P0323R4
std::expected

This paper revises [P0323r3] by applying feedback obtained from LEWG and EWG. The previous paper contains motivation, design rationale, implementability information, sample usage, history, alternative designs, and related types. This update only contains wording and open questions because its purpose is twofold:

• Present appropriate wording for including in the Library Fundamentals TS v3.

• List open questions which the TS should aim to answer.

1. Wording

Below, substitute the � character with a number or name the editor finds appropriate for the sub-section.

1.1. �.� Unexpected objects [unexpected]

1.2. �.�.1 General [unexpected.general]

This subclause describes class template unexpected that contain objects representing an unexpected outcome.

1.3. �.�.2 Header <experimental/unexpected> synopsis [unexpected.synop]

namespace std {
namespace experimental {
inline namespace fundamentals_v3 {
    // �.�.3, Unexpected object type
    template <class E>
      class unexpected;

    // �.�.4, Unexpected relational operators
    template <class E>
        constexpr bool
        operator==(const unexpected<E>&, const unexpected<E>&);
    template <class E>
        constexpr bool
        operator!=(const unexpected<E>&, const unexpected<E>&);

}}}

A program that needs the instantiation of template unexpected for a reference type or void is ill-formed.

1.4. �.�.3 Unexpected object type [unexpected.object]

template <class E>
class unexpected {
public:
    unexpected() = delete;
    constexpr explicit unexpected(const E&);
    constexpr explicit unexpected(E&&);
    constexpr const E& value() const &;
    constexpr E& value() &;
    constexpr E&& value() &&;
    constexpr E const&& value() const&&;
private:
    E val; // exposition only
};

If E is void the program is ill formed.

constexpr explicit unexpected(const E&);

Effects: Build an unexpected by copying the parameter to the internal storage val.

constexpr explicit unexpected(E &&);

Effects: Build an unexpected by moving the parameter to the internal storage val.

    constexpr const E& value() const &;
    constexpr E& value() &;

Returns: val.

    constexpr E&& value() &&;
    constexpr E const&& value() const&&;

Returns: move(val).

1.5. �.�.4 Unexpected relational operators [unexpected.relational_op]

    template <class E>
        constexpr bool operator==(const unexpected<E>& x, const unexpected<E>& y);

Requires: E shall meet the requirements of EqualityComparable.

Returns: x.value() == y.value().

Remarks: Specializations of this function template, for which x.value() == y.value() is a core constant expression, shall be constexpr functions.

    template <class E>
        constexpr bool operator!=(const unexpected<E>& x, const unexpected<E>& y);

Returns: x.value() != y.value().

Remarks: Specializations of this function template, for which x.value() != y.value() is a core constant expression, shall be constexpr functions.

1.6. �.� Expected objects [expected]

1.7. �.�.1 In general [expected.general]

This subclause describes class template expected that represents expected objects. An expected<T, E> object is an object that contains the storage for another object and manages the lifetime of this contained object T, alternatively it could contain the storage for another unexpected object E. The contained object may not be initialized after the expected object has been initialized, and may not be destroyed before the expected object has been destroyed. The initialization state of the contained object is tracked by the expected object.

1.8. �.�.2 Header <experimental/expected> synopsis [expected.synop]

namespace std {
namespace experimental {
inline namespace fundamentals_v3 {
    // �.�.4, Expected for object types
    template <class T, class E>
        class expected;

    // �.�.5, Expected specialization for void
    template <class E>
        class expected<void,E>;

    // �.�.6, unexpect tag
    struct unexpect_t {
       unexpect_t() = default;
    };
    inline constexpr unexpect_t unexpect{};

    // �.�.7, class bad_expected_access
    template <class E>
       class bad_expected_access;

    // �.�.8, Specialization for void.
    template <>
       class bad_expected_access<void>;

    // �.�.9, Expected relational operators
    template <class T, class E>
        constexpr bool operator==(const expected<T, E>&, const expected<T, E>&);
    template <class T, class E>
        constexpr bool operator!=(const expected<T, E>&, const expected<T, E>&);

    // �.�.10, Comparison with T
    template <class T, class E>
      constexpr bool operator==(const expected<T, E>&, const T&);
    template <class T, class E>
      constexpr bool operator==(const T&, const expected<T, E>&);
    template <class T, class E>
      constexpr bool operator!=(const expected<T, E>&, const T&);
    template <class T, class E>
      constexpr bool operator!=(const T&, const expected<T, E>&);

    // �.�.10, Comparison with unexpected<E>
    template <class T, class E>
      constexpr bool operator==(const expected<T, E>&, const unexpected<E>&);
    template <class T, class E>
      constexpr bool operator==(const unexpected<E>&, const expected<T, E>&);
    template <class T, class E>
      constexpr bool operator!=(const expected<T, E>&, const unexpected<E>&);
    template <class T, class E>
      constexpr bool operator!=(const unexpected<E>&, const expected<T, E>&);

    // �.�.11, Specialized algorithms
    void swap(expected<T, E>&, expected<T, E>&) noexcept(see below);

}}}

A program that necessitates the instantiation of template expected<T, E> with T for a reference type or for possibly cv-qualified types in_place_t, unexpect_t or unexpected<E> or E for a reference type or void is ill-formed.

1.9. �.�.3 Definitions [expected.defs]

An instance of expected<T, E> is said to be valued if it contains a value of type T. An instance of expected<T, E> is said to be unexpected if it contains an object of type E.

