Standard library header <expected> (C++23)

 

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Language support <cfloat> <climits> <compare>(C++20) <contracts>(C++26) <coroutine>(C++20) <csetjmp> <csignal> <cstdarg> <cstddef> <cstdint>(C++11) <cstdlib> <exception> <initializer_list>(C++11)    <limits> <new> <source_location>(C++20) <stdfloat>(C++23) <typeinfo> <version>(C++20) Concepts <concepts>(C++20) Diagnostics <cassert> <cerrno> <debugging>(C++26) <stacktrace>(C++23) <stdexcept> <system_error>(C++11) Memory management <memory> <memory_resource>(C++17) <scoped_allocator>(C++11) Metaprogramming <type_traits>(C++11) <ratio>(C++11)

General utilities <any>(C++17) <bitset> <bit>(C++20) <charconv>(C++17) <expected>(C++23) <format>(C++20) <functional> <optional>(C++17) <stdbit.h>(C++26) <tuple>(C++11) <typeindex>(C++11) <utility> <variant>(C++17) Containers <array>(C++11) <deque> <flat_map>(C++23) <flat_set>(C++23) <forward_list>(C++11) <hive>(C++26) <inplace_vector>(C++26)    <list> <map> <mdspan>(C++23) <queue> <set> <span>(C++20) <stack> <unordered_map>(C++11) <unordered_set>(C++11) <vector> Iterators <iterator> Ranges <generator>(C++23) <ranges>(C++20)

Algorithms <algorithm> <numeric> Strings <cctype> <cstring> <cuchar>(C++11) <cwchar> <cwctype> <string_view>(C++17) <string> Text processing <clocale> <codecvt>(C++11/17/26*) <locale> <regex>(C++11) <text_encoding>(C++26)    Numerics <cfenv>(C++11) <cmath> <complex> <linalg>(C++26) <numbers>(C++20) <random>(C++11) <simd>(C++26) <stdckdint.h>(C++26) <valarray> Time <chrono>(C++11) <ctime> C compatibility <ccomplex>(C++11/17/20*) <ciso646>(until C++20) <cstdalign>(C++11/17/20*) <cstdbool>(C++11/17/20*) <ctgmath>(C++11/17/20*)

Input/output <cinttypes>(C++11) <cstdio> <filesystem>(C++17) <fstream> <iomanip> <iosfwd> <iostream> <ios> <istream> <ostream> <print>(C++23) <spanstream>(C++23) <sstream> <streambuf> <strstream>(C++98/26*) <syncstream>(C++20) Concurrency support <atomic>(C++11) <barrier>(C++20) <condition_variable>(C++11) <future>(C++11) <hazard_pointer>(C++26) <latch>(C++20) <mutex>(C++11) <rcu>(C++26) <semaphore>(C++20) <shared_mutex>(C++14) <stdatomic.h>(C++23) <stop_token>(C++20) <thread>(C++11) Execution support <execution>(C++17)

 

This header is part of the general utility library.

Classes
expected (C++23)	a wrapper that contains either an expected or error value (class template)
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)
unexpectunexpect_t (C++23)	in-place construction tag for unexpected value in expected (tag)

Synopsis

namespace std {// class template unexpectedtemplate<class E>class unexpected;   // class template bad_expected_accesstemplate<class E>class bad_expected_access;   // specialization of bad_expected_access for voidtemplate<>class bad_expected_access<void>;   // in-place construction of unexpected valuesstruct unexpect_t {explicit unexpect_t()=default;};inlineconstexpr unexpect_t unexpect{};   // class template expectedtemplate<class T, class E>class expected;   // partial specialization of expected for void typestemplate<class T, class E> requires is_void_v<T>class expected<T, E>;}

