[optional.optional] # 22 General utilities library [[utilities]](./#utilities) ## 22.5 Optional objects [[optional]](optional#optional) ### 22.5.3 Class template optional [optional.optional] #### [22.5.3.1](#general) General [[optional.optional.general]](optional.optional.general) [🔗](#lib:optional) namespace std {templateclass optional {public:using value_type = T; using iterator = *implementation-defined*; // see [[optional.iterators]](#optional.iterators "22.5.3.6 Iterator support")using const_iterator = *implementation-defined*; // see [[optional.iterators]](#optional.iterators "22.5.3.6 Iterator support")// [[optional.ctor]](#optional.ctor "22.5.3.2 Constructors"), constructorsconstexpr optional() noexcept; constexpr optional(nullopt_t) noexcept; constexpr optional(const optional&); constexpr optional(optional&&) noexcept(*see below*); templateconstexpr explicit optional(in_place_t, Args&&...); templateconstexpr explicit optional(in_place_t, initializer_list, Args&&...); template>constexpr explicit(*see below*) optional(U&&); templateconstexpr explicit(*see below*) optional(const optional&); templateconstexpr explicit(*see below*) optional(optional&&); // [[optional.dtor]](#optional.dtor "22.5.3.3 Destructor"), destructorconstexpr ~optional(); // [[optional.assign]](#optional.assign "22.5.3.4 Assignment"), assignmentconstexpr optional& operator=(nullopt_t) noexcept; constexpr optional& operator=(const optional&); constexpr optional& operator=(optional&&) noexcept(*see below*); template> constexpr optional& operator=(U&&); template constexpr optional& operator=(const optional&); template constexpr optional& operator=(optional&&); template constexpr T& emplace(Args&&...); template constexpr T& emplace(initializer_list, Args&&...); // [[optional.swap]](#optional.swap "22.5.3.5 Swap"), swapconstexpr void swap(optional&) noexcept(*see below*); // [[optional.iterators]](#optional.iterators "22.5.3.6 Iterator support"), iterator supportconstexpr iterator begin() noexcept; constexpr const_iterator begin() const noexcept; constexpr iterator end() noexcept; constexpr const_iterator end() const noexcept; // [[optional.observe]](#optional.observe "22.5.3.7 Observers"), observersconstexpr const T* operator->() const noexcept; constexpr T* operator->() noexcept; constexpr const T& operator*() const & noexcept; constexpr T& operator*() & noexcept; constexpr T&& operator*() && noexcept; constexpr const T&& operator*() const && noexcept; constexpr explicit operator bool() const noexcept; constexpr bool has_value() const noexcept; constexpr const T& value() const &; // freestanding-deletedconstexpr T& value() &; // freestanding-deletedconstexpr T&& value() &&; // freestanding-deletedconstexpr const T&& value() const &&; // freestanding-deletedtemplate> constexpr T value_or(U&&) const &; template> constexpr T value_or(U&&) &&; // [[optional.monadic]](#optional.monadic "22.5.3.8 Monadic operations"), monadic operationstemplate constexpr auto and_then(F&& f) &; template constexpr auto and_then(F&& f) &&; template constexpr auto and_then(F&& f) const &; template constexpr auto and_then(F&& f) const &&; template constexpr auto transform(F&& f) &; template constexpr auto transform(F&& f) &&; template constexpr auto transform(F&& f) const &; template constexpr auto transform(F&& f) const &&; template constexpr optional or_else(F&& f) &&; template constexpr optional or_else(F&& f) const &; // [[optional.mod]](#optional.mod "22.5.3.9 Modifiers"), modifiersconstexpr void reset() noexcept; private: T* *val*; // *exposition only*}; template optional(T) -> optional;} [1](#general-1) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L3364) Any instance of optional at any given time either contains a value or does not contain a value[.](#general-1.sentence-1) When an instance of optional [*contains a value*](#def:contains_a_value,optional "22.5.3.1 General [optional.optional.general]"), it means that an object of type T, referred to as the optional object's [*contained value*](#def:contained_value,optional "22.5.3.1 General [optional.optional.general]"), is nested within ([[intro.object]](intro.object "6.8.2 Object model")) the optional object[.](#general-1.sentence-2) When an object of type optional is contextually converted to bool, the conversion returns true if the object contains a value; otherwise the conversion returns false[.](#general-1.sentence-3) [2](#general-2) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L3373) When an optional object contains a value, member val points to the contained value[.](#general-2.sentence-1) [3](#general-3) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L3377) A type X is a[*valid contained type*](#def:valid_contained_type,optional "22.5.3.1 General [optional.optional.general]") for optional if X is an lvalue reference type or a complete non-array object type, and remove_cvref_t is a type other than in_place_t or nullopt_t[.](#general-3.sentence-1) If a specialization of optional is instantiated with a type T that is not a valid contained type for optional, the program is ill-formed[.](#general-3.sentence-2) If T is an object type,T shall meet the *Cpp17Destructible* requirements (Table [35](utility.arg.requirements#tab:cpp17.destructible "Table 35: Cpp17Destructible requirements"))[.](#general-3.sentence-3) #### [22.5.3.2](#optional.ctor) Constructors [[optional.ctor]](optional.ctor) [1](#optional.ctor-1) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L3389) The