1.10. �.�.4 expected for object types [expected.object]

template <class T, class E>
class expected
{
public:
    typedef T value_type;
    typedef E error_type;
    typedef unexpected<E> unexpected_type;

    template <class U>
        struct rebind {
        using type = expected<U, error_type>;
      };

    // �.�.4.1, constructors
    constexpr expected();
    constexpr expected(const expected&);
    constexpr expected(expected&&) noexcept(see below);
    template <class U, class G>
        EXPLICIT constexpr expected(const expected<U, G>&);
    template <class U, class G>
        EXPLICIT constexpr expected(expected<U, G>&&);

    template <class U = T>
        EXPLICIT constexpr expected(U&& v);

    template <class... Args>
        constexpr explicit expected(in_place_t, Args&&...);
    template <class U, class... Args>
        constexpr explicit expected(in_place_t, initializer_list<U>, Args&&...);
    template <class G = E>
        constexpr expected(unexpected<G> const&);
    template <class G = E>
        constexpr expected(unexpected<G> &&);
    template <class... Args>
        constexpr explicit expected(unexpect_t, Args&&...);
    template <class U, class... Args>
        constexpr explicit expected(unexpect_t, initializer_list<U>, Args&&...);

    // �.�.4.2, destructor
    ~expected();

    // �.�.4.3, assignment
    expected& operator=(const expected&);
    expected& operator=(expected&&) noexcept(see below);
    template <class U = T> expected& operator=(U&&);
    template <class G = E>
        expected& operator=(const unexpected<G>&);
    template <class G = E>
        expected& operator=(unexpected<G>&&) noexcept(see below);

    template <class... Args>
        void emplace(Args&&...);
    template <class U, class... Args>
        void emplace(initializer_list<U>, Args&&...);

    // �.�.4.4, swap
    void swap(expected&) noexcept(see below);

    // �.�.4.5, observers
    constexpr const T* operator ->() const;
    constexpr T* operator ->();
    constexpr const T& operator *() const&;
    constexpr T& operator *() &;
    constexpr const T&& operator *() const &&;
    constexpr T&& operator *() &&;
    constexpr explicit operator bool() const noexcept;
    constexpr bool has_value() const noexcept;
    constexpr const T& value() const&;
    constexpr T& value() &;
    constexpr const T&& value() const &&;
    constexpr T&& value() &&;
    constexpr const E& error() const&;
    constexpr E& error() &;
    constexpr const E&& error() const &&;
    constexpr E&& error() &&;
    template <class U>
        constexpr T value_or(U&&) const&;
    template <class U>
        T value_or(U&&) &&;

private:
    bool has_val; // exposition only
    union
    {
        value_type val; // exposition only
        unexpected_type unexpect; // exposition only
    };
};

Valued instances of expected<T, E> where T and E are of object type shall contain a value of type T or a value of type E within its own storage. These values are referred to as the contained or the unexpected value of the expected object. Implementations are not permitted to use additional storage, such as dynamic memory, to allocate its contained or unexpected value. The contained or unexpected value shall be allocated in a region of the expected<T, E> storage suitably aligned for the type T and unexpected<E>. Members has_val, val and unexpect are provided for exposition only. Implementations need not provide those members. has_val indicates whether the expected object’s contained value has been initialized (and not yet destroyed); when has_val is true val points to the contained value, and when it is false unexpect points to the erroneous value.

T must be void or shall be object type and shall satisfy the requirements of Destructible (Table 27).

E shall be object type and shall satisfy the requirements of Destructible (Table 27).

1.11. �.�.4.1 Constructors [expected.object.ctor]

constexpr expected();

Effects: Initializes the contained value as if direct-non-list-initializing an object of type T with the expression T{} (if T is not void).

Postconditions: *this contains a value.

Throws: Any exception thrown by the default constructor of T (nothing if T is void).

Remarks: If value-initialization of T is a constexpr constructor or T is void this constructor shall be constexpr. This constructor shall be defined as deleted unless is_default_constructible_v<T> or T is void.

constexpr expected(const expected& rhs);

Effects: If rhs contains a value, initializes the contained value as if direct-non-list-initializing an object of type T with the expression *rhs (if T is not void).

If rhs does not contain a value initializes the unexpected value as if direct-non-list-initializing an object of type unexpected<E> with the expression unexpected(rhs.error()).

Postconditions: bool(rhs) == bool(*this).

Throws: Any exception thrown by the selected constructor of T if T is not void or by the selected constructor of unexpected<E>.

Remarks: This constructor shall be defined as deleted unless is_copy_constructible_v<T> or T is void and is_copy_constructible_v<E>. If is_trivially_copy_constructible_v<T> is true or T is void and is_trivially_copy_constructible_v<E> is true, this constructor shall be a constexpr constructor.

constexpr expected(expected && rhs) noexcept(see below);

Effects: If rhs contains a value initializes the contained value as if direct-non-list-initializing an object of type T with the expression std::move(*rhs) (if T is not void).

If rhs does not contain a value initializes the unexpected value as if direct-non-list-initializing an object of type unexpected<E> with the expression std::move(unexpected(rhs.error())).

bool(rhs) is unchanged.

Postconditions: bool(rhs) == bool(*this).

Throws: Any exception thrown by the selected constructor of T if T is not void or by the selected constructor of unexpected<E>.

Remarks: The expression inside noexcept is equivalent to: is_nothrow_move_constructible_v<T> orT is ``void and is_nothrow_move_constructible_v<E>. This constructor shall not participate in overload resolution unless is_move_constructible_v<T> and is_move_constructible_v<E>. If is_trivially_move_constructible_v<T> is true or T is void and is_trivially_move_constructible_v<E> is true, this constructor shall be a constexpr constructor.

    template <class U, class G>
    EXPLICIT constexpr expected(const expected<U,G>& rhs);

Effects: If rhs contains a value initializes the contained value as if direct-non-list-initializing an object of type T with the expression *rhs (if T is not void).

