std::nullopt_t

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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 {
    explicit constexpr nullopt_t(int) {}
};

See also

nullopt (C++17)

an object of type nullopt_t (constant)