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 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 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 anoptional
object that does not contain a value. - The member function
reset()
is called.
There are no optional references; a program is ill-formed if it instantiates an optional
with a reference 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 type | Definition |
---|---|
value_type | T |
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) |
|
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) |
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) |
Helper classes
(C++17) | hash support for std::optional (class template specialization) |
(C++17) | indicator of optional type with uninitialized state (class) |
(C++17) | exception indicating checked access to an optional that doesn't contain a value (class) |
Helpers
(C++17) | an object of type nullopt_t (constant) |
(C++17) | in-place construction tag (tag) |
Deduction guides
Notes
Feature-test macro | Value | Std | Feature |
---|---|---|---|
__cpp_lib_optional | 201606L | (C++17) |
std::optional |
__cpp_lib_optional | 202106L |
(C++20) (DR) | Fully constexpr |
__cpp_lib_optional | 202110L | (C++23) | Monadic operations |
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
See also
(C++17) | a type-safe discriminated union (class template) |
(C++17) | objects that hold instances of any CopyConstructible type (class) |
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