If rhs does not contain a value initializes the unexpected value as if direct-non-list-initializing an object of type unexpected<E> with the expression unexpected(rhs.error()).

Postconditions: bool(rhs) == bool(*this).

Throws: Any exception thrown by the selected constructor of T if T is not void or by the selected constructor of unexpected<E>.

Remarks: This constructor shall not participate in overload resolution unless:

• is_constructible_v<T, const U&> is true or T and U are void,

• is_constructible_v<E, const G&> is true,

• is_constructible_v<T, expected<U,G>&> is false or T and U are void,

• is_constructible_v<T, expected<U,G>&&> is false or T and U are void,

• is_constructible_v<T, const expected<U,G>&> is false or T and U are void,

• is_constructible_v<T, const expected<U,G>&&> is false or T and U are void,

• is_convertible_v<expected<U,G>&, T> is false or T and U are void,

• is_convertible_v<expected<U,G>&&, T> is false or T and U are void,

• is_convertible_v<const expected<U,G>&, T> is false or T and U are void, and

• is_convertible_v<const expected<U,G>&&, T> is false or T and U are void.

The constructor is explicit if and only if T is not void and is_convertible_v<U const&, T> is false or is_convertible_v<G const&, E> is false.

    template <class U, class G>
    EXPLICIT constexpr expected(expected<U,G>&& rhs);

Effects: If rhs contains a value initializes the contained value as if direct-non-list-initializing an object of type T with the expression std::move(*rhs) or nothing if Tis void.

If rhs does not contain a value initializes the unexpected value as if direct-non-list-initializing an object of type unexpected<E> with the expression std::move(unexpected(rhs.error())). bool(rhs) is unchanged

Postconditions: bool(rhs) == bool(*this).

Throws: Any exception thrown by the selected constructor of T if T is not void or by the selected constructor of unexpected<E>.

Remarks: This constructor shall not participate in overload resolution unless:

• is_constructible_v<T, U&&> is true,

• is_constructible_v<E, G&&> is true,

• is_constructible_v<T, expected<U,G>&> is false or T and U are void,

• is_constructible_v<T, expected<U,G>&&> is false or T and U are void,

• is_constructible_v<T, const expected<U,G>&> is false or T and U are void,

• is_constructible_v<T, const expected<U,G>&&> is false or T and U are void,

• is_convertible_v<expected<U,G>&, T> is false or T and U are void,

• is_convertible_v<expected<U,G>&&, T> is false or T and U are void,

• is_convertible_v<const expected<U,G>&, T> is false or T and U are void, and

• is_convertible_v<const expected<U,G>&&, T> is false or T and U are void.

The constructor is explicit if and only if is_convertible_v<U&&, T> is false or is_convertible_v<G&&, E> is false.

    template <class U = T>
    EXPLICIT constexpr expected(U&& v);

Effects: Initializes the contained value as if direct-non-list-initializing an object of type T with the expression std::forward<U>(v).

Postconditions: *this contains a value.

Throws: Any exception thrown by the selected constructor of T.

Remarks: If T's selected constructor is a constexpr constructor, this constructor shall be a constexpr constructor. This constructor shall not participate in overload resolution unless T is not void and is_constructible_v<T, U&&> is true, is_same_v<decay_t<U>, in_place_t> is false, is_same_v<expected<T, E>, decay_t<U>> is false, and is_same_v<unexpected<E>, decay_t<U>> is false. The constructor is explicit if and only if is_convertible_v<U&&, T> is false.

template <class... Args>
  constexpr explicit expected(in_place_t, Args&&... args);

Effects: Initializes the contained value as if direct-non-list-initializing an object of type T with the arguments std::forward<Args>(args)... if T is not void.

Postconditions: *this contains a value.

Throws: Any exception thrown by the selected constructor of T if T is not void.

Remarks: If T's constructor selected for the initialization is a constexpr constructor, this constructor shall be a constexpr constructor. This constructor shall not participate in overload resolution unless T is void and sizeof...Args)==0 or T is not void and is_constructible_v<T, Args&&...>.

template <class U, class... Args>
  constexpr explicit expected(in_place_t, initializer_list<U> il, Args&&... args);

Effects: Initializes the contained value as if direct-non-list-initializing an object of type T with the arguments il, std::forward<Args>(args)....

Postconditions: *this contains a value.

Throws: Any exception thrown by the selected constructor of T.

Remarks: If T's constructor selected for the initialization is a constexpr constructor, this constructor shall be a constexpr constructor. This constructor shall not participate in overload resolution unless T is not void and is_constructible<T, initializer_list_v<U>&, Args&&...>.

template <class G = E>
EXPLICIT constexpr expected(unexpected<G> const& e);

Effects: Initializes the unexpected value as if direct-non-list-initializing an object of type unexpected<E> with the expression e.

Postconditions: *this does not contain a value.

Throws: Any exception thrown by the selected constructor of unexpected<E>

Remark: If unexpected<E>'s selected constructor is a constexpr constructor, this constructor shall be a constexpr constructor. This constructor shall not participate in overload resolution unless is_constructible_v<E, const G&>. The constructor is explicit if and only if is_convertible_v<const G&, E> is false.

template <class G = E>
EXPLICIT constexpr expected(unexpected<G>&& e);

Effects: Initializes the unexpected value as if direct-non-list-initializing an object of type unexpected<E> with the expression std::move(e).

Postconditions: *this does not contain a value.

