std::optional

 

C++

Language

Standard library headers

Concepts

Utilities library

Strings library

Containers library

Algorithms library

Iterators library

Numerics library

Input/output library

Localizations library

Regular expressions library (C++11)

Atomic operations library (C++11)

Thread support library (C++11)

Filesystem library (C++17)

Technical Specifications

 

Utilities library

Type support (basic types, RTTI, type traits)

Dynamic memory management

Error handling

Program utilities

Variadic functions

Date and time

Function objects

initializer_list (C++11)

bitset

hash (C++11)

Relational operators

rel_ops::operator!=rel_ops::operator>rel_ops::operator<=rel_ops::operator>=

Optional and any

optional (C++17)

any (C++17)

Pairs and tuples

pair

tuple (C++11)

apply (C++17)

piecewise_construct_t (C++11)

piecewise_construct (C++11)

integer_sequence (C++14)

Swap, forward and move

swap

exchange (C++14)

forward (C++11)

move (C++11)

move_if_noexcept (C++11)

Type operations

declval (C++11)

as_const (C++17)

 

std::optional

Member functions

optional::optional

optional::~optional

optional::operator=

Observers

optional::operator->optional::operator*

optional::operator bool

optional::value

optional::value_or

Modifiers

optional::emplace

optional::swap

Non-member functions

operator==operator!=operator<operator<=operator>operator>=

make_optional

swap

Helper classes

hash

nullopt_t

in_place_t

bad_optional_access

Helper objects

nullopt

in_place

 

Defined in header <optional>
template< class T > class optional;		(since C++17)

The class template std::optional manages an optional contained value, i.e. a value that may or may not be present.

A common use case for optional is the return value of a function that may fail. As opposed to other approaches, such as std::pair<T,bool>, optional handles expensive to construct objects well and is more readable, as the intent is expressed explicitly.

Any instance of optional<T> at any given point in time either contains a value or does not contain a value.

If an optional<T> contains a value, the value is guaranteed to be allocated as part of the optional object footprint, i.e. no dynamic memory allocation ever takes place. Thus, an optional object models an object, not a pointer, even though the operator*() and operator->() are defined.

When an object of type optional<T> is contextually converted to bool, the conversion returns true if the object contains a value and false if it does not contain a value.

The optional object contains a value in the following conditions:

• The object is initialized with a value of type T
• The object is assigned from another optional that contains a value.

The object does not contain a value in the following conditions:

• The object is default-initialized.
• The object is initialized with a value of std::nullopt_t or an optional object that does not contain a value.
• The object is assigned from a value of std::nullopt_t or from an optional that does not contain a value

Template parameters

T

-

the type of the value to manage initialization state for. The type must meet the requirements of Destructible

Member types

Member type	Definition
value_type	T

Member functions

(constructor)	constructs the optional object (public member function)
(destructor)	destroys the contained value, if there is one (public member function)
operator=	assigns contents (public member function)
Observers
operator->operator*	accesses the contained value (public member function)
operator bool	checks whether the object contains a value (public member function)
value	returns the contained value (public member function)
value_or	returns the contained value if available, another value otherwise (public member function)
Modifiers
swap	exchanges the contents (public member function)
emplace	constructs the contained value in-place (public member function)

Member objects

Member name	Definition
val (private)	pointer to the contained value (which points at a data member of the same object), the name is for exposition only

Non-member functions

operator==operator!=operator<operator<=operator>operator>= (C++17)	compares optional objects (function template)
make_optional (C++17)	creates an optional object (function template)
std::swap(std::optional) (C++17)	specializes the std::swap algorithm (function)

Helper classes

std::hash<std::optional> (C++17)	specializes the std::hash algorithm (class template specialization)
nullopt_t (C++17)	indicator of optional type with uninitialized state (class)
in_place_t (C++17)	disambiguation tag type for in-place construction of optional types (class)
bad_optional_access (C++17)	exception indicating checked access to an optional that doesn't contain a value (class)

Helper objects

nullopt (C++17)	an object of type nullopt_t (constant)
in_place (C++17)	an object of type std::in_place_t (constant)