22 General utilities library [utilities]

22.8 Expected objects [expected]

22.8.6 Class template expected [expected.expected]

22.8.6.4 Assignment [expected.object.assign]

1
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This subclause makes use of the following exposition-only function template: template<class T, class U, class... Args> constexpr void reinit-expected(T& newval, U& oldval, Args&&... args) { // exposition only if constexpr (is_nothrow_constructible_v<T, Args...>) { destroy_at(addressof(oldval)); construct_at(addressof(newval), std::forward<Args>(args)...); } else if constexpr (is_nothrow_move_constructible_v<T>) { T tmp(std::forward<Args>(args)...); destroy_at(addressof(oldval)); construct_at(addressof(newval), std::move(tmp)); } else { U tmp(std::move(oldval)); destroy_at(addressof(oldval)); try { construct_at(addressof(newval), std::forward<Args>(args)...); } catch (...) { construct_at(addressof(oldval), std::move(tmp)); throw; } } }
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constexpr expected& operator=(const expected& rhs);
2
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Effects:
  • (2.1)
    If this->has_value() && rhs.has_value() is true, equivalent to val = *rhs.
  • (2.2)
    Otherwise, if this->has_value() is true, equivalent to: reinit-expected(unex, val, rhs.error())
  • (2.3)
    Otherwise, if rhs.has_value() is true, equivalent to: reinit-expected(val, unex, *rhs)
  • (2.4)
    Otherwise, equivalent to unex = rhs.error().
Then, if no exception was thrown, equivalent to: has_val = rhs.has_value(); return *this;
3
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Returns: *this.
4
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Remarks: This operator is defined as deleted unless:
  • (4.1)
    is_copy_assignable_v<T> is true and
  • (4.2)
    is_copy_constructible_v<T> is true and
  • (4.3)
    is_copy_assignable_v<E> is true and
  • (4.4)
    is_copy_constructible_v<E> is true and
  • (4.5)
    is_nothrow_move_constructible_v<T> || is_nothrow_move_constructible_v<E> is true.
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constexpr expected& operator=(expected&& rhs) noexcept(see below);
5
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Constraints:
  • (5.1)
    is_move_constructible_v<T> is true and
  • (5.2)
    is_move_assignable_v<T> is true and
  • (5.3)
    is_move_constructible_v<E> is true and
  • (5.4)
    is_move_assignable_v<E> is true and
  • (5.5)
    is_nothrow_move_constructible_v<T> || is_nothrow_move_constructible_v<E> is true.
6
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Effects:
  • (6.1)
    If this->has_value() && rhs.has_value() is true, equivalent to val = std​::​move(*rhs).
  • (6.2)
    Otherwise, if this->has_value() is true, equivalent to: reinit-expected(unex, val, std::move(rhs.error()))
  • (6.3)
    Otherwise, if rhs.has_value() is true, equivalent to: reinit-expected(val, unex, std::move(*rhs))
  • (6.4)
    Otherwise, equivalent to unex = std​::​move(rhs.error()).
Then, if no exception was thrown, equivalent to: has_val = rhs.has_value(); return *this;
7
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Returns: *this.
8
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Remarks: The exception specification is equivalent to: is_nothrow_move_assignable_v<T> && is_nothrow_move_constructible_v<T> && is_nothrow_move_assignable_v<E> && is_nothrow_move_constructible_v<E>
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template<class U = T> constexpr expected& operator=(U&& v);
9
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Constraints:
  • (9.1)
    is_same_v<expected, remove_cvref_t<U>> is false; and
  • (9.2)
    remove_cvref_t<U> is not a specialization of unexpected; and
  • (9.3)
    is_constructible_v<T, U> is true; and
  • (9.4)
    is_assignable_v<T&, U> is true; and
  • (9.5)
    is_nothrow_constructible_v<T, U> || is_nothrow_move_constructible_v<T> ||
    is_nothrow_move_constructible_v<E>     is true.
10
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Effects:
  • (10.1)
    If has_value() is true, equivalent to: val = std​::​forward<U>(v);
  • (10.2)
    Otherwise, equivalent to: reinit-expected(val, unex, std::forward<U>(v)); has_val = true;
11
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Returns: *this.
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template<class G> constexpr expected& operator=(const unexpected<G>& e); template<class G> constexpr expected& operator=(unexpected<G>&& e);
12
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Let GF be const G& for the first overload and G for the second overload.
13
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Constraints:
  • (13.1)
    is_constructible_v<E, GF> is true; and
  • (13.2)
    is_assignable_v<E&, GF> is true; and
  • (13.3)
    is_nothrow_constructible_v<E, GF> || is_nothrow_move_constructible_v<T> ||
    is_nothrow_move_constructible_v<E>     is true.
14
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Effects:
  • (14.1)
    If has_value() is true, equivalent to: reinit-expected(unex, val, std::forward<GF>(e.error())); has_val = false;
  • (14.2)
    Otherwise, equivalent to: unex = std​::​forward<GF>(e.error());
15
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Returns: *this.
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template<class... Args> constexpr T& emplace(Args&&... args) noexcept;
16
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Constraints: is_nothrow_constructible_v<T, Args...> is true.
17
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Effects: Equivalent to: if (has_value()) { destroy_at(addressof(val)); } else { destroy_at(addressof(unex)); has_val = true; } return *construct_at(addressof(val), std::forward<Args>(args)...);
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template<class U, class... Args> constexpr T& emplace(initializer_list<U> il, Args&&... args) noexcept;
18
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Constraints: is_nothrow_constructible_v<T, initializer_list<U>&, Args...> is true.
19
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Effects: Equivalent to: if (has_value()) { destroy_at(addressof(val)); } else { destroy_at(addressof(unex)); has_val = true; } return *construct_at(addressof(val), il, std::forward<Args>(args)...);