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Showing content from https://en.cppreference.com/w/cpp/language/../preprocessor/../utility/optional/nullopt_t.html below:

std::nullopt_t - cppreference.com

struct 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.

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) {}
};

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