std:: bad_typeid

From cppreference.com
C++
Utilities library
General utilities
Relational operators (deprecated in C++20)
Type support
Defined in header <typeinfo>
class bad_typeid : public std:: exception ;

An exception of this type is thrown when a typeid operator is applied to a dereferenced null pointer value of a polymorphic type.

cpp/error/exception std-bad typeid-inheritance.svg

Inheritance diagram

Member functions

(constructor)
constructs a new bad_typeid object
(public member function)
operator=
replaces the bad_typeid object
(public member function)
what
returns the explanatory string
(public member function)

std::bad_typeid:: bad_typeid

(1)
bad_typeid ( ) throw ( ) ;
(until C++11)
bad_typeid ( ) noexcept ;
(since C++11)
(2)
bad_typeid ( const bad_typeid & other ) throw ( ) ;
(until C++11)
bad_typeid ( const bad_typeid & other ) noexcept ;
(since C++11)

Constructs a new bad_typeid object with an implementation-defined null-terminated byte string which is accessible through what() .

1) Default constructor.
2) Copy constructor. If * this and other both have dynamic type std::bad_typeid then std:: strcmp ( what ( ) , other. what ( ) ) == 0 . (since C++11)

Parameters

other - another exception object to copy

std::bad_typeid:: operator=

bad_typeid & operator = ( const bad_typeid & other ) throw ( ) ;
(until C++11)
bad_typeid & operator = ( const bad_typeid & other ) noexcept ;
(since C++11)

Assigns the contents with those of other . If * this and other both have dynamic type std::bad_typeid then std:: strcmp ( what ( ) , other. what ( ) ) == 0 after assignment. (since C++11)

Parameters

other - another exception object to assign with

Return value

* this

std::bad_typeid:: what

virtual const char * what ( ) const throw ( ) ;
(until C++11)
virtual const char * what ( ) const noexcept ;
(since C++11)
(constexpr since C++26)

Returns the explanatory string.

Parameters

(none)

Return value

Pointer to a null-terminated string with explanatory information. The string is suitable for conversion and display as a std::wstring . The pointer is guaranteed to be valid at least until the exception object from which it is obtained is destroyed, or until a non-const member function (e.g. copy assignment operator) on the exception object is called.

The returned string is encoded with the ordinary literal encoding during constant evaluation.

(since C++26)

Notes

Implementations are allowed but not required to override what() .

Inherited from std:: exception

Member functions

[virtual]
destroys the exception object
(virtual public member function of std::exception )
[virtual]
returns an explanatory string
(virtual public member function of std::exception )

Notes

Feature-test macro Value Std Feature
__cpp_lib_constexpr_exceptions 202411L (C++26) constexpr for exception types

Example

Run this code 
#include <iostream>
#include <typeinfo>
 
struct S // The type has to be polymorphic
{
    virtual void f();
}; 
 
int main()
{
    S* p = nullptr;
    try
    {
        std::cout << typeid(*p).name() << '\n';
    }
    catch (const std::bad_typeid& e)
    {
        std::cout << e.what() << '\n';
    }
}

Possible output: 

Attempted a typeid of NULL pointer!
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