std::bad_cast

Defined in header `<typeinfo>`
class bad_cast : public std::exception;

An exception of this type is thrown when a dynamic_cast to a reference type fails the run-time check (e.g. because the types are not related by inheritance), and also from std::use_facet if the requested facet does not exist in the locale.

Inheritance diagram

Member functions

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

std::bad_cast::bad_cast

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

Constructs a new bad_cast 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_cast then std::strcmp(what(), other.what()) == 0.(since C++11)

Parameters

other

another exception object to copy

std::bad_cast::operator=

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

Assigns the contents with those of other. If *this and other both have dynamic type std::bad_cast 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_cast::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.

Return value

Pointer to an implementation-defined 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

(destructor) [virtual]	destroys the exception object (virtual public member function of `std::exception`)
what [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

#include <iostream>
#include <typeinfo>
 
struct Foo { virtual ~Foo() {} };
struct Bar { virtual ~Bar() { std::cout << "~Bar\n"; } };
struct Pub : Bar { ~Pub() override { std::cout << "~Pub\n"; } };
 
int main()
{
    Pub pub;
    try
    {
        [[maybe_unused]]
        Bar& r1 = dynamic_cast<Bar&>(pub); // OK, upcast
 
        [[maybe_unused]]
        Foo& r2 = dynamic_cast<Foo&>(pub); // throws
    }
    catch (const std::bad_cast& e)
    {
        std::cout << "e.what(): " << e.what() << '\n';
    }
}

Possible output:

e.what(): std::bad_cast
~Pub
~Bar

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https://en.cppreference.com/cpp/types/bad_cast