22 General utilities library [utilities]

22.8 Expected objects [expected]

22.8.7 Partial specialization of expected for void types [expected.void]

22.8.7.1 General [expected.void.general]

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>; // [expected.void.cons], constructors constexpr expected() noexcept; constexpr expected(const expected&); constexpr expected(expected&&) noexcept(see below); template<class U, class G> constexpr explicit(see below) expected(const expected<U, G>&); template<class U, class G> constexpr explicit(see below) expected(expected<U, G>&&); template<class G> constexpr explicit(see below) expected(const unexpected<G>&); template<class G> constexpr explicit(see below) expected(unexpected<G>&&); constexpr explicit expected(in_place_t) noexcept; template<class... Args> constexpr explicit expected(unexpect_t, Args&&...); template<class U, class... Args> constexpr explicit expected(unexpect_t, initializer_list, Args&&...); // [expected.void.dtor], destructor constexpr ~expected(); // [expected.void.assign], assignment constexpr expected& operator=(const expected&); constexpr expected& operator=(expected&&) noexcept(see below); template<class G> constexpr expected& operator=(const unexpected<G>&); template<class G> constexpr expected& operator=(unexpected<G>&&); constexpr void emplace() noexcept; // [expected.void.swap], swap constexpr void swap(expected&) noexcept(see below); friend constexpr void swap(expected& x, expected& y) noexcept(noexcept(x.swap(y))); // [expected.void.obs], observers constexpr explicit operator bool() const noexcept; constexpr bool has_value() const noexcept; constexpr void operator*() const noexcept; constexpr void value() const &; constexpr void value() &&; constexpr const E& error() const & noexcept; constexpr E& error() & noexcept; constexpr const E&& error() const && noexcept; constexpr E&& error() && noexcept; template<class G = E> constexpr E error_or(G&&) const &; template<class G = E> constexpr E error_or(G&&) &&; // [expected.void.monadic], monadic operations template<class F> constexpr auto and_then(F&& f) &; template<class F> constexpr auto and_then(F&& f) &&; template<class F> constexpr auto and_then(F&& f) const &; template<class F> constexpr auto and_then(F&& f) const &&; template<class F> constexpr auto or_else(F&& f) &; template<class F> constexpr auto or_else(F&& f) &&; template<class F> constexpr auto or_else(F&& f) const &; template<class F> constexpr auto or_else(F&& f) const &&; template<class F> constexpr auto transform(F&& f) &; template<class F> constexpr auto transform(F&& f) &&; template<class F> constexpr auto transform(F&& f) const &; template<class F> constexpr auto transform(F&& f) const &&; template<class F> constexpr auto transform_error(F&& f) &; template<class F> constexpr auto transform_error(F&& f) &&; template<class F> constexpr auto transform_error(F&& f) const &; template<class F> constexpr auto transform_error(F&& f) const &&; // [expected.void.eq], equality operators template<class T2, class E2> requires is_void_v<T2> friend constexpr bool operator==(const expected& x, const expected<T2, E2>& y); template<class E2> friend constexpr bool operator==(const expected&, const unexpected<E2>&); private: bool has_val; // exposition only union { E unex; // exposition only }; };

Any object of type expected<T, E> either represents a value of type T, or contains a value of type E within its own storage.

Implementations are not permitted to use additional storage, such as dynamic memory, to allocate the object of type E.

Member has_val indicates whether the expected<T, E> object represents a value of type T.

A program that instantiates the definition of the template expected<T, E> with a type for the E parameter that is not a valid template argument for unexpected is ill-formed.

E shall meet the requirements of Cpp17Destructible (Table 35).

22.8.7.2 Constructors [expected.void.cons]

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constexpr expected() noexcept;

Postconditions: has_value() is true.

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constexpr expected(const expected& rhs);

Effects: If rhs.has_value() is false, direct-non-list-initializes unex with rhs.error().

Postconditions: rhs.has_value() == this->has_value().

Throws: Any exception thrown by the initialization of unex.

