22 General utilities library [utilities]

22.8 Expected objects [expected]

22.8.6 Class template expected [expected.expected]

22.8.6.2 Constructors [expected.object.cons]

The exposition-only variable template converts-from-any-cvref defined in [optional.ctor] is used by some constructors for expected.

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constexpr expected();

Constraints: is_default_constructible_v<T> is true.

Effects: Value-initializes val.

Postconditions: has_value() is true.

Throws: Any exception thrown by the initialization of val.

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constexpr expected(const expected& rhs);

Effects: If rhs.has_value() is true, direct-non-list-initializes val with *rhs.

Otherwise, direct-non-list-initializes unex with rhs.error().

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

Throws: Any exception thrown by the initialization of val or unex.

Remarks: This constructor is defined as deleted unless

(9.1)
is_copy_constructible_v<T> is true and
(9.2)
is_copy_constructible_v<E> is true.

This constructor is trivial if

(10.1)
is_trivially_copy_constructible_v<T> is true and
(10.2)
is_trivially_copy_constructible_v<E> is true.

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

Constraints:

(11.1)
is_move_constructible_v<T> is true and
(11.2)
is_move_constructible_v<E> is true.

Effects: If rhs.has_value() is true, direct-non-list-initializes val with std::move(*rhs).

Otherwise, 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 val or unex.

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

This constructor is trivial if

(16.1)
is_trivially_move_constructible_v<T> is true and
(16.2)
is_trivially_move_constructible_v<E> is true.

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template<class U, class G>
  constexpr explicit(see below) expected(const expected<U, G>& rhs);
template<class U, class G>
  constexpr explicit(see below) expected(expected<U, G>&& rhs);

Let:

(17.1)
UF be const U& for the first overload and U for the second overload.
(17.2)
GF be const G& for the first overload and G for the second overload.

Constraints:

(18.1)
is_constructible_v<T, UF> is true; and
(18.2)
is_constructible_v<E, GF> is true; and
(18.3)
if T is not cv bool, converts-from-any-cvref<T, expected<U, G>> is false; and
(18.4)
is_constructible_v<unexpected<E>, expected<U, G>&> is false; and
(18.5)
is_constructible_v<unexpected<E>, expected<U, G>> is false; and
(18.6)
is_constructible_v<unexpected<E>, const expected<U, G>&> is false; and
(18.7)
is_constructible_v<unexpected<E>, const expected<U, G>> is false.

Effects: If rhs.has_value(), direct-non-list-initializes val with std::forward<UF>(*rhs).

Otherwise, 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 val or unex.

Remarks: The expression inside explicit is equivalent to !is_convertible_v<UF, T> || !is_convertible_v<GF, E>.

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template<class U = T>
  constexpr explicit(!is_convertible_v<U, T>) expected(U&& v);

Constraints:

(23.1)
is_same_v<remove_cvref_t, in_place_t> is false; and
(23.2)
is_same_v<expected, remove_cvref_t> is false; and
(23.3)
remove_cvref_t is not a specialization of unexpected; and
(23.4)
is_constructible_v<T, U> is true; and
(23.5)
if T is cv bool, remove_cvref_t is not a specialization of expected.

Effects: Direct-non-list-initializes val with std::forward(v).

Postconditions: has_value() is true.

Throws: Any exception thrown by the initialization of val.

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

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

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

Postconditions: has_value() is true.

Throws: Any exception thrown by the initialization of val.

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

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

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

Postconditions: has_value() is true.

Throws: Any exception thrown by the initialization of val.

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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.