std::optional (3) - Linux Manuals
Defined in header <optional>
template< class T > (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 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.
T - the type of the value to manage initialization state for. The type must meet the requirements of Destructible
Member type Definition
constructor (public member function)
destructor (public member function)
operator= (public member function)
operator-> (public member function)
operator_bool (public member function)
value (public member function)
value_or (public member function)
swap (public member function)
reset (public member function)
emplace (public member function)
operator< compares optional objects
operator<= (function template)
make_optional creates an optional object
std::swap(std::optional) specializes the std::swap algorithm
std::hash<std::optional> specializes the std::hash algorithm
nullopt_t indicator of optional type with uninitialized state
bad_optional_access exception indicating checked access to an optional that doesn't contain a value
nullopt an object of type nullopt_t
in_place_index_ in-place construction tag
in_place_t_ (class template)
// Run this code
variant a type-safe discriminated union
any Objects that hold instances of any CopyConstructible type.