Remarks: This constructor is defined as deleted unless is_copy_constructible_v<E> is true.

This constructor is trivial if is_trivially_copy_constructible_v<E> is true.

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constexpr expected(expected&& rhs) noexcept(is_nothrow_move_constructible_v<E>);

Constraints: is_move_constructible_v<E> is true.

Effects: If rhs.has_value() is false, direct-non-list-initializes unex with std::move(rhs.error()).

Postconditions: rhs.has_value() is unchanged; rhs.has_value() == this->has_value() is true.

Throws: Any exception thrown by the initialization of unex.

Remarks: This constructor is trivial if is_trivially_move_constructible_v<E> is true.

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template<class U, class G>
  constexpr explicit(!is_convertible_v<const G&, E>) expected(const expected<U, G>& rhs);
template<class U, class G>
  constexpr explicit(!is_convertible_v<G, E>) expected(expected<U, G>&& rhs);

Let GF be const G& for the first overload and G for the second overload.

Constraints:

(13.1)
is_void_v is true; and
(13.2)
is_constructible_v<E, GF> is true; and
(13.3)
is_constructible_v<unexpected<E>, expected<U, G>&> is false; and
(13.4)
is_constructible_v<unexpected<E>, expected<U, G>> is false; and
(13.5)
is_constructible_v<unexpected<E>, const expected<U, G>&> is false; and
(13.6)
is_constructible_v<unexpected<E>, const expected<U, G>> is false.

Effects: If rhs.has_value() is false, direct-non-list-initializes unex with std::forward<GF>(rhs.error()).

Postconditions: rhs.has_value() is unchanged; rhs.has_value() == this->has_value() is true.

Throws: Any exception thrown by the initialization of unex.

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template<class G>
  constexpr explicit(!is_convertible_v<const G&, E>) expected(const unexpected<G>& e);
template<class G>
  constexpr explicit(!is_convertible_v<G, E>) expected(unexpected<G>&& e);

Let GF be const G& for the first overload and G for the second overload.

Constraints: is_constructible_v<E, GF> is true.

Effects: Direct-non-list-initializes unex with std::forward<GF>(e.error()).

Postconditions: has_value() is false.

Throws: Any exception thrown by the initialization of unex.

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constexpr explicit expected(in_place_t) noexcept;

Postconditions: has_value() is true.

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

Constraints: is_constructible_v<E, Args...> is true.

Effects: Direct-non-list-initializes unex with std::forward<Args>(args)....

Postconditions: has_value() is false.

Throws: Any exception thrown by the initialization of unex.

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

Constraints: is_constructible_v<E, initializer_list&, Args...> is true.

Effects: Direct-non-list-initializes unex with il, std::forward<Args>(args)....

Postconditions: has_value() is false.

Throws: Any exception thrown by the initialization of unex.

22.8.7.3 Destructor [expected.void.dtor]

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constexpr ~expected();

Effects: If has_value() is false, destroys unex.

Remarks: If is_trivially_destructible_v<E> is true, then this destructor is a trivial destructor.

22.8.7.4 Assignment [expected.void.assign]

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constexpr expected& operator=(const expected& rhs);

Effects:

(1.1)
If this->has_value() && rhs.has_value() is true, no effects.
(1.2)
Otherwise, if this->has_value() is true, equivalent to: construct_at(addressof(unex), rhs.unex); has_val = false;
(1.3)
Otherwise, if rhs.has_value() is true, destroys unex and sets has_val to true.
(1.4)
Otherwise, equivalent to unex = rhs.error().

Returns: *this.

Remarks: This operator is defined as deleted unless is_copy_assignable_v<E> is true and is_copy_constructible_v<E> is true.

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

Effects:

(4.1)
If this->has_value() && rhs.has_value() is true, no effects.
(4.2)
Otherwise, if this->has_value() is true, equivalent to: construct_at(addressof(unex), std::move(rhs.unex)); has_val = false;
(4.3)
Otherwise, if rhs.has_value() is true, destroys unex and sets has_val to true.
(4.4)
Otherwise, equivalent to unex = std::move(rhs.error()).

