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.
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.
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
  • is_copy_constructible_v<T> is true and
  • is_copy_constructible_v<E> is true.
This constructor is trivial if
  • is_trivially_copy_constructible_v<T> is true and
  • is_trivially_copy_constructible_v<E> is true.
constexpr expected(expected&& rhs) noexcept(see below);
Constraints:
  • is_move_constructible_v<T> is true and
  • 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
  • is_trivially_move_constructible_v<T> is true and
  • is_trivially_move_constructible_v<E> is true.
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:
  • UF be const U& for the first overload and U for the second overload.
  • GF be const G& for the first overload and G for the second overload.
Constraints:
  • is_constructible_v<T, UF> is true; and
  • is_constructible_v<E, GF> is true; and
  • if T is not cv bool, converts-from-any-cvref<T, expected<U, G>> is false; and
  • is_constructible_v<unexpected<E>, expected<U, G>&> is false; and
  • is_constructible_v<unexpected<E>, expected<U, G>> is false; and
  • is_constructible_v<unexpected<E>, const expected<U, G>&> is false; and
  • 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>.
template<class U = T> constexpr explicit(!is_convertible_v<U, T>) expected(U&& v);
Constraints:
  • is_same_v<remove_cvref_t<U>, in_place_t> is false; and
  • is_same_v<expected, remove_cvref_t<U>> is false; and
  • remove_cvref_t<U> is not a specialization of unexpected; and
  • is_constructible_v<T, U> is true; and
  • if T is cv bool, remove_cvref_t<U> is not a specialization of expected.
Effects: Direct-non-list-initializes val with std​::​forward<U>(v).
Postconditions: has_value() is true.
Throws: Any exception thrown by the initialization of val.
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.
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.
template<class U, class... Args> constexpr explicit expected(in_place_t, initializer_list<U> il, Args&&... args);
Constraints: is_constructible_v<T, initializer_list<U>&, 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.
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.
template<class U, class... Args> constexpr explicit expected(unexpect_t, initializer_list<U> il, Args&&... args);
Constraints: is_constructible_v<E, initializer_list<U>&, 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.