[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)