std::experimental::in_place_t

From cppreference.com
Defined in header <experimental/optional>
struct in_place_t {};
(library fundamentals TS)

std::experimental::in_place_t is an empty class type used to disambiguate the overloads of constructors and member functions of that take arguments (possibly a parameter pack) for in-place construction of some value.

[edit] See also

(library fundamentals TS)
an object of type std::experimental::in_place_t
(function)
(library fundamentals TS)
a wrapper that may or may not hold an object
(class template)