std::experimental::nullopt_t
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                    < cpp | experimental | optional
                    
                                                            
                    | Defined in header  <experimental/optional> | ||
| struct nullopt_t; | (library fundamentals TS) | |
std::experimental::nullopt_t is an empty class type used to indicate optional type with uninitialized state. In particular, std::experimental::optional has a constructor with nullopt_t as a single argument, which creates an optional that does not contain a value.
std::experimental::nullopt_t must be a LiteralType and cannot have a default constructor.
It must have a constexpr constructor that takes some implementation-defined literal type.
[edit] Notes
nullopt_t is not DefaultConstructible 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 nullopt_t(int) {} };
[edit] See also
| (C++17) | an object of type nullopt_t(constant) | 


