Std::optional (3) Linux Manual Page

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 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

Member types

Member type Definition
value_type T

Member functions

              constructs the optional object
constructor (public member function)
              destroys the contained value, if there is one
destructor (public member function)
              assigns contents
operator= (public member function)

Observers

              accesses the contained value
operator-> (public member function)
operator*
              checks whether the object contains a value
operator_bool (public member function)
has_value
              returns the contained value
value (public member function)
              returns the contained value if available, another value otherwise
value_or (public member function)

Modifiers

              exchanges the contents
swap (public member function)
              destroys any contained value
reset (public member function)
              constructs the contained value in-place
emplace (public member function)

Non-member functions

operator==
operator!=
operator< compares optional objects
operator<= (function template)
operator>
operator>=

(C++17)

make_optional creates an optional object
(function template)
(C++17)

std::swap(std::optional) specializes the std::swap algorithm
(function)
(C++17)

Helper classes

std::hash<std::optional> specializes the std::hash algorithm
(class template specialization)
(C++17)

nullopt_t indicator of optional type with uninitialized state
(class)
(C++17)

bad_optional_access exception indicating checked access to an optional that doesn’t contain a value
(class)
(C++17)

Helpers

nullopt an object of type nullopt_t
(constant)
(C++17)

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

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

std::optional (3) Linux Manual Page