exposition-only variable template *converts-from-any-cvref* is used by some constructors for optional[.](#optional.ctor-1.sentence-1) templateconstexpr bool *converts-from-any-cvref* = // *exposition only* disjunction_v, is_convertible, is_constructible, is_convertible, is_constructible, is_convertible, is_constructible, is_convertible>; [🔗](#lib:optional,constructor) `constexpr optional() noexcept; constexpr optional(nullopt_t) noexcept; ` [2](#optional.ctor-2) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L3408) *Postconditions*: *this does not contain a value[.](#optional.ctor-2.sentence-1) [3](#optional.ctor-3) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L3412) *Remarks*: No contained value is initialized[.](#optional.ctor-3.sentence-1) For every object type T these constructors are constexpr constructors ([[dcl.constexpr]](dcl.constexpr "9.2.6 The constexpr and consteval specifiers"))[.](#optional.ctor-3.sentence-2) [🔗](#lib:optional,constructor_) `constexpr optional(const optional& rhs); ` [4](#optional.ctor-4) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L3424) *Effects*: If rhs contains a value, direct-non-list-initializes the contained value with *rhs[.](#optional.ctor-4.sentence-1) [5](#optional.ctor-5) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L3429) *Postconditions*: rhs.has_value() == this->has_value()[.](#optional.ctor-5.sentence-1) [6](#optional.ctor-6) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L3433) *Throws*: Any exception thrown by the selected constructor of T[.](#optional.ctor-6.sentence-1) [7](#optional.ctor-7) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L3437) *Remarks*: This constructor is defined as deleted unlessis_copy_constructible_v is true[.](#optional.ctor-7.sentence-1) If is_trivially_copy_constructible_v is true, this constructor is trivial[.](#optional.ctor-7.sentence-2) [🔗](#lib:optional,constructor__) `constexpr optional(optional&& rhs) noexcept(see below); ` [8](#optional.ctor-8) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L3451) *Constraints*: is_move_constructible_v is true[.](#optional.ctor-8.sentence-1) [9](#optional.ctor-9) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L3455) *Effects*: If rhs contains a value, direct-non-list-initializes the contained value with *std​::​move(rhs)[.](#optional.ctor-9.sentence-1) rhs.has_value() is unchanged[.](#optional.ctor-9.sentence-2) [10](#optional.ctor-10) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L3461) *Postconditions*: rhs.has_value() == this->has_value()[.](#optional.ctor-10.sentence-1) [11](#optional.ctor-11) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L3465) *Throws*: Any exception thrown by the selected constructor of T[.](#optional.ctor-11.sentence-1) [12](#optional.ctor-12) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L3469) *Remarks*: The exception specification is equivalent tois_nothrow_move_constructible_v[.](#optional.ctor-12.sentence-1) If is_trivially_move_constructible_v is true, this constructor is trivial[.](#optional.ctor-12.sentence-2) [🔗](#lib:optional,constructor___) `template constexpr explicit optional(in_place_t, Args&&... args); ` [13](#optional.ctor-13) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L3483) *Constraints*: is_constructible_v is true[.](#optional.ctor-13.sentence-1) [14](#optional.ctor-14) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L3487) *Effects*: Direct-non-list-initializes the contained value with std​::​forward(args)...[.](#optional.ctor-14.sentence-1) [15](#optional.ctor-15) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L3491) *Postconditions*: *this contains a value[.](#optional.ctor-15.sentence-1) [16](#optional.ctor-16) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L3495) *Throws*: Any exception thrown by the selected constructor of T[.](#optional.ctor-16.sentence-1) [17](#optional.ctor-17) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L3499) *Remarks*: If T's constructor selected for the initialization is a constexpr constructor, this constructor is a constexpr constructor[.](#optional.ctor-17.sentence-1) [🔗](#lib:optional,constructor____) `template constexpr explicit optional(in_place_t, initializer_list il, Args&&... args); ` [18](#optional.ctor-18) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L3511) *Constraints*: is_constructible_v&, Args...> is true[.](#optional.ctor-18.sentence-1) [19](#optional.ctor-19) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L3515) *Effects*: Direct-non-list-initializes the contained value with il, std​::​forward(args)...