Returns: *this.

Remarks: The exception specification is equivalent to is_nothrow_move_constructible_v<E> && is_nothrow_move_assignable_v<E>.

This operator is defined as deleted unless is_move_constructible_v<E> is true and is_move_assignable_v<E> is true.

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

Let GF be const G& for the first overload and G for the second overload.

Constraints: is_constructible_v<E, GF> is true and is_assignable_v<E&, GF> is true.

Effects:

(10.1)
If has_value() is true, equivalent to: construct_at(addressof(unex), std::forward<GF>(e.error())); has_val = false;
(10.2)
Otherwise, equivalent to: unex = std::forward<GF>(e.error());

Returns: *this.

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constexpr void emplace() noexcept;

Effects: If has_value() is false, destroys unex and sets has_val to true.

22.8.7.5 Swap [expected.void.swap]

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

Constraints: is_swappable_v<E> is true and is_move_constructible_v<E> is true.

Effects: See Table 64 .

Table 64: swap(expected&) effects [tab:expected.void.swap]

🔗		this->has_value()	!this->has_value()
🔗	rhs.has_value()	no effects	calls rhs.swap(*this)
🔗	!rhs.has_value()	see below	equivalent to: using std::swap; swap(unex, rhs.unex);

For the case where rhs.value() is false and this->has_value() is true, equivalent to: construct_at(addressof(unex), std::move(rhs.unex)); destroy_at(addressof(rhs.unex)); has_val = false; rhs.has_val = true;

Throws: Any exception thrown by the expressions in the Effects.

Remarks: The exception specification is equivalent to is_nothrow_move_constructible_v<E> && is_nothrow_swappable_v<E>.

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friend constexpr void swap(expected& x, expected& y) noexcept(noexcept(x.swap(y)));

Effects: Equivalent to x.swap(y).

22.8.7.6 Observers [expected.void.obs]

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constexpr explicit operator bool() const noexcept;
constexpr bool has_value() const noexcept;

Returns: has_val.

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constexpr void operator*() const noexcept;

Preconditions: has_value() is true.

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constexpr void value() const &;

Throws: bad_expected_access(error()) if has_value() is false.

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constexpr void value() &&;

Throws: bad_expected_access(std::move(error())) if has_value() is false.

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

Preconditions: has_value() is false.

Returns: unex.

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

Preconditions: has_value() is false.

Returns: std::move(unex).

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template<class G = E> constexpr E error_or(G&& e) const &;

Mandates: is_copy_constructible_v<E> is true and is_convertible_v<G, E> is true.

Returns: std::forward<G>(e) if has_value() is true, error() otherwise.

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template<class G = E> constexpr E error_or(G&& e) &&;

Mandates: is_move_constructible_v<E> is true and is_convertible_v<G, E> is true.

Returns: std::forward<G>(e) if has_value() is true, std::move(error()) otherwise.

22.8.7.7 Monadic operations [expected.void.monadic]

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template<class F> constexpr auto and_then(F&& f) &;
template<class F> constexpr auto and_then(F&& f) const &;

Let U be remove_cvref_t<invoke_result_t<F>>.

Constraints: is_constructible_v<E, decltype(error())>> is true.

Mandates: U is a specialization of expected and is_same_v<U::error_type, E> is true.

Effects: Equivalent to: if (has_value()) return invoke(std::forward<F>(f)); else return U(unexpect, error());

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template<class F> constexpr auto and_then(F&& f) &&;
template<class F> constexpr auto and_then(F&& f) const &&;

Let U be remove_cvref_t<invoke_result_t<F>>.

Constraints: is_constructible_v<E, decltype(std::move(error()))> is true.

Mandates: U is a specialization of expected and is_same_v<U::error_type, E> is true.