Throws: Any exception thrown by the selected constructor of unexpected<E>

Remark: If unexpected<E>'s selected constructor is a constexpr constructor, this constructor shall be a constexpr constructor. The expression inside noexcept is equivalent to: is_nothrow_constructible_v<E, G&&>. This constructor shall not participate in overload resolution unless is_constructible_v<E, G&&>. The constructor is explicit if and only if is_convertible_v<G&&, E> is false.

template <class... Args>
  constexpr explicit expected(unexpect_t, Args&&... args);

Effects: Initializes the unexpected value as if direct-non-list-initializing an object of unexpected<E>with the arguments std::forward<Args>(args)....

Postconditions: *this does not contain a value.

Throws: Any exception thrown by the selected constructor of unexpected<E>

Remarks: If unexpected<E>'s constructor selected for the initialization is a constexpr constructor, this constructor shall be a constexpr constructor. This constructor shall not participate in overload resolution unless is_constructible_v<E, Args&&...>.

template <class U, class... Args>
  constexpr explicit expected(unexpect_t, initializer_list<U> il, Args&&... args);

Effects: Initializes the unexpected value as if direct-non-list-initializing an object of unexpected<E> with the arguments il, std::forward<Args>(args)....

Postconditions: *this does not contain a value.

Throws: Any exception thrown by the selected constructor of unexpected<E>.

Remarks: If unexpected<E>'s constructor selected for the initialization is a constexpr constructor, this constructor shall be a constexpr constructor. This constructor shall not participate in overload resolution unless is_constructible_v<E, initializer_list<U>&, Args&&...>.

1.12. �.�.4.2 Destructor [expected.object.dtor]

~expected();

Effects: If T is not void and is_trivially_destructible_v<T> != true and *this contains a value, calls val.~T(). If is_trivially_destructible_v<E> != true and *this does not contain a value, calls unexpect.~unexpected<E>().

Remarks: If T is void or is_trivially_destructible_v<T> and is_trivially_destructible_v<E> is true then this destructor shall be a trivial destructor.

1.13. �.�.4.3 Assignment [expected.object.assign]

expected<T, E>& operator=(const expected<T, E>& rhs) noexcept(see below);

Effects:

If *this contains a value and rhs contains a value,

• assigns *rhs to the contained value val if T is not void;

otherwise, if *this does not contain a value and rhs does not contain a value,

• assigns unexpected(rhs.error()) to the unexpected value unexpect;

otherwise, if *this contains a value and rhs does not contain a value,

• if T is void

  • initializes the unexpected value as if direct-non-list-initializing an object of type unexpected<E> with unexpected(rhs.error()). Either

  • The didn’t throw, so mark the expected as holding a unexpected<E> (which can’t throw), or

  • the constructor did throw, an nothing was changed.

• otherwise T is not void, if is_nothrow_copy_constructible_v<E>

  • destroys the contained value by calling val.T::~T(),

  • initializes the unexpected value as if direct-non-list-initializing an object of type unexpected<E> with unexpected(rhs.error()).

• otherwise if is_nothrow_move_constructible_v<E>

  • constructs a new unexpected<E> tmp on the stack from *rhs (this can throw),

  • destroys the contained value by calling val.T::~T(),

  • initializes the unexpected value as if direct-non-list-initializing an object of type unexpected<E> with unexpected(rhs.error()).

otherwise as is_nothrow_move_constructible_v<T>

• constructs a new T tmp on the stack from *this (this can throw),

• destroys the contained value by calling val.T::~T(),

• initializes the contained value as if direct-non-list-initializing an object of type unexpected<E> with unexpected(rhs.error()). Either,

  • the last constructor didn’t throw, so mark the expected as holding a unexpected<E> (which can’t throw), or

  • the last constructor did throw, so move-construct the T from the stack tmp back into the expected storage (which can’t throw as is_nothrow_move_constructible_v<T> is true), and rethrow the exception.

otherwise *this does not contain a value and rhs contains a value

• if T is void destroys the unexpected value by calling unexpect.~unexpected<E>()

• otherwise T is not void, if is_nothrow_copy_constructible_v<T>

  • destroys the unexpected value by calling unexpect.~unexpected<E>()

  • initializes the contained value as if direct-non-list-initializing an object of type T with *rhs;

• otherwise T is not void, if is_nothrow_copy_constructible_v<T>

  • destroys the unexpected value by calling unexpect.~unexpected<E>()

  • initializes the contained value as if direct-non-list-initializing an object of type T with move(*rhs);

• otherwise if is_nothrow_move_constructible_v<T>

  • constructs a new T tmp on the stack from *rhs (this can throw),

  • destroys the unexpected value by calling unexpect.~unexpected<E>()

  • initializes the contained value as if direct-non-list-initializing an object of type T with move(tmp);

• otherwise as is_nothrow_move_constructible_v<E>

  • constructs a new unexpected<E> tmp on the stack from unexpected(this->error()) (which can throw),

  • destroys the unexpected value by calling unexpect.~unexpected<E>(),

  • initializes the contained value as if direct-non-list-initializing an object of type T with *rhs. Either,

  • the constructor didn’t throw, so mark the expected as holding a T (which can’t throw), or

  • the constructor did throw, so move-construct the unexpected<E> from the stack tmp back into the expected storage (which can’t throw as is_nothrow_move_constructible_v<E> is true), and rethrow the exception.

Returns: *this.

Postconditions: bool(rhs) == bool(*this).

Throws: any exception throw by the selected operations.

Remarks: If any exception is thrown, the values of bool(*this) and bool(rhs) remain unchanged.

If an exception is thrown during the call to T's or unexpected<E>'s copy constructor, no effect. If an exception is thrown during the call to T's or unexpected<E>'s copy assignment, the state of its contained value is as defined by the exception safety guarantee of T's or unexpected<E>'s copy assignment.

