std::optional
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 anotheroptional
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 |
(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.
Contents |
[edit] 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). |
[edit] Member types
Member name | Definition |
value_type
|
T
|
iterator (since C++26)
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implementation-defined LegacyRandomAccessIterator, ConstexprIterator, and contiguous_iterator whose value_type and reference are std::remove_cv_t<T> and T&, respectively.
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const_iterator (since C++26)
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implementation-defined LegacyRandomAccessIterator, ConstexprIterator, and contiguous_iterator whose value_type and reference are std::remove_cv_t<T> and const T&, respectively.
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All requirements on the iterator types of a Container apply to the iterator
type of optional
as well.
[edit] Member functions
constructs the optional object (public member function) | |
destroys the contained value, if there is one (public member function) | |
assigns contents (public member function) | |
Iterators | |
(C++26) |
returns an iterator to the beginning (public member function) |
(C++26) |
returns an iterator to the end (public member function) |
Observers | |
accesses the contained value (public member function) | |
checks whether the object contains a value (public member function) | |
returns the contained value (public member function) | |
returns the contained value if available, another value otherwise (public member function) | |
Monadic operations | |
(C++23) |
returns the result of the given function on the contained value if it exists, or an empty optional otherwise (public member function) |
(C++23) |
returns an optional containing the transformed contained value if it exists, or an empty optional otherwise (public member function) |
(C++23) |
returns the optional itself if it contains a value, or the result of the given function otherwise (public member function) |
Modifiers | |
exchanges the contents (public member function) | |
destroys any contained value (public member function) | |
constructs the contained value in-place (public member function) |
[edit] Non-member functions
(C++17)(C++17)(C++17)(C++17)(C++17)(C++17)(C++20) |
compares optional objects (function template) |
(C++17) |
creates an optional object (function template) |
(C++17) |
specializes the std::swap algorithm (function template) |
[edit] Helper classes
(C++17) |
hash support for std::optional (class template specialization) |
(C++17) |
indicator of an std::optional that does not contain a value (class) |
(C++17) |
exception indicating checked access to an optional that doesn't contain a value (class) |
[edit] Helpers
(C++17) |
an object of type nullopt_t (constant) |
in-place construction tag (tag) |
[edit] 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
.
[edit] Deduction guides
[edit] 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
|
[edit] Example
#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
[edit] See also
(C++17) |
a type-safe discriminated union (class template) |
(C++17) |
objects that hold instances of any CopyConstructible type (class) |
(C++23) |
a wrapper that contains either an expected or error value (class template) |
a view that contains a single element of a specified value(class template) (customization point object) | |
an empty view with no elements(class template) (variable template) |