std::bad_expected_access

 

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Defined in header <expected>
template< class E > class bad_expected_access : public std::bad_expected_access<void>	(1)	(since C++23)
template<> class bad_expected_access<void> : public std::exception	(2)	(since C++23)

1) Defines a type of object to be thrown by std::expected::value when accessing an expected object that contains an unexpected value. bad_expected_access<E> stores a copy of the unexpected value.

2) bad_expected_access<void> is the base class of all other bad_expected_access specializations.

All member functions of std::bad_expected_access are constexpr: it is possible to create and use std::bad_expected_access objects in the evaluation of a constant expression. However, std::bad_expected_access objects generally cannot be constexpr, because any dynamically allocated storage must be released in the same evaluation of constant expression.

(since C++26)

Members of the primary template

(constructor)	constructs a bad_expected_access object (public member function)
error	returns the stored value (public member function)
what	returns the explanatory string (public member function)

std::bad_expected_access::bad_expected_access

explicit bad_expected_access( E e );		(since C++23) (constexpr since C++26)

Constructs a new bad_expected_access<E> object. Initializes the stored value with std::move(e).

std::bad_expected_access::error

const E& error() const & noexcept;	(1)	(since C++23) (constexpr since C++26)
E& error() & noexcept;	(2)	(since C++23) (constexpr since C++26)
const E&& error() const && noexcept;	(3)	(since C++23) (constexpr since C++26)
E&& error() && noexcept;	(4)	(since C++23) (constexpr since C++26)

Returns a reference to the stored value.

std::bad_expected_access::what

const char* what() const noexcept override;		(since C++23) (constexpr since C++26)

Returns the explanatory string.

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::bad_expected_access<void>

Members of the bad_expected_access<void> specialization

(constructor)	constructs a bad_expected_access<void> object (protected member function)
(destructor)	destroys the bad_expected_access<void> object (protected member function)
operator=	replaces the bad_expected_access<void> object (protected member function)
what	returns the explanatory string (public member function)

Special member functions of bad_expected_access<void> are protected. They can only be called by derived classes.

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	202502L	(C++26)	constexpr std::bad_expected_access

See also

bad_optional_access (C++17)	exception indicating checked access to an optional that doesn't contain a value (class)
bad_variant_access (C++17)	exception thrown on invalid accesses to the value of a variant (class)