This operator shall be defined as deleted unless

• T is void and is_copy_assignable_v<E> and is_copy_constructible_v<E> or

• T is not void and is_copy_assignable_v<T> and is_copy_constructible_v<T> and is_copy_assignable_v<E> and is_copy_constructible_v<E> and (is_nothrow_move_constructible_v<E> or is_nothrow_move_constructible_v<T>).

expected<T, E>& operator=(expected<T, E>&& rhs) noexcept(see below);

Effects:

If *this contains a value and rhs contains a value,

• move assign *rhs to the contained value val if T is not void;

otherwise, if *this does not contain a value and rhs does not contain a value,

• move assign unexpected(rhs.error()) to the unexpected value unexpect;

otherwise, if *this contains a value and rhs does not contain a value,

• if T is void

  • initializes the unexpected value as if direct-non-list-initializing an object of type unexpected<E> with unexpected(move(rhs).error()). Either

  • the constructor didn’t throw, so mark the expected as holding a unexpected<E> (which can’t throw), or

  • the constructor did throw, and nothing was changed.

• otherwise if is_nothrow_move_constructible_v<E>

  • destroys the contained value by calling val.T::~T(),

  • initializes the unexpected value as if direct-non-list-initializing an object of type unexpected<E> with unexpected(move(rhs).error()));

• otherwise as is_nothrow_move_constructible_v<T>

  • move constructs a new T tmp on the stack from *this (which can’t throw as T is nothrow-move-constructible),

  • destroys the contained value by calling val.T::~T(),

  • initializes the unexpected value as if direct-non-list-initializing an object of type unexpected_type<E> with unexpected(move(rhs).error())). Either,

  • The constructor didn’t throw, so mark the expected as holding a unexpected_type<E> (which can’t throw), or

  • The constructor did throw, so move-construct the T from the stack tmp back into the expected storage (which can’t throw as T is nothrow-move-constructible), and rethrow the exception.

otherwise *this does not contain a value and rhs contains a value,

• if T is void destroys the unexpected value by calling unexpect.~unexpected<E>()

• otherwise, if is_nothrow_move_constructible_v<T>

  • destroys the unexpected value by calling unexpect.~unexpected<E>(),

  • initializes the contained value as if direct-non-list-initializing an object of type T with *move(rhs);

• otherwise as is_nothrow_move_constructible_v<E>

  • move constructs a new unpepected_type<E> tmp on the stack from unexpected(this->error()) (which can’t throw as E is nothrow-move-constructible),

  • destroys the unexpected value by calling unexpect.~unexpected<E>(),

  • initializes the contained value as if direct-non-list-initializing an object of type T with *move(rhs). Either,

  • The constructor didn’t throw, so mark the expected as holding a T (which can’t throw), or

  • The constructor did throw, so move-construct the unexpected<E> from the stack tmp back into the expected storage (which can’t throw as E is nothrow-move-constructible), and rethrow the exception.

Returns: *this.

Postconditions: bool(rhs) == bool(*this).

Remarks: The expression inside noexcept is equivalent to: is_nothrow_move_assignable_v<T> && is_nothrow_move_constructible_v<T>.

If any exception is thrown, the values of bool(*this) and bool(rhs) remain unchanged. If an exception is thrown during the call to T's copy constructor, no effect. If an exception is thrown during the call to T's copy assignment, the state of its contained value is as defined by the exception safety guarantee of T's copy assignment. If an exception is thrown during the call to E's copy assignment, the state of its contained unexpected value is as defined by the exception safety guarantee of E's copy assignment.

This operator shall be defined as deleted unless is_move_constructible_v<T> and is_move_assignable_v<T> and is_nothrow_move_constructible_v<E> and is_nothrow_move_assignable_v<E>.

template <class U>
  expected<T, E>& operator=(U&& v);

Effects:

If *this contains a value, assigns forward<U>(v) to the contained value val;

otherwise, if is_nothrow_constructible_v<T, U&&>

• destroys the unexpected value by calling unexpect.~unexpected<E>(),

• initializes the contained value as if direct-non-list-initializing an object of type T with forward<U>(v) and

• set has_valto true;

otherwise as is_nothrow_constructible_v<E>

• move constructs a new unexpected<E> tmp on the stack from unexpected(this->error()) (which can’t throw as E is nothrow-move-constructible),

• destroys the contained value by calling unexpect.~unexpected<E>(),

• initializes the contained value as if direct-non-list-initializing an object of type T with forward<U>(v). Either,

  • the constructor didn’t throw, so mark the expected as holding a T (which can’t throw), that is set has_val to true, or

  • the constructor did throw, so move construct the unexpected<E> from the stack tmp back into the expected storage (which can’t throw as E is nothrow-move-constructible), and re-throw the exception.

Returns: *this.

Postconditions: *this contains a value.

Remarks: If any exception is thrown, the value of bool(*this) remains unchanged. If an exception is thrown during the call to T's constructor, no effect. If an exception is thrown during the call to T's copy assignment, the state of its contained value is as defined by the exception safety guarantee of T's copy assignment.

This function shall not participate in overload resolution unless:

• is_same_v<T,void> is false and

• is_same_v<expected<T,E>, decay_t<U>> is false and

• conjunction_v<is_scalar<T>, is_same<T, decay_t<U>>> is false,

• is_constructible_v<T, U> is true,

• is_assignable_v<T&, U> is true and

• is_nothrow_move_constructible_v<E> is true.

expected<T, E>& operator=(unexpected<E> const& e) noexcept(see below);

Effects:

If *this does not contain a value, assigns unexpected(rhs.error()) to the unexpected value unexpect;

otherwise,

• destroys the contained value by calling val.~T() if T is not void,

• initializes the unexpected value as if direct-non-list-initializing an object of type unexpected<E> with unexpected(forward<expected<T, E>>(rhs)) and set has_val to false.

