# std::nullopt_t

From cppreference.com
< cpp‎ | utility‎ | optional

Defined in header `<optional>` |  |
struct nullopt_t; |  | (since C++17)

`std::nullopt_t` is an empty class type used to indicate `optional` type with uninitialized state. In particular, std::optional has a constructor with `nullopt_t` as a single argument, which creates an optional that does not contain a value.

`std::nullopt_t` must be a non-aggregate LiteralType and cannot have a default constructor or an initializer-list constructor.

It must have a `constexpr` constructor that takes some implementation-defined literal type.

### Notes

The constraints on `nullopt_t`'s constructors exist to support both op = {}; and op = nullopt; as the syntax for disengaging an optional object.

A possible implementation of this class is

    struct nullopt_t {
        constexpr explicit nullopt_t(int) {}
    };

### See also

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

Retrieved from "https://en.cppreference.com/mwiki/index.php?title=cpp/utility/optional/nullopt_t&oldid=154484"