22 General utilities library [utilities]

22.5 Optional objects [optional]

22.5.3 Class template optional [optional.optional]

22.5.3.7 Monadic operations [optional.monadic]

template<class F> constexpr auto and_then(F&& f) &; template<class F> constexpr auto and_then(F&& f) const &;
Let U be invoke_result_t<F, decltype(*val)>.
Mandates: remove_cvref_t<U> is a specialization of optional.
Effects: Equivalent to: if (*this) { return invoke(std::forward<F>(f), *val); } else { return remove_cvref_t<U>(); }
template<class F> constexpr auto and_then(F&& f) &&; template<class F> constexpr auto and_then(F&& f) const &&;
Let U be invoke_result_t<F, decltype(std​::​move(*val))>.
Mandates: remove_cvref_t<U> is a specialization of optional.
Effects: Equivalent to: if (*this) { return invoke(std::forward<F>(f), std::move(*val)); } else { return remove_cvref_t<U>(); }
template<class F> constexpr auto transform(F&& f) &; template<class F> constexpr auto transform(F&& f) const &;
Let U be remove_cv_t<invoke_result_t<F, decltype(*val)>>.
Mandates: U is a non-array object type other than in_place_t or nullopt_t.
The declaration U u(invoke(std::forward<F>(f), *val)); is well-formed for some invented variable u.
[Note 1: 
There is no requirement that U is movable ([dcl.init.general]).
— end note]
Returns: If *this contains a value, an optional<U> object whose contained value is direct-non-list-initialized with invoke(std​::​forward<F>(f), *val); otherwise, optional<U>().
template<class F> constexpr auto transform(F&& f) &&; template<class F> constexpr auto transform(F&& f) const &&;
Let U be remove_cv_t<invoke_result_t<F, decltype(std​::​move(*val))>>.
Mandates: U is a non-array object type other than in_place_t or nullopt_t.
The declaration U u(invoke(std::forward<F>(f), std::move(*val))); is well-formed for some invented variable u.
[Note 2: 
There is no requirement that U is movable ([dcl.init.general]).
— end note]
Returns: If *this contains a value, an optional<U> object whose contained value is direct-non-list-initialized with invoke(std​::​forward<F>(f), std​::​move(*val)); otherwise, optional<U>().
template<class F> constexpr optional or_else(F&& f) const &;
Constraints: F models invocable<> and T models copy_constructible.
Mandates: is_same_v<remove_cvref_t<invoke_result_t<F>>, optional> is true.
Effects: Equivalent to: if (*this) { return *this; } else { return std::forward<F>(f)(); }
template<class F> constexpr optional or_else(F&& f) &&;
Constraints: F models invocable<> and T models move_constructible.
Mandates: is_same_v<remove_cvref_t<invoke_result_t<F>>, optional> is true.
Effects: Equivalent to: if (*this) { return std::move(*this); } else { return std::forward<F>(f)(); }