Returns: *this.

Postconditions: *this does not contain a value.

Remarks: If any exception is thrown, value of valued remains unchanged.

This signature shall not participate in overload resolution unless is_nothrow_copy_constructible_v<E> and is_assignable_v<E&, E>.

expected<T, E>& operator=(unexpected<E> && e);

Effects:

If *this does not contain a value, move assign unexpected(rhs.error()) to the unexpected value unexpect;

otherwise,

• destroys the contained value by calling val.~T() if T is not void,

• initializes the unexpected value as if direct-non-list-initializing an object of type unexpected<E> with unexpected(move(forward<expected<T, E>>(rhs))) and set has_val to false.

Returns: *this.

Postconditions: *this does not contain a value.

Remarks: If any exception is thrown, value of valued remains unchanged.

This signature shall not participate in overload resolution unless is_nothrow_move_constructible_v<E> and is_move_assignable_v<E&, E>.

  void expected<void,E>::emplace();

Effects:

If *this doesn’t contains a value

• destroys the unexpected value by calling unexpect.~unexpected<E>(),

• set has_valto true

Postconditions: *this contains a value.

Throws: nothing

template <class... Args>
  void emplace(Args&&... args);

Effects:

If *this contains a value, assigns the contained value val as if constructing an object of type T with the arguments std::forward<Args>(args)...

otherwise, if is_nothrow_constructible_v<T, Args&&...>

• destroys the unexpected value by calling unexpect.~unexpected<E>(),

• initializes the contained value as if direct-non-list-initializing an object of type T with std::forward<Args>(args)... and

• set has_val to true;

otherwise if is_nothrow_move_constructible_v<T>

• constructs a new T tmp on the stack from std::forward<Args>(args)... (which can throw),

• destroys the unexpected value by calling unexpect.~unexpected<E>(),

• initializes the contained value as if direct-non-list-initializing an object of type T with move(tmp) (which can not throw) and

• set has_val to true;

otherwise as is_nothrow_move_constructible_v<E>

• move constructs a new unexpected<E> tmp on the stack from unexpected(this->error()) (which can’t throw),

• destroys the unexpected value by calling unexpect.~unexpected<E>(),

• initializes the contained value as if direct-non-list-initializing an object of type T with std::forward<Args>(args).... Either,

  • the constructor didn’t throw, so mark the expected as holding a T (which can’t throw), that is set has_val to true, or

  • the constructor did throw, so move-construct the unexpected<E> from the stack tmp back into the expected storage (which can’t throw as E is nothrow-move-constructible), and re-throw the exception.

Postconditions: *this contains a value.

Throws: Any exception thrown by the selected assignment of T.

Remarks: If an exception is thrown during the call to T's assignment, nothing changes.

This signature shall not participate in overload resolution unless is_no_throw_constructible_v<T, Args&&...>.

template <class U, class... Args>
  void emplace(initializer_list<U> il, Args&&... args);

Effects: if *this contains a value, assigns the contained value val as if constructing an object of type T with the arguments il, std::forward<Args>(args)..., otherwise destroys the unexpected value by calling unexpect.~unexpected<E>() and initializes the contained value as if constructing an object of type T with the arguments il, std::forward<Args>(args)....

Postconditions: *this contains a value.

Throws: Any exception thrown by the selected assignment of T.

Remarks: If an exception is thrown during the call to T's assignment nothing changes.

The function shall not participate in overload resolution unless: T is not void and is_no_throw_constructible_v<T, initializer_list<U>&, Args&&...>.

1.14. �.�.4.4 Swap [expected.object.swap]

void swap(expected<T, E>& rhs) noexcept(see below);

Check swap Effects.

Effects: if *this contains a value and rhs contains a value,

• calls using std::swap; swap(val, rhs.val),

otherwise if *this does not contain a value and rhs does not contain a value,

• calls using std::swap; swap(err, rhs.err),

otherwise if *this does not contains a value and rhs contains a value,

• calls rhs.swap(*this),

otherwise, *this contains a value and rhs does not contains a value,

• if T is void

  • initializes the unexpected value as if direct-non-list-initializing an object of type unexpected<E> with unexpected(move(rhs)). Either

  • the constructor didn’t throw, so mark the expected as holding a unexpected<E>, destroys the unexpected value by calling rhs.unexpect.~unexpected<E>() set rhs.has_val to true.

  • the constructor did throw, rethrow the exception.

• otherwise T is not void, if is_nothrow_move_constructible_v<E>,

  • the unexpected value of rhs is moved to a temporary variable tmp of type unpected_type,

  • followed by destruction of the unexpected value as if by rhs.unexpect.unexpected<E>::~unexpected<E>(),

  • the contained value of rhs is direct-initialized from std::move(*other),

  • followed by destruction of the contained value as if by rhs.val->T::~T(),

  • the unexpected value of this is direct-initialized from std::move(tmp), after this, this does not contain a value; and rhs contains a value.

• otherwise if is_nothrow_move_constructible_v<T>,

  • the contained value of rhs is moved to a temporary variable tmp of type T,

  • followed by destruction of the contained value as if by rhs.val.T::~T(),

  • the unexpected value of rhs is direct-initialized from unexpected(std::move(other.error())),

  • followed by destruction of the unexpected value as if by rhs.unexpect->unexpected<E>::~unexpected<E>(),

  • the contained value of this is direct-initialized from std::move(tmp), after this, this does not contain a value; and rhs contains a value.

