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