std::nullopt_t - cppreference.com
From cppreference.com
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(since C++17) | |
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) {} };