type_info_less(const std::type_info* a, const std::type_info* b)
{
  return a->before(*b);
}
   
// Print a type_info name on a line.
void print(const std::type_info* x)
{
  std::cout << x->name(  ) << '\n';
}
   
void demo(  )
{
  // Construct and initialize the set.
  typeset types(&type_info_less);
   
  types.insert(&typeid(int));
  types.insert(&typeid(char));
  types.insert(&typeid(std::type_info));
  types.insert(&typeid(std::bad_alloc));
  types.insert(&typeid(std::exception));
   . . . 
  // Print the types in the set.
  std::for_each(types.begin(  ), types.end(  ), print);
}

The members of type_info are:

bool before(const type_info& rhs) const

Returns true if this type_info object comes before rhs in the implementation-defined order. The relative order of types can vary between programs, even for the same types.

const char* name( ) const

Returns the type's name as a null-terminated string, which might be a multibyte string. The contents of the name string are implementation-defined.

bool operator==(const type_info& rhs) const

bool operator!=(const type_info& rhs) const

Compares type_info objects, which are equal when the types they describe are the same.

See Also

typeid operator