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std:: bad_expected_access

From cppreference.net
C++
Utilities library
std::expected
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)
This section is incomplete
Reason: inheritance diagram

Contents

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

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

Example

This section is incomplete
Reason: no example

See also

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