Throws: Any exceptions that the expressions in the Effects clause throw.

Adapt swap Remarks once Effects are good.

Remarks: The expression inside noexcept is equivalent to: is_nothrow_move_constructible_v<T> and noexcept(swap(declval<T&>(), declval<T&>())) and is_nothrow_move_constructible_v<E> and noexcept(swap(declval<E&>(), declval<E&>())). The function shall not participate in overload resolution unless: LValues of type T shall be Swappable, LValues of type E shall be Swappable and is_move_constructible_v<E> is_move_constructible_v<E> is_move_constructible_v<E> or is_move_constructible_v<T>.

1.15. �.�.4.5 Observers [expected.object.observe]

constexpr const T* operator->() const;
T* operator->();

Requires: *this contains a value.

Returns: &val.

Remarks: Unless T is a user-defined type with overloaded unary operator&, the first operator shall be a constexpr function. The operator shall not participate in overload resolution unless: T is not void.

constexpr const T& operator *() const&;
T& operator *() &;

Requires: *this contains a value.

Returns: val.

Remarks: The first operator shall be a constexpr function. The operator shall not participate in overload resolution unless: T is not void.

constexpr T&& operator *() &&;
constexpr const T&& operator *() const&&;

Requires: *this contains a value.

Returns: move(val).

Remarks: This operator shall be a constexpr function. The operator shall not participate in overload resolution unless: T is not void.

constexpr explicit operator bool() noexcept;

Returns: has_val.

Remarks: This operator shall be a constexpr function.

    constexpr bool has_value() const noexcept;

Returns: has_val.

Remarks: This function shall be a constexpr function.

constexpr void expected<void, E>::value() const;

Throws: bad_expected_access(err) if *this does not contain a value.

constexpr const T& expected::value() const&;
constexpr T& expected::value() &;

Returns: val, if *this contains a value.

Throws: bad_expected_access(err) if *this does not contain a value.

Remarks: These functions shall be constexpr functions. The operator shall not participate in overload resolution unless: T is not void.

constexpr T&& expected::value() &&;
constexpr const T&& expected::value() const&&;

Returns: move(val), if *this contains a value.

Throws: bad_expected_access(err) if *this does not contain a value.

Remarks: These functions shall be constexpr functions. The operator shall not participate in overload resolution unless: T is not void.

constexpr const E& error() const&;
constexpr E& error() &;

Requires: *this does not contain a value.

Returns: unexpect.value().

Remarks: The first function shall be a constexpr function.

constexpr E&& error() &&;
constexpr const E&& error() const &&;

Requires: *this does not contain a value.

Returns: move(unexpect.value()).

Remarks: The first function shall be a constexpr function.

template <class U>
  constexpr T value_or(U&& v) const&;

Effects: Equivalent to return bool(*this) ? **this : static_cast<T>(std::forward<U>(v));.

Remarks: If is_copy_constructible_v<T> and is_convertible_v<U&&, T> is false the program is ill-formed.

template <class U>
  T value_or(U&& v) &&;

Effects: Equivalent to return bool(*this) ? std::move(**this) : static_cast<T>(std::forward<U>(v));.

Remarks: If is_move_constructible_v<T> and is_convertible_v<U&&, T> is false the program is ill-formed.

1.16. �.�.6 unexpect tag [expected.unexpect]

struct unexpect_t {
    explicit unexpect_t() = default;
};
inline constexpr unexpect_t unexpect{};

1.17. �.�.7 Template Class bad_expected_access [expected.bad_expected_access]

template <class E>
class bad_expected_access : public bad_expected_access<void> {
public:
    explicit bad_expected_access(E);
    virtual const char* what() const noexcept override;
    E& error() &;
    const E& error() const&;
    E&& error() &&;
    const E&&  error() const&&;
private:
    E val; // exposition only
};

Wondering if we just need an const & overload as we do for system_error.

The template class bad_expected_access defines the type of objects thrown as exceptions to report the situation where an attempt is made to access the value of a unexpected expected object.

bad_expected_access::bad_expected_access(E e);

Effects: Constructs an object of class bad_expected_access storing the parameter.

Postconditions: what() returns an implementation-defined NTBS.

    const E& error() const&;
    E& error() &;

Effects: Equivalent to: return val;

    E&& error() &&;
    const E&& error() const &&;

Effects: Equivalent to: return move(val);

virtual const char* what() const noexcept override;

Returns: An implementation-defined NTBS.

1.18. �.�.7 Template Class bad_expected_access<void> [expected.bad_expected_access_base]

template <>
class bad_expected_access<void> : public exception {
public:
    explicit bad_expected_access();
};

The template class bad_expected_access<void> defines the type of objects thrown as exceptions to report the situation where an attempt is made to access the value of a unexpected expected object.

1.19. �.�.8 Expected Relational operators [expected.relational_op]

    template <class T, class E>
        constexpr bool operator==(const expected<T, E>& x, const expected<T, E>& y);

Requires: T (if not void) and unexpected<E> shall meet the requirements of EqualityComparable.

Returns: If bool(x) != bool(y), false; otherwise if bool(x) == false, unexpected(x.error()) == unexpected(y.error()); otherwise true if T is void or *x == *y otherwise.

Remarks: Specializations of this function template, for which T is void or *x == *y and unexpected(x.error()) == unexpected(y.error()) are core constant expression, shall be constexpr functions.

    template <class T, class E>
        constexpr bool operator!=(const expected<T,    E>& x, const expected<T, E>& y);

Requires: T (if not void) and unexpected<E> shall meet the requirements of EqualityComparable.

