std::optional(3)

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)