[.](#optional.ctor-19.sentence-1) [20](#optional.ctor-20) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L3519) *Postconditions*: *this contains a value[.](#optional.ctor-20.sentence-1) [21](#optional.ctor-21) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L3523) *Throws*: Any exception thrown by the selected constructor of T[.](#optional.ctor-21.sentence-1) [22](#optional.ctor-22) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L3527) *Remarks*: If T's constructor selected for the initialization is a constexpr constructor, this constructor is a constexpr constructor[.](#optional.ctor-22.sentence-1) [🔗](#lib:optional,constructor_____) `template> constexpr explicit(see below) optional(U&& v); ` [23](#optional.ctor-23) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L3538) *Constraints*: - [(23.1)](#optional.ctor-23.1) is_constructible_v is true, - [(23.2)](#optional.ctor-23.2) is_same_v, in_place_t> is false, - [(23.3)](#optional.ctor-23.3) is_same_v, optional> is false, and - [(23.4)](#optional.ctor-23.4) if T is cv bool,remove_cvref_t is not a specialization of optional[.](#optional.ctor-23.sentence-1) [24](#optional.ctor-24) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L3548) *Effects*: Direct-non-list-initializes the contained value with std​::​forward(v)[.](#optional.ctor-24.sentence-1) [25](#optional.ctor-25) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L3552) *Postconditions*: *this contains a value[.](#optional.ctor-25.sentence-1) [26](#optional.ctor-26) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L3556) *Throws*: Any exception thrown by the selected constructor of T[.](#optional.ctor-26.sentence-1) [27](#optional.ctor-27) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L3560) *Remarks*: If T's selected constructor is a constexpr constructor, this constructor is a constexpr constructor[.](#optional.ctor-27.sentence-1) The expression inside explicit is equivalent to:!is_convertible_v [🔗](#lib:optional,constructor______) `template constexpr explicit(see below) optional(const optional& rhs); ` [28](#optional.ctor-28) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L3576) *Constraints*: - [(28.1)](#optional.ctor-28.1) is_constructible_v is true, and - [(28.2)](#optional.ctor-28.2) if T is not cv bool,*converts-from-any-cvref*> is false[.](#optional.ctor-28.sentence-1) [29](#optional.ctor-29) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L3584) *Effects*: If rhs contains a value, direct-non-list-initializes the contained value with *rhs[.](#optional.ctor-29.sentence-1) [30](#optional.ctor-30) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L3589) *Postconditions*: rhs.has_value() == this->has_value()[.](#optional.ctor-30.sentence-1) [31](#optional.ctor-31) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L3593) *Throws*: Any exception thrown by the selected constructor of T[.](#optional.ctor-31.sentence-1) [32](#optional.ctor-32) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L3597) *Remarks*: The expression inside explicit is equivalent to:!is_convertible_v [🔗](#lib:optional,constructor_______) `template constexpr explicit(see below) optional(optional&& rhs); ` [33](#optional.ctor-33) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L3611) *Constraints*: - [(33.1)](#optional.ctor-33.1) is_constructible_v is true, and - [(33.2)](#optional.ctor-33.2) if T is not cv bool,*converts-from-any-cvref*> is false[.](#optional.ctor-33.sentence-1) [34](#optional.ctor-34) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L3619) *Effects*: If rhs contains a value, direct-non-list-initializes the contained value with *std​::​move(rhs)[.](#optional.ctor-34.sentence-1) rhs.has_value() is unchanged[.](#optional.ctor-34.sentence-2) [35](#optional.ctor-35) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L3625) *Postconditions*: rhs.has_value() == this->has_value()[.](#optional.ctor-35.sentence-1) [36](#optional.ctor-36) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L3629) *Throws*: Any exception thrown by the selected constructor of T[.](#optional.ctor-36.sentence-1) [37](#optional.ctor-37) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L3633) *Remarks*: The expression inside explicit is equivalent to:!is_convertible_v #### [22.5.3.3](#optional.dtor) Destructor [[optional.dtor]](optional.dtor) [🔗](#lib:optional,destructor) `constexpr ~optional(); ` [1](#optional.dtor-1) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L3649) *Effects*: If is_trivially_destructible_v != true and *this contains a value, callsval->T::~T() [2](#optional.dtor-2) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L3656) *Remarks*: If is_trivially_destructible_v is true, then this destructor is trivial[.](#optional.dtor-2.sentence-1) #### [22.5.3.4](#optional.assign) Assignment [[optional.assign]](optional.assign) [🔗](#lib:operator=,optional) `constexpr optional& operator=(nullopt_t) noexcept; ` [1](#optional.assign-1) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L3669) *Effects*: If *this contains a value, calls val->T​::​~T() to destroy the contained value; otherwise no effect[.](#optional.assign-1.sentence-1) [2](#optional.assign-2) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L3673) *Postconditions*: *this does not contain a value[.](#optional.assign-2.sentence-1) [3](#optional.assign-3) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L3677) *Returns*: *this[.](#optional.assign-3.sentence-1) [🔗](#lib:operator=,optional_) `constexpr optional& operator=(const optional& rhs); ` [4](#optional.assign-4) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L3688) *Effects*: See Table [67](#tab:optional.assign.copy "Table 67: optional​::​operator=(const optional&) effects")[.](#optional.assign-4.sentence-1) Table [67](#tab:optional.assign.copy) — optional​::​operator=(const optional&) effects [[tab:optional.assign.copy]](./tab:optional.assign.copy) | [🔗](#tab:optional.assign.copy-row-1) | | ***this contains a value** | ***this does not contain a value** | | --- | --- | --- | --- | | [🔗](#tab:optional.assign.copy-row-2)