Returns: If bool(x) != bool(y), true; otherwise if bool(x) == false, unexpected(x.error()) != unexpected(y.error()); otherwise true if T is void or *x != *y.

Remarks: Specializations of this function template, for which T is void or *x != *y and unexpected(x.error()) != unexpected(y.error()) are core constant expression, shall be constexpr functions.

1.20. �.�.9 Comparison with T [expected.comparison_T]

    template <class T, class E> constexpr bool operator==(const expected<T, E>& x, const T& v);
    template <class T, class E> constexpr bool operator==(const T& v, const expected<T, E>& x);

Requires: T is not void and the expression *x == v shall be well-formed and its result shall be convertible to bool. [ Note: T need not be EqualityComparable. - end note]

Effects: Equivalent to: return bool(x) ? *x == v : false;.

    template <class T, class E> constexpr bool operator!=(const expected<T, E>& x, const T& v);
    template <class T, class E> constexpr bool operator!=(const T& v, const expected<T, E>& x);

Requires: T is not void and the expression *x == v shall be well-formed and its result shall be convertible to bool. [ Note: T need not be EqualityComparable. - end note]

Effects: Equivalent to: return bool(x) ? *x != v : false;.

1.21. �.�.10 Comparison with unexpected<E> [expected.comparison_unexpected_E]

    template <class T, class E> constexpr bool operator==(const expected<T, E>& x, const unexpected<E>& e);
    template <class T, class E> constexpr bool operator==(const unexpected<E>& e, const expected<T, E>& x);

Requires: The expression unexpected(x.error()) == e shall be well-formed and its result shall be convertible to bool. [ Note: E need not be EqualityComparable. - end note]

Effects: Equivalent to: return bool(x) ? true : unexpected(x.error()) == e;.

    template <class T, class E> constexpr bool operator!=(const expected<T, E>& x, const unexpected<E>& e);
    template <class T, class E> constexpr bool operator!=(const unexpected<E>& e, const expected<T, E>& x);

Requires: The expression unexpected(x.error()) != e shall be well-formed and its result shall be convertible to bool. [ Note: E need not be EqualityComparable. - end note]

Effects: Equivalent to: return bool(x) ? false : unexpected(x.error()) != e;.

1.22. �.�.11 Specialized algorithms [expected.specalg]

template <class T, class E>
void swap(expected<T, E>& x, expected<T, E>& y) noexcept(noexcept(x.swap(y)));

Effects: Calls x.swap(y).

Remarks: This function shall not participate in overload resolution unless T is void or is_move_constructible_v<T> is true, is_swappable_v<T> is true and is_move_constructible_v<E> is true and is_swappable_v<E> is true

2. Open Questions

std::expected is a vocabulary type with an opinionated design and a proven record under varied forms in a multitude of codebases. Its current form has undergone multiple revisions and received substantial feedback, falling roughly in the following categories:

1. Ergonomics: is this the right way to expose such functionality?

2. Disappointment: should we expose this in the Standard, given C++'s existing error handling mechanisms?

3. STL usage: should the Standard Template Library adopt this class, at which pace, and where?

LEWG and EWG have nonetheless reached consensus that a class of this general approach is probably desirable, and the only way to truly answer these questions is to try it out in a TS and ask for explicit feedback from developers. The authors hope that developers will provide new information which they’ll be able to communicate to the Committee.

Here are open questions, and questions which the Committee thinks are settled and which new information can justify revisiting.

2.1. Ergonomics

1. Name:

   • Is expected the right name?

   • Does it express intent both as a consumer and a producer?

2. Is E a salient property of expected?

3. Is expected<void, E> clear on what it expresses as a return type?

4. Would it make sense for expected to support containing both T and E (in some designs, either one of them being optional), or is this usecase better handled by a separate proposal?

5. Is the order of parameters <T, E> appropriate?

6. Is usage of expected "viral" in a codebase, or can it be adopted incrementally?

7. Comparisons:

   • Are == and != useful?

   • Should other comparisons be provided?

   • What usages of expected mandate putting instances in a map, or other such container?

   • Should hash be provided?

   • What usages of expected mandate putting instances in an unordered_map, or other such container?

   • Should expected<T, E> always be comparable if T is comparable, even if E is not comparable?

8. Error type E:

   • E has no default. Should it?

   • Should expected be specialized for particular E types such as exception_ptr, and how?

   • Should expected handle E types with a built-in "success" value any differently, and how?

   • expected is not implicitly constructible from an E, even when unambiguous from T, because as a vocabulary type it wants unexpected error construction to be verbose, and require hopping through an unexpected. Is the verbosity extraneous?

9. Does usage of this class cause a meaningful performance impact compared to using error codes?

10. The accessor design offers a terse unchecked dereference operator (expected to be used alongside the implicit bool conversion), as well as value() and error() accessors which are checked. Is that a gotcha, or is it similar enough to classes such as optional to be unsurprising?

11. Is bad_expected_access the right thing to throw?

12. Should some members be [[nodiscard]]?

2.2. Disappointment

C++ already supports exceptions and error codes, expected would be a third kind of error handling.

1. where does expected work better than either exceptions or error handling?

2. expected was designed to be particularly well suited to APIs which require their immediate caller to consider an error scenario. Do it succeed in that purpose?

3. Do codebases successfully compose these three types of error handling?

4. Is debuggability any harder?

5. Is it easy to teach C++ as a whole with a third type of error handling?

2.3. STL Usage

1. Should expected be used in the STL at the same time as it gets standardized?

2. Where, considering std2 may be a good place to change APIs?