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