22 General utilities library [utilities]

22.5 Optional objects [optional]

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

22.5.4.6 Observers [optional.ref.observe]

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constexpr T* operator->() const noexcept;
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Hardened preconditions: has_value() is true.
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Returns: val.
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constexpr T& operator*() const noexcept;
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Hardened preconditions: has_value() is true.
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Returns: *val.
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constexpr explicit operator bool() const noexcept;
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Returns: val != nullptr.
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constexpr bool has_value() const noexcept;
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Returns: val != nullptr.
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constexpr T& value() const;
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Effects: Equivalent to: return has_value() ? *val : throw bad_optional_access();
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template<class U = remove_cv_t<T>> constexpr remove_cv_t<T> value_or(U&& u) const;
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Constraints: T is a non-array object type.
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Let X be remove_cv_t<T>.
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Mandates: is_constructible_v<X, T&> && is_convertible_v<U, X> is true.
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Effects: Equivalent to: return has_value() ? *val : static_cast<X>(std::forward<U>(u));
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Remarks: The return type is unspecified if T is an array type or a non-object type.
[Note 1: 
This is to avoid the declaration being ill-formed.
— end note]