Class template std::unexpected

namespace std {template<class E>class unexpected {public:// constructorsconstexpr unexpected(const unexpected&)=default;constexpr unexpected(unexpected&&)=default;template<class... Args>constexprexplicit unexpected(in_place_t, Args&&...);template<class U, class... Args>constexprexplicit unexpected(in_place_t, initializer_list<U>, Args&&...);template<class Err = E>constexprexplicit unexpected(Err&&);   // assignmentconstexpr unexpected& operator=(const unexpected&)=default;constexpr unexpected& operator=(unexpected&&)=default;   // observerconstexprconst E& error()const&noexcept;constexpr E& error()&noexcept;constexprconst E&& error()const&&noexcept;constexpr E&& error()&&noexcept;   // swapconstexprvoid swap(unexpected& other)noexcept(/* see description */);   friendconstexprvoid swap(unexpected& x, unexpected& y)noexcept(noexcept(x.swap(y)));   // equality operatortemplate<class E2>friendconstexprbool operator==(const unexpected&, const unexpected<E2>&);   private: E unex;// exposition only};   template<class E> unexpected(E)-> unexpected<E>;}

Class template std::bad_expected_access

namespace std {template<class E>class bad_expected_access :public bad_expected_access<void>{public:   // explicit constructorexplicit bad_expected_access(E);   // observersconstchar* what()constnoexcept override; E& error()&noexcept;const E& error()const&noexcept; E&& error()&&noexcept;const E&& error()const&&noexcept;   private: E unex;// exposition only};}

Class template specialization std::bad_expected_access<void>

namespace std {template<>class bad_expected_access<void>:public exception {protected:// constructors bad_expected_access()noexcept; bad_expected_access(const bad_expected_access&); bad_expected_access(bad_expected_access&&); bad_expected_access& operator=(const bad_expected_access&); bad_expected_access& operator=(bad_expected_access&&);   ~bad_expected_access();   public:constchar* what()constnoexcept override;};}

Class template std::expected

namespace std {template<class T, class E>class expected {public:using value_type = T;using error_type = E;using unexpected_type = unexpected<E>;   template<class U>using rebind = expected<U, error_type>;   // constructorsconstexpr expected();constexprexplicit(/* see description */) expected(const expected&);constexprexplicit(/* see description */) expected(expected&&)noexcept(/* see description */);template<class U, class G>constexprexplicit(/* see description */) expected(const expected<U, G>&);template<class U, class G>constexprexplicit(/* see description */) expected(expected<U, G>&&);   template<class U = remove_cv_t<T>>constexprexplicit(/* see description */) expected(U&& v);   template<class G>constexpr expected(const unexpected<G>&);template<class G>constexpr expected(unexpected<G>&&);   template<class... Args>constexprexplicit expected(in_place_t, Args&&...);template<class U, class... Args>constexprexplicit expected(in_place_t, initializer_list<U>, Args&&...);template<class... Args>constexprexplicit expected(unexpect_t, Args&&...);template<class U, class... Args>constexprexplicit expected(unexpect_t, initializer_list<U>, Args&&...);   // destructorconstexpr ~expected();   // assignmentconstexpr expected& operator=(const expected&);constexpr expected& operator=(expected&&)noexcept(/* see description */);template<class U = remove_cv_t<T>>constexpr expected& operator=(U&&);template<class G>constexpr expected& operator=(const unexpected<G>&);template<class G>constexpr expected& operator=(unexpected<G>&&);   template<class... Args>constexpr T& emplace(Args&&...)noexcept;template<class U, class... Args>constexpr T& emplace(initializer_list<U>, Args&&...)noexcept;   // swapconstexprvoid swap(expected&)noexcept(/* see description */);friendconstexprvoid swap(expected&, expected&)noexcept(/* see description */);   // observersconstexprconst T* operator->()constnoexcept;constexpr T* operator->()noexcept;constexprconst T& operator*()const&noexcept;constexpr T& operator*()&noexcept;constexprconst T&& operator*()const&&noexcept;constexpr T&& operator*()&&noexcept;constexprexplicit operator bool()constnoexcept;constexprbool has_value()constnoexcept;constexprconst T& value()const&;constexpr T& value()&;constexprconst T&& value()const&&;constexpr T&& value()&&;constexprconst E& error()const&;constexpr E& error()&;constexprconst E&& error()const&&;constexpr E&& error()&&;template<class U = remove_cv_t<T>>constexpr T value_or(U&&)const&;template<class U = remove_cv_t<T>>constexpr T value_or(U&&)&&;template<class G = E>constexpr E error_or(G&&)const&;template<class G = E>constexpr E error_or(G&&)&&;   // monadic operationstemplate<class F>constexprauto and_then(F&& f)&;template<class F>constexprauto and_then(F&& f)&&;template<class F>constexprauto and_then(F&& f)const&;template<class F>constexprauto and_then(F&& f)const&&;template<class F>constexprauto or_else(F&& f)&;template<class F>constexprauto or_else(F&& f)&&;template<class F>constexprauto or_else(F&& f)const&;template<class F>constexprauto or_else(F&& f)const&&;template<class F>constexprauto transform(F&& f)&;template<class F>constexprauto transform(F&& f)&&;template<class F>constexprauto transform(F&& f)const&;template<class F>constexprauto transform(F&& f)const&&;template<class F>constexprauto transform_error(F&& f)&;template<class F>constexprauto transform_error(F&& f)&&;template<class F>constexprauto transform_error(F&& f)const&;template<class F>constexprauto transform_error(F&& f)const&&;   // equality operatorstemplate<class T2, class E2> requires (!is_void_v<T2>)friendconstexprbool operator==(const expected& x, const expected<T2, E2>& y);template<class T2>friendconstexprbool operator==(const expected&, const T2&);template<class E2>friendconstexprbool operator==(const expected&, const unexpected<E2>&);   private:bool has_val;// exposition onlyunion{ T val;// exposition only E unex;// exposition only};};}