Effects: Equivalent to: if (has_value()) return invoke(std::forward<F>(f)); else return U(unexpect, std::move(error()));

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template<class F> constexpr auto or_else(F&& f) &;
template<class F> constexpr auto or_else(F&& f) const &;

Let G be remove_cvref_t<invoke_result_t<F, decltype(error())>>.

Mandates: G is a specialization of expected and is_same_v<G::value_type, T> is true.

Effects: Equivalent to: if (has_value()) return G(); else return invoke(std::forward<F>(f), error());

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template<class F> constexpr auto or_else(F&& f) &&;
template<class F> constexpr auto or_else(F&& f) const &&;

Let G be remove_cvref_t<invoke_result_t<F, decltype(std::move(error()))>>.

Mandates: G is a specialization of expected and is_same_v<G::value_type, T> is true.

Effects: Equivalent to: if (has_value()) return G(); else return invoke(std::forward<F>(f), std::move(error()));

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template<class F> constexpr auto transform(F&& f) &;
template<class F> constexpr auto transform(F&& f) const &;

Let U be remove_cv_t<invoke_result_t<F>>.

Constraints: is_constructible_v<E, decltype(error())> is true.

Mandates: U is a valid value type for expected.

If is_void_v is false, the declaration U u(invoke(std::forward<F>(f))); is well-formed.

Effects:

(18.1)
If has_value() is false, returns expected<U, E>(unexpect, error()).
(18.2)
Otherwise, if is_void_v is false, returns an expected<U, E> object whose has_val member is true and val member is direct-non-list-initialized with invoke(std::forward<F>(f)).
(18.3)
Otherwise, evaluates invoke(std::forward<F>(f)) and then returns expected<U, E>().

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template<class F> constexpr auto transform(F&& f) &&;
template<class F> constexpr auto transform(F&& f) const &&;

Let U be remove_cv_t<invoke_result_t<F>>.

Constraints: is_constructible_v<E, decltype(std::move(error()))> is true.

Mandates: U is a valid value type for expected.

If is_void_v is false, the declaration U u(invoke(std::forward<F>(f))); is well-formed.

Effects:

(22.1)
If has_value() is false, returns expected<U, E>(unexpect, std::move(error())).
(22.2)
Otherwise, if is_void_v is false, returns an expected<U, E> object whose has_val member is true and val member is direct-non-list-initialized with invoke(std::forward<F>(f)).
(22.3)
Otherwise, evaluates invoke(std::forward<F>(f)) and then returns expected<U, E>().

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template<class F> constexpr auto transform_error(F&& f) &;
template<class F> constexpr auto transform_error(F&& f) const &;

Let G be remove_cv_t<invoke_result_t<F, decltype(error())>>.

Mandates: G is a valid template argument for unexpected ([expected.un.general]) and the declaration G g(invoke(std::forward<F>(f), error())); is well-formed.

Returns: If has_value() is true, expected<T, G>(); otherwise, an expected<T, G> object whose has_val member is false and unex member is direct-non-list-initialized with invoke(std::forward<F>(f), error()).

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template<class F> constexpr auto transform_error(F&& f) &&;
template<class F> constexpr auto transform_error(F&& f) const &&;

Let G be remove_cv_t<invoke_result_t<F, decltype(std::move(error()))>>.

Mandates: G is a valid template argument for unexpected ([expected.un.general]) and the declaration G g(invoke(std::forward<F>(f), std::move(error()))); is well-formed.

22.8.7.8 Equality operators [expected.void.eq]

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template<class T2, class E2> requires is_void_v<T2>
  friend constexpr bool operator==(const expected& x, const expected<T2, E2>& y);

Mandates: The expression x.error() == y.error() is well-formed and its result is convertible to bool.

Returns: If x.has_value() does not equal y.has_value(), false; otherwise x.has_value() || static_cast<bool>(x.error() == y.error()).

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template<class E2>
  friend constexpr bool operator==(const expected& x, const unexpected<E2>& e);

Mandates: The expression x.error() == e.error() is well-formed and its result is convertible to bool.

Returns: !x.has_value() && static_cast<bool>(x.error() == e.error()).