**rhs contains a value** | | assigns *rhs to the contained value | direct-non-list-initializes the contained value with *rhs | | [🔗](#tab:optional.assign.copy-row-3)
**rhs does not contain a value** | | destroys the contained value by calling val->T​::​~T() | no effect | [5](#optional.assign-5) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L3705) *Postconditions*: rhs.has_value() == this->has_value()[.](#optional.assign-5.sentence-1) [6](#optional.assign-6) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L3709) *Returns*: *this[.](#optional.assign-6.sentence-1) [7](#optional.assign-7) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L3713) *Remarks*: If any exception is thrown, the result of the expression this->has_value() remains unchanged[.](#optional.assign-7.sentence-1) If an exception is thrown during the call to T's copy constructor, no effect[.](#optional.assign-7.sentence-2) If an exception is thrown during the call to T's copy assignment, the state of its contained value is as defined by the exception safety guarantee of T's copy assignment[.](#optional.assign-7.sentence-3) This operator is defined as deleted unlessis_copy_constructible_v is true andis_copy_assignable_v is true[.](#optional.assign-7.sentence-4) If is_trivially_copy_constructible_v &&is_trivially_copy_assignable_v &&is_trivially_destructible_v is true, this assignment operator is trivial[.](#optional.assign-7.sentence-5) [🔗](#lib:operator=,optional__) `constexpr optional& operator=(optional&& rhs) noexcept(see below); ` [8](#optional.assign-8) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L3734) *Constraints*: is_move_constructible_v is true andis_move_assignable_v is true[.](#optional.assign-8.sentence-1) [9](#optional.assign-9) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L3739) *Effects*: See Table [68](#tab:optional.assign.move "Table 68: optional​::​operator=(optional&&) effects")[.](#optional.assign-9.sentence-1) The result of the expression rhs.has_value() remains unchanged[.](#optional.assign-9.sentence-2) Table [68](#tab:optional.assign.move) — optional​::​operator=(optional&&) effects [[tab:optional.assign.move]](./tab:optional.assign.move) | [🔗](#tab:optional.assign.move-row-1) | | ***this contains a value** | ***this does not contain a value** | | --- | --- | --- | --- | | [🔗](#tab:optional.assign.move-row-2)
**rhs contains a value** | | assigns *std​::​move(rhs) to the contained value | direct-non-list-initializes the contained value with *std​::​move(rhs) | | [🔗](#tab:optional.assign.move-row-3)
**rhs does not contain a value** | | destroys the contained value by calling val->T​::​~T() | no effect | [10](#optional.assign-10) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L3757) *Postconditions*: rhs.has_value() == this->has_value()[.](#optional.assign-10.sentence-1) [11](#optional.assign-11) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L3761) *Returns*: *this[.](#optional.assign-11.sentence-1) [12](#optional.assign-12) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L3765) *Remarks*: The exception specification is equivalent to:is_nothrow_move_assignable_v && is_nothrow_move_constructible_v [13](#optional.assign-13) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L3772) If any exception is thrown, the result of the expression this->has_value() remains unchanged[.](#optional.assign-13.sentence-1) If an exception is thrown during the call to T's move constructor, the state of *rhs.val is determined by the exception safety guarantee of T's move constructor[.](#optional.assign-13.sentence-2) If an exception is thrown during the call to T's move assignment, the state of *val and *rhs.val is determined by the exception safety guarantee of T's move assignment[.](#optional.assign-13.sentence-3) If is_trivially_move_constructible_v &&is_trivially_move_assignable_v &&is_trivially_destructible_v is true, this assignment operator is trivial[.](#optional.assign-13.sentence-4) [🔗](#lib:operator=,optional___) `template> constexpr optional& operator=(U&& v); ` [14](#optional.assign-14) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L3790) *Constraints*: - [(14.1)](#optional.assign-14.1) is_same_v, optional> is false, - [(14.2)](#optional.assign-14.2) conjunction_v, is_same>> is false, - [(14.3)](#optional.assign-14.3) is_constructible_v is true, and - [(14.4)](#optional.assign-14.4) is_assignable_v is true[.](#optional.assign-14.sentence-1) [15](#optional.assign-15) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L3799) *Effects*: If *this contains a value, assigns std​::​forward(v) to the contained value; otherwise direct-non-list-initializes the contained value with std​::​forward(v)[.](#optional.assign-15.sentence-1) [16](#optional.assign-16) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L3803) *Postconditions*: *this contains a value[.](#optional.assign-16.sentence-1) [17](#optional.assign-17) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L3807) *Returns*: *this[.](#optional.assign-17.sentence-1) [18](#optional.assign-18) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L3811) *Remarks*: If any exception is thrown, the result of the expression this->has_value() remains unchanged[.](#optional.assign-18.sentence-1) If an exception is thrown during the call to T's constructor, the state of v is determined by the exception safety guarantee of T's constructor[.](#optional.assign-18.sentence-2) If an exception is thrown during the call to T's assignment, the state of *val and v is determined by the exception safety guarantee of