std::expected

 

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std::expected

Member functions

expected::expected

expected::~expected

expected::operator=

Observers

expected::operator->expected::operator*

expected::operator boolexpected::has_value

expected::value

expected::error

expected::value_or

Modifiers

expected::emplace

expected::swap

Non-member functions

operator==

swap

Helper classes

unexpected

bad_expected_access

unexpect_tunexpect

 

Defined in header <expected>
template< class T, class E > class expected;		(since C++23)

The class template std::expected provides a way to store either of two values. An object of std::expected at any given time either holds an expected value of type T, or an unexpected value of type E. std::expected is never valueless.

The stored value is allocated directly within the storage occupied by the expected object. No dynamic memory allocation takes place.

A program is ill-formed if it instantiates an expected with a reference type, a function type, or a specialization of std::unexpected. In addition, T must not be std::in_place_t or std::unexpect_t.

Template parameters

T	-	the type of the expected value. The type must either be (possibly cv-qualified) void, or meet the Destructible requirements (in particular, array and reference types are not allowed).
E	-	the type of the unexpected value. The type must meet the Destructible requirements, and must be a valid template argument for std::unexpected (in particular, arrays, non-object types, and cv-qualified types are not allowed).

Member types

Member type	Definition
value_type	T
error_type	E
unexpected_type	std::unexpected<E>
rebind	template< class U > using rebind = expected<U, error_type>;

Member functions

(constructor) (C++23)	constructs the expected object (public member function)
(destructor) (C++23)	destroys the expected object, along with its contained value (public member function)
operator= (C++23)	assigns contents (public member function)
Observers
operator->operator* (C++23)	accesses the expected value (public member function)
operator boolhas_value (C++23)	checks whether the object contains an expected value (public member function)
value (C++23)	returns the expected value (public member function)
error (C++23)	returns the unexpected value (public member function)
value_or (C++23)	returns the expected value if present, another value otherwise (public member function)
Monadic operations
and_then (C++23)	returns the result of the given function on the expected value if it exists; otherwise, returns the expected itself (public member function)
transform (C++23)	returns an expected containing the transformed expected value if it exists; otherwise, returns the expected itself (public member function)
or_else (C++23)	returns the expected itself if it contains an expected value; otherwise, returns the result of the given function on the unexpected value (public member function)
transform_error (C++23)	returns the expected itself if it contains an expected value; otherwise, returns an expected containing the transformed unexpected value (public member function)
Modifiers
emplace (C++23)	constructs the expected value in-place (public member function)
swap (C++23)	exchanges the contents (public member function)

Non-member functions

operator== (C++23)	compares expected objects (function template)
swap(std::expected) (C++23)	specializes the std::swap algorithm (function)

Helper classes

unexpected (C++23)	represented as an unexpected value (class template)
bad_expected_access (C++23)	exception indicating checked access to an expected that contains an unexpected value (class template)
unexpect_tunexpect (C++23)	in-place construction tag for unexpected value in expected (class) (constant)

Notes

Types with the same functionality are called Result in Rust and Either in Haskell.

Feature-test macro	Value	Std	Comment
__cpp_lib_expected	202202L	(C++23)	class template std::expected and associated helper classes
	202211L	(C++23)	Monadic functions for std::expected

See also

variant (C++17)	a type-safe discriminated union (class template)
optional (C++17)	a wrapper that may or may not hold an object (class template)