std::optional

 

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std::optional

Member functions

optional::optional

optional::~optional

optional::operator=

Observers

optional::operator->optional::operator*

optional::operator booloptional::has_value

optional::value

optional::value_or

Iterators

optional::begin (C++26)

optional::end (C++26)

Monadic operations

optional::and_then (C++23)

optional::transform (C++23)

optional::or_else (C++23)

Modifiers

optional::emplace

optional::swap

optional::reset

Non-member functions

operator==operator!=operator<operator<=operator>operator>=operator<=> (C++20)

make_optional

swap(std::optional)

Deduction guides

Helper classes

hash<std::optional>

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 nested within the optional object, which means that no dynamic memory allocation ever takes place. Thus, an optional object models an object, not a pointer, even though 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/assigned from a value of type T or 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/assigned from a value of type std::nullopt_t or an optional object that does not contain a value.
• The member function reset() is called.

The optional object is a view that contains either one element if it contains a value, or otherwise zero elements if it does not contain a value. The lifetime of the contained element is bound to the object.

(since C++26)

There are no optional references, functions, arrays, or cv void; a program is ill-formed if it instantiates an optional with such a type. In addition, a program is ill-formed if it instantiates an optional with the (possibly cv-qualified) tag types std::nullopt_t or std::in_place_t.

Template parameters

T

-

the type of the value to manage initialization state for. The type must meet the requirements of Destructible (in particular, array and reference types are not allowed).

Member types

Member name	Definition
value_type	T
iterator (since C++26)	implementation-defined LegacyRandomAccessIterator, ConstexprIterator, and contiguous_iterator whose value_type and reference are std::remove_cv_t<T> and T&, respectively.
const_iterator (since C++26)	implementation-defined LegacyRandomAccessIterator, ConstexprIterator, and contiguous_iterator whose value_type and reference are std::remove_cv_t<T> and const T&, respectively.

All requirements on the iterator types of a Container apply to the iterator type of optional as well.

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)
Iterators
begin (C++26)	returns an iterator to the beginning (public member function)
end (C++26)	returns an iterator to the end (public member function)
Observers
operator->operator*	accesses the contained value (public member function)
operator boolhas_value	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)
Monadic operations
and_then (C++23)	returns the result of the given function on the contained value if it exists, or an empty optional otherwise (public member function)
transform (C++23)	returns an optional containing the transformed contained value if it exists, or an empty optional otherwise (public member function)
or_else (C++23)	returns the optional itself if it contains a value, or the result of the given function otherwise (public member function)
Modifiers
swap	exchanges the contents (public member function)
reset	destroys any contained value (public member function)
emplace	constructs the contained value in-place (public member function)

Non-member functions

operator==operator!=operator<operator<=operator>operator>=operator<=> (C++17)(C++17)(C++17)(C++17)(C++17)(C++17)(C++20)	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 template)

Helper classes

std::hash<std::optional> (C++17)	hash support for std::optional (class template specialization)
nullopt_t (C++17)	indicator of an std::optional that does not contain a value (class)
bad_optional_access (C++17)	exception indicating checked access to an optional that doesn't contain a value (class)

Helpers

nullopt (C++17)	an object of type nullopt_t (constant)
in_placein_place_typein_place_indexin_place_tin_place_type_tin_place_index_t (C++17)	in-place construction tag (tag)

Helper specializations

template< class T > constexpr bool ranges::enable_view<std::optional<T>> = true;		(since C++26)

This specialization of ranges::enable_view makes optional satisfy view.

template< class T > constexpr auto format_kind<std::optional<T>> = range_format::disabled;		(since C++26)

This specialization of format_kind disables the range formatting support of optional.

Notes

Feature-test macro	Value	Std	Feature
__cpp_lib_optional	201606L	(C++17)	std::optional
	202106L	(C++20) (DR20)	Fully constexpr
	202110L	(C++23)	Monadic operations
__cpp_lib_optional_range_support	202406L	(C++26)	Range support for std::optional

Example

Run this code

#include <iostream>
#include <optional>
#include <string>
 
// optional can be used as the return type of a factory that may fail
std::optional<std::string> create(bool b)
{
    if (b)
        return "Godzilla";
    return {};
}
 
// std::nullopt can be used to create any (empty) std::optional
auto create2(bool b)
{
    return b ? std::optional<std::string>{"Godzilla"} : std::nullopt;
}
 
int main()
{
    std::cout << "create(false) returned "
              << create(false).value_or("empty") << '\n';
 
    // optional-returning factory functions are usable as conditions of while and if
    if (auto str = create2(true))
        std::cout << "create2(true) returned " << *str << '\n';
}

Output:

create(false) returned empty
create2(true) returned Godzilla

See also

variant (C++17)	a type-safe discriminated union (class template)
any (C++17)	objects that hold instances of any CopyConstructible type (class)
expected (C++23)	a wrapper that contains either an expected or error value (class template)
ranges::single_viewviews::single (C++20)	a view that contains a single element of a specified value (class template) (customization point object)
ranges::empty_viewviews::empty (C++20)	an empty view with no elements (class template) (variable template)