T's assignment[.](#optional.assign-18.sentence-3) [🔗](#lib:operator=,optional____) `template constexpr optional& operator=(const optional& rhs); ` [19](#optional.assign-19) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L3822) *Constraints*: - [(19.1)](#optional.assign-19.1) is_constructible_v is true, - [(19.2)](#optional.assign-19.2) is_assignable_v is true, - [(19.3)](#optional.assign-19.3) *converts-from-any-cvref*> is false, - [(19.4)](#optional.assign-19.4) is_assignable_v&> is false, - [(19.5)](#optional.assign-19.5) is_assignable_v&&> is false, - [(19.6)](#optional.assign-19.6) is_assignable_v&> is false, and - [(19.7)](#optional.assign-19.7) is_assignable_v&&> is false[.](#optional.assign-19.sentence-1) [20](#optional.assign-20) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L3834) *Effects*: See Table [69](#tab:optional.assign.copy.templ "Table 69: optional​::​operator=(const optional&) effects")[.](#optional.assign-20.sentence-1) Table [69](#tab:optional.assign.copy.templ) — optional​::​operator=(const optional&) effects [[tab:optional.assign.copy.templ]](./tab:optional.assign.copy.templ) | [🔗](#tab:optional.assign.copy.templ-row-1) | | ***this contains a value** | ***this does not contain a value** | | --- | --- | --- | --- | | [🔗](#tab:optional.assign.copy.templ-row-2)
**rhs contains a value** | | assigns *rhs to the contained value | direct-non-list-initializes the contained value with *rhs | | [🔗](#tab:optional.assign.copy.templ-row-3)
**rhs does not contain a value** | | destroys the contained value by calling val->T​::​~T() | no effect | [21](#optional.assign-21) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L3851) *Postconditions*: rhs.has_value() == this->has_value()[.](#optional.assign-21.sentence-1) [22](#optional.assign-22) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L3855) *Returns*: *this[.](#optional.assign-22.sentence-1) [23](#optional.assign-23) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L3859) *Remarks*: If any exception is thrown, the result of the expression this->has_value() remains unchanged[.](#optional.assign-23.sentence-1) If an exception is thrown during the call to T's constructor, the state of *rhs.val is determined by the exception safety guarantee of T's constructor[.](#optional.assign-23.sentence-2) If an exception is thrown during the call to T's assignment, the state of *val and *rhs.val is determined by the exception safety guarantee of T's assignment[.](#optional.assign-23.sentence-3) [🔗](#lib:operator=,optional_____) `template constexpr optional& operator=(optional&& rhs); ` [24](#optional.assign-24) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L3877) *Constraints*: - [(24.1)](#optional.assign-24.1) is_constructible_v is true, - [(24.2)](#optional.assign-24.2) is_assignable_v is true, - [(24.3)](#optional.assign-24.3) *converts-from-any-cvref*> is false, - [(24.4)](#optional.assign-24.4) is_assignable_v&> is false, - [(24.5)](#optional.assign-24.5) is_assignable_v&&> is false, - [(24.6)](#optional.assign-24.6) is_assignable_v&> is false, and - [(24.7)](#optional.assign-24.7) is_assignable_v&&> is false[.](#optional.assign-24.sentence-1) [25](#optional.assign-25) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L3889) *Effects*: See Table [70](#tab:optional.assign.move.templ "Table 70: optional​::​operator=(optional&&) effects")[.](#optional.assign-25.sentence-1) The result of the expression rhs.has_value() remains unchanged[.](#optional.assign-25.sentence-2) Table [70](#tab:optional.assign.move.templ) — optional​::​operator=(optional&&) effects [[tab:optional.assign.move.templ]](./tab:optional.assign.move.templ) | [🔗](#tab:optional.assign.move.templ-row-1) | | ***this contains a value** | ***this does not contain a value** | | --- | --- | --- | --- | | [🔗](#tab:optional.assign.move.templ-row-2)
**rhs contains a value** | | assigns *std​::​move(rhs) to the contained value | direct-non-list-initializes the contained value with *std​::​move(rhs) | | [🔗](#tab:optional.assign.move.templ-row-3)
**rhs does not contain a value** | | destroys the contained value by calling val->T​::​~T() | no effect | [26](#optional.assign-26) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L3907) *Postconditions*: rhs.has_value() == this->has_value()[.](#optional.assign-26.sentence-1) [27](#optional.assign-27) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L3911) *Returns*: *this[.](#optional.assign-27.sentence-1) [28](#optional.assign-28) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L3915) *Remarks*: If any exception is thrown, the result of the expression this->has_value() remains unchanged[.](#optional.assign-28.sentence-1) If an exception is thrown during the call to T's constructor, the state of *rhs.val is determined by the exception safety guarantee of T's constructor[.](#optional.assign-28.sentence-2) If an exception is thrown during the call to T's assignment, the state of *val and *rhs.val is determined by the exception safety guarantee of T's assignment[.](#optional.assign-28.sentence-3) [🔗](#lib:emplace,optional) `template constexpr T& emplace(Args&&... args); ` [29](#optional.assign-29) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L3933) *Mandates*: is_constructible_v is true[.](#optional.assign-29.sentence-1) [30](#optional.assign-30) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L3937) *Effects*: Calls *this = nullopt[.](#optional.assign-30.sentence-1) Then direct-non-list-initializes the contained value with std​::​forward​(args)...