std::optional (3) - Linux Manuals

std::optional: std::optional

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 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