cppdraft_translate/cppdraft/optional/monadic.md

[optional.monadic]

# 22 General utilities library [[utilities]](./#utilities)

## 22.5 Optional objects [[optional]](optional#monadic)

### 22.5.3 Class template optional [[optional.optional]](optional.optional#optional.monadic)

#### 22.5.3.8 Monadic operations [optional.monadic]

[🔗](#lib:and_then,optional)

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

[1](#1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L4266)

Let U be invoke_result_t<F, decltype(**val*)>[.](#1.sentence-1)

[2](#2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L4269)

*Mandates*: remove_cvref_t<U> is a specialization of optional[.](#2.sentence-1)

[3](#3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L4273)

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

[🔗](#lib:and_then,optional_)

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

[4](#4)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L4292)

Let U be invoke_result_t<F, decltype(std​::​move(**val*))>[.](#4.sentence-1)

[5](#5)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L4295)

*Mandates*: remove_cvref_t<U> is a specialization of optional[.](#5.sentence-1)

[6](#6)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L4299)

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

[🔗](#lib:transform,optional)

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

[7](#7)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L4318)

Let U be remove_cv_t<invoke_result_t<F, decltype(**val*)>>[.](#7.sentence-1)

[8](#8)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L4321)

*Mandates*: U is a valid contained type for optional[.](#8.sentence-1)

The declarationU u(invoke(std::forward<F>(f), **val*)); is well-formed for some invented variable u[.](#8.sentence-2)

[*Note [1](#note-1)*:

There is no requirement that U is movable ([[dcl.init.general]](dcl.init.general "9.5.1 General"))[.](#8.sentence-3)

— *end note*]

[9](#9)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L4333)

*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>()[.](#9.sentence-1)

[🔗](#lib:transform,optional_)

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

[10](#10)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L4348)

Let U beremove_cv_t<invoke_result_t<F, decltype(std​::​move(**val*))>>[.](#10.sentence-1)

[11](#11)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L4352)

*Mandates*: U is a valid contained type for optional[.](#11.sentence-1)

The declarationU u(invoke(std::forward<F>(f), std::move(**val*))); is well-formed for some invented variable u[.](#11.sentence-2)

[*Note [2](#note-2)*:

There is no requirement that U is movable ([[dcl.init.general]](dcl.init.general "9.5.1 General"))[.](#11.sentence-3)

— *end note*]

[12](#12)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L4364)

*Returns*: If *this contains a value, an optional<U> object
whose contained value is direct-non-list-initialized withinvoke(std​::​forward<F>(f), std​::​move(**val*));
otherwise, optional<U>()[.](#12.sentence-1)

[🔗](#lib:or_else,optional)

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

[13](#13)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L4378)

*Constraints*: F models [invocable](concept.invocable#concept:invocable "18.7.2 Concept invocable [concept.invocable]") andT models [copy_constructible](concept.copyconstructible#concept:copy_constructible "18.4.14 Concept copy_­constructible [concept.copyconstructible]")[.](#13.sentence-1)

[14](#14)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L4383)

*Mandates*: is_same_v<remove_cvref_t<invoke_result_t<F>>, optional> is true[.](#14.sentence-1)

[15](#15)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L4387)

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

[🔗](#lib:or_else,optional_)

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

[16](#16)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L4405)

*Constraints*: F models [invocable](concept.invocable#concept:invocable "18.7.2 Concept invocable [concept.invocable]") andT models [move_constructible](concept.moveconstructible#concept:move_constructible "18.4.13 Concept move_­constructible [concept.moveconstructible]")[.](#16.sentence-1)

[17](#17)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L4410)

*Mandates*: is_same_v<remove_cvref_t<invoke_result_t<F>>, optional> is true[.](#17.sentence-1)

[18](#18)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L4414)

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