[.](#optional.assign-30.sentence-2) [31](#optional.assign-31) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L3942) *Postconditions*: *this contains a value[.](#optional.assign-31.sentence-1) [32](#optional.assign-32) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L3946) *Returns*: A reference to the new contained value[.](#optional.assign-32.sentence-1) [33](#optional.assign-33) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L3950) *Throws*: Any exception thrown by the selected constructor of T[.](#optional.assign-33.sentence-1) [34](#optional.assign-34) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L3954) *Remarks*: If an exception is thrown during the call to T's constructor, *this does not contain a value, and the previous *val (if any) has been destroyed[.](#optional.assign-34.sentence-1) [🔗](#lib:emplace,optional_) `template constexpr T& emplace(initializer_list il, Args&&... args); ` [35](#optional.assign-35) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L3965) *Constraints*: is_constructible_v&, Args...> is true[.](#optional.assign-35.sentence-1) [36](#optional.assign-36) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L3969) *Effects*: Calls *this = nullopt[.](#optional.assign-36.sentence-1) Then direct-non-list-initializes the contained value withil, std​::​​forward(args)...[.](#optional.assign-36.sentence-2) [37](#optional.assign-37) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L3974) *Postconditions*: *this contains a value[.](#optional.assign-37.sentence-1) [38](#optional.assign-38) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L3978) *Returns*: A reference to the new contained value[.](#optional.assign-38.sentence-1) [39](#optional.assign-39) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L3982) *Throws*: Any exception thrown by the selected constructor of T[.](#optional.assign-39.sentence-1) [40](#optional.assign-40) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L3986) *Remarks*: If an exception is thrown during the call to T's constructor, *this does not contain a value, and the previous *val (if any) has been destroyed[.](#optional.assign-40.sentence-1) #### [22.5.3.5](#optional.swap) Swap [[optional.swap]](optional.swap) [🔗](#lib:swap,optional) `constexpr void swap(optional& rhs) noexcept(see below); ` [1](#optional.swap-1) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L3999) *Mandates*: is_move_constructible_v is true[.](#optional.swap-1.sentence-1) [2](#optional.swap-2) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L4003) *Preconditions*: T meets the [*Cpp17Swappable*](swappable.requirements#:Cpp17Swappable "16.4.4.3 Swappable requirements [swappable.requirements]") requirements ([[swappable.requirements]](swappable.requirements "16.4.4.3 Swappable requirements"))[.](#optional.swap-2.sentence-1) [3](#optional.swap-3) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L4007) *Effects*: See Table [71](#tab:optional.swap "Table 71: optional​::​swap(optional&) effects")[.](#optional.swap-3.sentence-1) Table [71](#tab:optional.swap) — optional​::​swap(optional&) effects [[tab:optional.swap]](./tab:optional.swap) | [🔗](#tab:optional.swap-row-1) | | ***this contains a value** | ***this does not contain a value** | | --- | --- | --- | --- | | [🔗](#tab:optional.swap-row-2)
**rhs contains a value** | | calls swap(*(*this), *rhs) | direct-non-list-initializes the contained value of *this with std​::​move(*rhs), followed by rhs.val->T​::​~T(); postcondition is that *this contains a value and rhs does not contain a value | | [🔗](#tab:optional.swap-row-3)
**rhs does not contain a value** | | direct-non-list-initializes the contained value of rhs with std​::​move(*(*this)), followed by val->T​::​~T(); postcondition is that *this does not contain a value and rhs contains a value | no effect | [4](#optional.swap-4) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L4030) *Throws*: Any exceptions thrown by the operations in the relevant part of Table [71](#tab:optional.swap "Table 71: optional​::​swap(optional&) effects")[.](#optional.swap-4.sentence-1) [5](#optional.swap-5) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L4034) *Remarks*: The exception specification is equivalent to:is_nothrow_move_constructible_v && is_nothrow_swappable_v [6](#optional.swap-6) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L4041) If any exception is thrown, the results of the expressions this->has_value() and rhs.has_value() remain unchanged[.](#optional.swap-6.sentence-1) If an exception is thrown during the call to function swap, the state of *val and *rhs.val is determined by the exception safety guarantee of swap for lvalues of T[.](#optional.swap-6.sentence-2) If an exception is thrown during the call to T's move constructor, the state of *val and *rhs.val is determined by the exception safety guarantee of T's move constructor[.](#optional.swap-6.sentence-3) #### [22.5.3.6](#optional.iterators) Iterator support [[optional.iterators]](optional.iterators) [🔗](#lib:iterator,optional) `using iterator = implementation-defined; using const_iterator = implementation-defined; ` [1](#optional.iterators-1) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L4059) These types model [contiguous_iterator](iterator.concept.contiguous#concept:contiguous_iterator "24.3.4.14 Concept