[optional.ref.monadic] (original) (raw)

22 General utilities library [utilities]

22.5 Optional objects [optional]

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

22.5.4.7 Monadic operations [optional.ref.monadic]

template<class F> constexpr auto and_then(F&& f) const;

Let U be invoke_result_t<F, T&>.

Mandates: remove_cvref_t<U> is a specialization of optional.

Effects: Equivalent to:if (has_value()) { return invoke(std::forward<F>(f), *val);} else { return remove_cvref_t<U>();}

template<class F> constexpr optional<remove_cv_t<invoke_result_t<F, T&>>> transform(F&& f) const;

Let U be remove_cv_t<invoke_result_t<F, T&>>.

Mandates: The declarationU u(invoke(std::forward<F>(f), *val));is well-formed for some invented variable u.

Returns: If *this contains a value, an optional<U> object whose contained value is direct-non-list-initialized withinvoke(std​::​forward<F>(f), *val); otherwise, optional<U>().

template<class F> constexpr optional or_else(F&& f) const;

Mandates: is_same_v<remove_cvref_t<invoke_result_t<F>>, optional> is true.

Effects: Equivalent to:if (has_value()) { return *val;} else { return std::forward<F>(f)();}