std:: nullopt_t

std::nullopt_t is an empty class type used to indicate that an std::optional does not contain a value.

std::nullopt_t is a non-aggregate LiteralType that has no default constructor, no initializer-list constructor, but does have a constexpr constructor that takes an 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