contiguous_­iterator [iterator.concept.contiguous]") ([[iterator.concept.contiguous]](iterator.concept.contiguous "24.3.4.14 Concept contiguous_­iterator")), meet the [*Cpp17RandomAccessIterator*](random.access.iterators#:Cpp17RandomAccessIterator "24.3.5.7 Random access iterators [random.access.iterators]") requirements ([[random.access.iterators]](random.access.iterators "24.3.5.7 Random access iterators")), and meet the requirements for constexpr iterators ([[iterator.requirements.general]](iterator.requirements.general "24.3.1 General")), with value type remove_cv_t[.](#optional.iterators-1.sentence-1) The reference type is T& for iterator andconst T& for const_iterator[.](#optional.iterators-1.sentence-2) [2](#optional.iterators-2) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L4068) All requirements on container iterators ([[container.reqmts]](container.reqmts "23.2.2.2 Container requirements")) apply tooptional​::​iterator and optional​::​const_iterator as well[.](#optional.iterators-2.sentence-1) [3](#optional.iterators-3) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L4072) Any operation that initializes or destroys the contained value of an optional object invalidates all iterators into that object[.](#optional.iterators-3.sentence-1) [🔗](#lib:begin,optional) `constexpr iterator begin() noexcept; constexpr const_iterator begin() const noexcept; ` [4](#optional.iterators-4) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L4083) *Returns*: If has_value() is true, an iterator referring to the contained value[.](#optional.iterators-4.sentence-1) Otherwise, a past-the-end iterator value[.](#optional.iterators-4.sentence-2) [🔗](#lib:end,optional) `constexpr iterator end() noexcept; constexpr const_iterator end() const noexcept; ` [5](#optional.iterators-5) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L4097) *Returns*: begin() + has_value()[.](#optional.iterators-5.sentence-1) #### [22.5.3.7](#optional.observe) Observers [[optional.observe]](optional.observe) [🔗](#lib:operator-%3e,optional) `constexpr const T* operator->() const noexcept; constexpr T* operator->() noexcept; ` [1](#optional.observe-1) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L4111) *Hardened preconditions*: has_value() is true[.](#optional.observe-1.sentence-1) [2](#optional.observe-2) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L4115) *Returns*: val[.](#optional.observe-2.sentence-1) [3](#optional.observe-3) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L4119) *Remarks*: These functions are constexpr functions[.](#optional.observe-3.sentence-1) [🔗](#lib:operator*,optional) `constexpr const T& operator*() const & noexcept; constexpr T& operator*() & noexcept; ` [4](#optional.observe-4) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L4131) *Hardened preconditions*: has_value() is true[.](#optional.observe-4.sentence-1) [5](#optional.observe-5) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L4135) *Returns*: *val[.](#optional.observe-5.sentence-1) [6](#optional.observe-6) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L4139) *Remarks*: These functions are constexpr functions[.](#optional.observe-6.sentence-1) [🔗](#lib:operator*,optional_) `constexpr T&& operator*() && noexcept; constexpr const T&& operator*() const && noexcept; ` [7](#optional.observe-7) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L4151) *Hardened preconditions*: has_value() is true[.](#optional.observe-7.sentence-1) [8](#optional.observe-8) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L4155) *Effects*: Equivalent to: return std​::​move(*val); [🔗](#lib:operator_bool,optional) `constexpr explicit operator bool() const noexcept; ` [9](#optional.observe-9) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L4166) *Returns*: true if and only if *this contains a value[.](#optional.observe-9.sentence-1) [10](#optional.observe-10) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L4170) *Remarks*: This function is a constexpr function[.](#optional.observe-10.sentence-1) [🔗](#lib:has_value,optional) `constexpr bool has_value() const noexcept; ` [11](#optional.observe-11) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L4181) *Returns*: true if and only if *this contains a value[.](#optional.observe-11.sentence-1) [12](#optional.observe-12) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L4185) *Remarks*: This function is a constexpr function[.](#optional.observe-12.sentence-1) [🔗](#lib:value,optional) `constexpr const T& value() const &; constexpr T& value() &; ` [13](#optional.observe-13) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L4197) *Effects*: Equivalent to:return has_value() ? *val : throw bad_optional_access(); [🔗](#lib:value,optional_) `constexpr T&& value() &&; constexpr const T&& value() const &&; ` [14](#optional.observe-14) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L4213) *Effects*: Equivalent to:return has_value() ? std::move(*val) : throw bad_optional_access(); [🔗](#lib:value_or,optional) `template> constexpr T value_or(U&& v) const &; ` [15](#optional.observe-15) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L4227) *Mandates*: is_copy_constructible_v && is_convertible_v is true[.](#optional.observe-15.sentence-1) [16](#optional.observe-16) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L4231) *Effects*: Equivalent to:return has_value() ? **this : static_cast(std::forward(v)); [🔗](#lib:value_or,optional_) `template> constexpr T value_or(U&& v) &&; ` [17](#optional.observe-17) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L4245) *Mandates*: is_move_constructible_v && is_convertible_v is true[.](#optional.observe-17.sentence-1) [18](#optional.observe-18) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L4249) *Effects*: Equivalent to:return has_value() ? std::move(**this) : static_cast(std::forward(v)); #### [22.5.3.8](#optional.monadic) Monadic operations [[optional.monadic]](optional.monadic) [🔗](#lib:and_then,optional) `template constexpr auto and_then(F&& f) &; template constexpr auto and_then(F&& f) const &; ` [1](#optional.monadic-1) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L4266) Let U be invoke_result_t[.](#optional.monadic-1.sentence-1) [2](#optional.monadic-2) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L4269) *Mandates*: remove_cvref_t is a specialization of optional[.](#optional.monadic-2.sentence-1) [3](#optional.monadic-3) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L4273) *Effects*: Equivalent to:if (*this) {return invoke(std::forward(f), **val*);} else {return remove_cvref_t();} [🔗](#lib:and_then,optional_) `template constexpr auto and_then(F&& f) &&; template constexpr auto and_then(F&& f) const &&; ` [4](#optional.monadic-4) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L4292) Let U be invoke_result_t[.](#optional.monadic-4.sentence-1) [5](#optional.monadic-5) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L4295) *Mandates*: remove_cvref_t is a specialization of optional[.](#optional.monadic-5.sentence-1) [6](#optional.monadic-6) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L4299) *Effects*: Equivalent to:if (*this) {return invoke(std::forward(f), std::move(**val*));} else {return remove_cvref_t();} [🔗](#lib:transform,optional) `template constexpr auto transform(F&& f) &; template constexpr auto transform(F&& f) const &; ` [7](#optional.monadic-7) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L4318) Let U be remove_cv_t>[.](#optional.monadic-7.sentence-1) [8](#optional.monadic-8) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L4321) *Mandates*: U is a valid contained type for optional[.](#optional.monadic-8.sentence-1) The declarationU u(invoke(std::forward(f), **val*)); is well-formed for some invented variable u[.](#optional.monadic-8.sentence-2) [*Note [1](#optional.monadic-note-1)*: There is no requirement that U is movable ([[dcl.init.general]](dcl.init.general "9.5.1 General"))[.](#optional.monadic-8.sentence-3) — *end note*] [9](#optional.monadic-9) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L4333) *Returns*: If *this contains a value, an optional object whose contained value is direct-non-list-initialized withinvoke(std​::​forward(f), **val*); otherwise, optional()[.](#optional.monadic-9.sentence-1) [🔗](#lib:transform,optional_) `template constexpr auto transform(F&& f) &&; template constexpr auto transform(F&& f) const &&; ` [10](#optional.monadic-10) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L4348) Let U beremove_cv_t>[.](#optional.monadic-10.sentence-1) [11](#optional.monadic-11) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L4352) *Mandates*: U is a valid contained type for optional[.](#optional.monadic-11.sentence-1) The declarationU u(invoke(std::forward(f), std::move(**val*))); is well-formed for some invented variable u[.](#optional.monadic-11.sentence-2) [*Note [2](#optional.monadic-note-2)*: There is no requirement that U is movable ([[dcl.init.general]](dcl.init.general "9.5.1 General"))[.](#optional.monadic-11.sentence-3) — *end note*] [12](#optional.monadic-12) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L4364) *Returns*: If *this contains a value, an optional object whose contained value is direct-non-list-initialized withinvoke(std​::​forward(f), std​::​move(**val*)); otherwise, optional()[.](#optional.monadic-12.sentence-1) [🔗](#lib:or_else,optional) `template constexpr optional or_else(F&& f) const &; ` [13](#optional.monadic-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]")[.](#optional.monadic-13.sentence-1) [14](#optional.monadic-14) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L4383) *Mandates*: is_same_v>, optional> is true[.](#optional.monadic-14.sentence-1) [15](#optional.monadic-15) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L4387) *Effects*: Equivalent to:if (*this) {return *this;} else {return std::forward(f)();} [🔗](#lib:or_else,optional_) `template constexpr optional or_else(F&& f) &&; ` [16](#optional.monadic-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]")[.](#optional.monadic-16.sentence-1) [17](#optional.monadic-17) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L4410) *Mandates*: is_same_v>, optional> is true[.](#optional.monadic-17.sentence-1) [18](#optional.monadic-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)();} #### [22.5.3.9](#optional.mod) Modifiers [[optional.mod]](optional.mod) [🔗](#lib:reset,optional) `constexpr void reset() noexcept; ` [1](#optional.mod-1) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L4434) *Effects*: If *this contains a value, calls val->T​::​~T() to destroy the contained value; otherwise no effect[.](#optional.mod-1.sentence-1) [2](#optional.mod-2) [#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L4439) *Postconditions*: *this does not contain a value[.](#optional.mod-2.sentence-1)