Partial specialization of std::expected for void types

namespace std {template<class T, class E> requires is_void_v<T>class expected<T, E>{public:using value_type = T;using error_type = E;using unexpected_type = unexpected<E>;   template<class U>using rebind = expected<U, error_type>;   // constructorsconstexpr expected()noexcept;constexprexplicit(/* see description */) expected(const expected&);constexprexplicit(/* see description */) expected(expected&&)noexcept(/* see description */);template<class U, class G>constexprexplicit(/* see description */) expected(const expected<U, G>&);template<class U, class G>constexprexplicit(/* see description */) expected(expected<U, G>&&);   template<class G>constexpr expected(const unexpected<G>&);template<class G>constexpr expected(unexpected<G>&&);   constexprexplicit expected(in_place_t)noexcept;template<class... Args>constexprexplicit expected(unexpect_t, Args&&...);template<class U, class... Args>constexprexplicit expected(unexpect_t, initializer_list<U>, Args&&...);   // destructorconstexpr ~expected();   // assignmentconstexpr expected& operator=(const expected&);constexpr expected& operator=(expected&&)noexcept(/* see description */);template<class G>constexpr expected& operator=(const unexpected<G>&);template<class G>constexpr expected& operator=(unexpected<G>&&);constexprvoid emplace()noexcept;   // swapconstexprvoid swap(expected&)noexcept(/* see description */);friendconstexprvoid swap(expected&, expected&)noexcept(/* see description */);   // observersconstexprexplicit operator bool()constnoexcept;constexprbool has_value()constnoexcept;constexprvoid operator*()constnoexcept;constexprvoid value()const&;constexprvoid value()&&;constexprconst E& error()const&;constexpr E& error()&;constexprconst E&& error()const&&;constexpr E&& error()&&;template<class G = E>constexpr E error_or(G&&)const&;template<class G = E>constexpr E error_or(G&&)&&;   // monadic operationstemplate<class F>constexprauto and_then(F&& f)&;template<class F>constexprauto and_then(F&& f)&&;template<class F>constexprauto and_then(F&& f)const&;template<class F>constexprauto and_then(F&& f)const&&;template<class F>constexprauto or_else(F&& f)&;template<class F>constexprauto or_else(F&& f)&&;template<class F>constexprauto or_else(F&& f)const&;template<class F>constexprauto or_else(F&& f)const&&;template<class F>constexprauto transform(F&& f)&;template<class F>constexprauto transform(F&& f)&&;template<class F>constexprauto transform(F&& f)const&;template<class F>constexprauto transform(F&& f)const&&;template<class F>constexprauto transform_error(F&& f)&;template<class F>constexprauto transform_error(F&& f)&&;template<class F>constexprauto transform_error(F&& f)const&;template<class F>constexprauto transform_error(F&& f)const&&;   // equality operatorstemplate<class T2, class E2> requires is_void_v<T2>friendconstexprbool operator==(const expected& x, const expected<T2, E2>& y);template<class E2>friendconstexprbool operator==(const expected&, const unexpected<E2>&);   private:bool has_val;// exposition onlyunion{ E unex;// exposition only};};}