440 lines
16 KiB
Markdown
440 lines
16 KiB
Markdown
[optional.assign]
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# 22 General utilities library [[utilities]](./#utilities)
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## 22.5 Optional objects [[optional]](optional#assign)
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### 22.5.3 Class template optional [[optional.optional]](optional.optional#optional.assign)
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#### 22.5.3.4 Assignment [optional.assign]
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[ð](#lib:operator=,optional)
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`constexpr optional<T>& operator=(nullopt_t) noexcept;
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`
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[1](#1)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L3669)
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*Effects*: If *this contains a value, calls val->T::~T() to destroy the contained value; otherwise no effect[.](#1.sentence-1)
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[2](#2)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L3673)
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*Postconditions*: *this does not contain a value[.](#2.sentence-1)
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[3](#3)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L3677)
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*Returns*: *this[.](#3.sentence-1)
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[ð](#lib:operator=,optional_)
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`constexpr optional<T>& operator=(const optional& rhs);
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`
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[4](#4)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L3688)
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*Effects*: See Table [67](#tab:optional.assign.copy "Table 67: optional::operator=(const optional&) effects")[.](#4.sentence-1)
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Table [67](#tab:optional.assign.copy) — optional::operator=(const optional&) effects [[tab:optional.assign.copy]](./tab:optional.assign.copy)
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| [ð](#tab:optional.assign.copy-row-1) | | ***this contains a value** | ***this does not contain a value** |
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| --- | --- | --- | --- |
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| [ð](#tab:optional.assign.copy-row-2)<br>**rhs contains a value** | | assigns *rhs to the contained value | direct-non-list-initializes the contained value with *rhs |
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| [ð](#tab:optional.assign.copy-row-3)<br>**rhs does not contain a value** | | destroys the contained value by calling val->T::~T() | no effect |
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[5](#5)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L3705)
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*Postconditions*: rhs.has_value() == this->has_value()[.](#5.sentence-1)
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[6](#6)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L3709)
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*Returns*: *this[.](#6.sentence-1)
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[7](#7)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L3713)
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*Remarks*: If any exception is thrown, the result of the expression this->has_value() remains unchanged[.](#7.sentence-1)
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If an exception is thrown during the call to T's copy constructor, no effect[.](#7.sentence-2)
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If an exception is thrown during the call to T's copy assignment,
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the state of its contained value is as defined by the exception safety guarantee of T's copy assignment[.](#7.sentence-3)
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This operator is defined as deleted unlessis_copy_constructible_v<T> is true andis_copy_assignable_v<T> is true[.](#7.sentence-4)
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If is_trivially_copy_constructible_v<T> &&is_trivially_copy_assignable_v<T> &&is_trivially_destructible_v<T> is true,
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this assignment operator is trivial[.](#7.sentence-5)
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[ð](#lib:operator=,optional__)
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`constexpr optional& operator=(optional&& rhs) noexcept(see below);
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`
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[8](#8)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L3734)
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*Constraints*: is_move_constructible_v<T> is true andis_move_assignable_v<T> is true[.](#8.sentence-1)
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[9](#9)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L3739)
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*Effects*: See Table [68](#tab:optional.assign.move "Table 68: optional::operator=(optional&&) effects")[.](#9.sentence-1)
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The result of the expression rhs.has_value() remains unchanged[.](#9.sentence-2)
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Table [68](#tab:optional.assign.move) — optional::operator=(optional&&) effects [[tab:optional.assign.move]](./tab:optional.assign.move)
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| [ð](#tab:optional.assign.move-row-1) | | ***this contains a value** | ***this does not contain a value** |
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| --- | --- | --- | --- |
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| [ð](#tab:optional.assign.move-row-2)<br>**rhs contains a value** | | assigns *std::move(rhs) to the contained value | direct-non-list-initializes the contained value with *std::move(rhs) |
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| [ð](#tab:optional.assign.move-row-3)<br>**rhs does not contain a value** | | destroys the contained value by calling val->T::~T() | no effect |
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[10](#10)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L3757)
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*Postconditions*: rhs.has_value() == this->has_value()[.](#10.sentence-1)
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[11](#11)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L3761)
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*Returns*: *this[.](#11.sentence-1)
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[12](#12)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L3765)
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*Remarks*: The exception specification is equivalent to:is_nothrow_move_assignable_v<T> && is_nothrow_move_constructible_v<T>
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[13](#13)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L3772)
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If any exception is thrown, the result of the expression this->has_value() remains unchanged[.](#13.sentence-1)
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If an exception is thrown during the call to T's move constructor,
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the state of *rhs.val is determined by the exception safety guarantee of T's move constructor[.](#13.sentence-2)
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If an exception is thrown during the call to T's move assignment,
