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(value())>.

Mandates: remove_cvref_t is a specialization of optional.

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

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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(value()))>.

Mandates: remove_cvref_t is a specialization of optional.

Effects: Equivalent to: if (*this) { return invoke(std::forward<F>(f), std::move(value())); } else { return remove_cvref_t(); }

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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(value())>>.

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), value())); 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 object whose contained value is direct-non-list-initialized with invoke(std::forward<F>(f), value()); otherwise, optional().

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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(value()))>>.

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(value()))); 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 object whose contained value is direct-non-list-initialized with invoke(std::forward<F>(f), std::move(value())); otherwise, optional().

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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)(); }

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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)(); }