std::bad_cast

 

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type_info type_index (C++11)	bad_typeid bad_cast

 

Defined in header <typeinfo>
class bad_cast :publicstd::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

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

virtualconstchar* what()constthrow();		(until C++11)
virtualconstchar* what()constnoexcept;		(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

Run this code

#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