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the state of *val and *rhs.val is determined by the exception safety guarantee of T's move assignment[.](#13.sentence-3)
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If is_trivially_move_constructible_v<T> &&is_trivially_move_assignable_v<T> &&is_trivially_destructible_v<T> is true,
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this assignment operator is trivial[.](#13.sentence-4)
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[ð](#lib:operator=,optional___)
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`template<class U = remove_cv_t<T>> constexpr optional& operator=(U&& v);
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`
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[14](#14)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L3790)
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*Constraints*:
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- [(14.1)](#14.1)
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is_same_v<remove_cvref_t<U>, optional> is false,
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- [(14.2)](#14.2)
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conjunction_v<is_scalar<T>, is_same<T, decay_t<U>>> is false,
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- [(14.3)](#14.3)
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is_constructible_v<T, U> is true, and
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- [(14.4)](#14.4)
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is_assignable_v<T&, U> is true[.](#14.sentence-1)
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[15](#15)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L3799)
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*Effects*: If *this contains a value, assigns std::forward<U>(v) to the contained value; otherwise direct-non-list-initializes the contained value with std::forward<U>(v)[.](#15.sentence-1)
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[16](#16)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L3803)
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*Postconditions*: *this contains a value[.](#16.sentence-1)
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[17](#17)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L3807)
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*Returns*: *this[.](#17.sentence-1)
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[18](#18)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L3811)
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*Remarks*: If any exception is thrown, the result of the expression this->has_value() remains unchanged[.](#18.sentence-1)
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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[.](#18.sentence-2)
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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[.](#18.sentence-3)
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[ð](#lib:operator=,optional____)
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`template<class U> constexpr optional<T>& operator=(const optional<U>& rhs);
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`
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[19](#19)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L3822)
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*Constraints*:
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- [(19.1)](#19.1)
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is_constructible_v<T, const U&> is true,
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- [(19.2)](#19.2)
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is_assignable_v<T&, const U&> is true,
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- [(19.3)](#19.3)
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*converts-from-any-cvref*<T, optional<U>> is false,
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- [(19.4)](#19.4)
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is_assignable_v<T&, optional<U>&> is false,
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- [(19.5)](#19.5)
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is_assignable_v<T&, optional<U>&&> is false,
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- [(19.6)](#19.6)
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is_assignable_v<T&, const optional<U>&> is false, and
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- [(19.7)](#19.7)
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is_assignable_v<T&, const optional<U>&&> is false[.](#19.sentence-1)
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[20](#20)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L3834)
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*Effects*: See Table [69](#tab:optional.assign.copy.templ "Table 69: optional::operator=(const optional<U>&) effects")[.](#20.sentence-1)
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Table [69](#tab:optional.assign.copy.templ) — optional::operator=(const optional<U>&) effects [[tab:optional.assign.copy.templ]](./tab:optional.assign.copy.templ)
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| [ð](#tab:optional.assign.copy.templ-row-1) | | ***this contains a value** | ***this does not contain a value** |
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| --- | --- | --- | --- |
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| [ð](#tab:optional.assign.copy.templ-row-2)<br>**rhs contains a value** | | assigns *rhs to the contained value | direct-non-list-initializes the contained value with *rhs |
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| [ð](#tab:optional.assign.copy.templ-row-3)<br>**rhs does not contain a value** | | destroys the contained value by calling val->T::~T() | no effect |
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[21](#21)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L3851)
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*Postconditions*: rhs.has_value() == this->has_value()[.](#21.sentence-1)
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[22](#22)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L3855)
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*Returns*: *this[.](#22.sentence-1)
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[23](#23)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L3859)
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*Remarks*: If any exception is thrown,
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the result of the expression this->has_value() remains unchanged[.](#23.sentence-1)
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If an exception is thrown during the call to T's constructor,
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the state of *rhs.val is determined by
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the exception safety guarantee of T's constructor[.](#23.sentence-2)
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If an exception is thrown during the call to T's assignment,
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the state of *val and *rhs.val is determined by
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the exception safety guarantee of T's assignment[.](#23.sentence-3)
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[ð](#lib:operator=,optional_____)
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`template<class U> constexpr optional<T>& operator=(optional<U>&& rhs);
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`
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[24](#24)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L3877)
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*Constraints*:
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- [(24.1)](#24.1)
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is_constructible_v<T, U> is true,
