NAME¶

std::optional - std::optional

Synopsis¶

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

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 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.

There are no optional references; a program is ill-formed if it instantiates an
optional with a reference type. Alternatively, an optional of a
std::reference_wrapper of type T may be used to hold a reference. 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¶

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

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-> accesses the contained value
operator* (public member function)
operator bool checks whether the object contains a value
has_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 returns the result of the given function on the contained value if it
(C++23) exists, or an empty optional otherwise
(public member function)
transform returns an optional containing the transformed contained value if it
(C++23) exists, or an empty optional otherwise
(public member function)
or_else returns the optional itself if it contains a value, or the result of
(C++23) 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<=> compares optional objects
(C++17) (function template)
(C++17)
(C++17)
(C++17)
(C++17)
(C++17)
(C++20)
make_optional creates an optional object
(C++17) (function template)
std::swap(std::optional) specializes the std::swap algorithm
(C++17) (function template)

Helper classes¶

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

Helpers

nullopt an object of type nullopt_t
(C++17) (constant)
in_place
in_place_type
in_place_index in-place construction tag
in_place_t (class template)
in_place_type_t
in_place_index_t
(C++17)

Deduction guides

Notes¶

Feature-test macro: __cpp_lib_optional

Example¶

// Run this code

#include <string>
#include <functional>
#include <iostream>
#include <optional>

// 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;
}

// std::reference_wrapper may be used to return a reference
auto create_ref(bool b) {
static std::string value = "Godzilla";
return b ? std::optional<std::reference_wrapper<std::string>>{value}
: 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';
}

if (auto str = create_ref(true)) {
// using get() to access the reference_wrapper's value
std::cout << "create_ref(true) returned " << str->get() << '\n';
str->get() = "Mothra";
std::cout << "modifying it changed it to " << str->get() << '\n';
}
}

Output:¶

create(false) returned empty
create2(true) returned Godzilla
create_ref(true) returned Godzilla
modifying it changed it to Mothra

See also¶

variant a type-safe discriminated union
(C++17) (class template)
any Objects that hold instances of any CopyConstructible type.
(C++17) (class)