22 General utilities library [utilities]

22.5 Optional objects [optional]

22.5.4 Partial specialization of optional for reference types [optional.optional.ref]

22.5.4.3 Assignment [optional.ref.assign]

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constexpr optional& operator=(nullopt_t) noexcept;
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Effects: Assigns nullptr to val.
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Postconditions: *this does not contain a value.
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Returns: *this.
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template<class U> constexpr T& emplace(U&& u) noexcept(is_nothrow_constructible_v<T&, U>);
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Constraints:
  • (4.1)
    is_constructible_v<T&, U> is true, and
  • (4.2)
    reference_constructs_from_temporary_v<T&, U> is false.
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Effects: Equivalent to: convert-ref-init-val(std​::​forward<U>(u)).