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- [(24.2)](#24.2)
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is_assignable_v<T&, U> is true,
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- [(24.3)](#24.3)
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*converts-from-any-cvref*<T, optional<U>> is false,
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- [(24.4)](#24.4)
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is_assignable_v<T&, optional<U>&> is false,
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- [(24.5)](#24.5)
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is_assignable_v<T&, optional<U>&&> is false,
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- [(24.6)](#24.6)
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is_assignable_v<T&, const optional<U>&> is false, and
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- [(24.7)](#24.7)
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is_assignable_v<T&, const optional<U>&&> is false[.](#24.sentence-1)
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[25](#25)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L3889)
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*Effects*: See Table [70](#tab:optional.assign.move.templ "Table 70: optional::operator=(optional<U>&&) effects")[.](#25.sentence-1)
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The result of the expression rhs.has_value() remains unchanged[.](#25.sentence-2)
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Table [70](#tab:optional.assign.move.templ) — optional::operator=(optional<U>&&) effects [[tab:optional.assign.move.templ]](./tab:optional.assign.move.templ)
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| [ð](#tab:optional.assign.move.templ-row-1) | | ***this contains a value** | ***this does not contain a value** |
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| --- | --- | --- | --- |
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| [ð](#tab:optional.assign.move.templ-row-2)<br>**rhs contains a value** | | assigns *std::move(rhs) to the contained value | direct-non-list-initializes the contained value with *std::move(rhs) |
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| [ð](#tab:optional.assign.move.templ-row-3)<br>**rhs does not contain a value** | | destroys the contained value by calling val->T::~T() | no effect |
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[26](#26)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L3907)
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*Postconditions*: rhs.has_value() == this->has_value()[.](#26.sentence-1)
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[27](#27)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L3911)
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*Returns*: *this[.](#27.sentence-1)
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[28](#28)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L3915)
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*Remarks*: If any exception is thrown,
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the result of the expression this->has_value() remains unchanged[.](#28.sentence-1)
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If an exception is thrown during the call to T's constructor,
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the state of *rhs.val is determined by
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the exception safety guarantee of T's constructor[.](#28.sentence-2)
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If an exception is thrown during the call to T's assignment,
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the state of *val and *rhs.val is determined by
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the exception safety guarantee of T's assignment[.](#28.sentence-3)
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[ð](#lib:emplace,optional)
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`template<class... Args> constexpr T& emplace(Args&&... args);
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`
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[29](#29)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L3933)
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*Mandates*: is_constructible_v<T, Args...> is true[.](#29.sentence-1)
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[30](#30)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L3937)
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*Effects*: Calls *this = nullopt[.](#30.sentence-1)
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Then direct-non-list-initializes the contained value
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with std::forward<Args>(args)...[.](#30.sentence-2)
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[31](#31)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L3942)
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*Postconditions*: *this contains a value[.](#31.sentence-1)
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[32](#32)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L3946)
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*Returns*: A reference to the new contained value[.](#32.sentence-1)
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[33](#33)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L3950)
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*Throws*: Any exception thrown by the selected constructor of T[.](#33.sentence-1)
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[34](#34)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L3954)
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*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[.](#34.sentence-1)
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[ð](#lib:emplace,optional_)
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`template<class U, class... Args> constexpr T& emplace(initializer_list<U> il, Args&&... args);
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`
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[35](#35)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L3965)
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*Constraints*: is_constructible_v<T, initializer_list<U>&, Args...> is true[.](#35.sentence-1)
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[36](#36)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L3969)
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*Effects*: Calls *this = nullopt[.](#36.sentence-1)
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Then direct-non-list-initializes the contained value withil, std::forward<Args>(args)...[.](#36.sentence-2)
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||
[37](#37)
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||
|
||
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L3974)
|
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|
||
*Postconditions*: *this contains a value[.](#37.sentence-1)
|
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|
||
[38](#38)
|
||
|
||
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L3978)
|
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|
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*Returns*: A reference to the new contained value[.](#38.sentence-1)
|
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|
||
[39](#39)
|
||
|
||
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L3982)
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|
||
*Throws*: Any exception thrown by the selected constructor of T[.](#39.sentence-1)
|
||
|
||
[40](#40)
|
||
|
||
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L3986)
|
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|
||
*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[.](#40.sentence-1)
|