std::experimental::in_place_t

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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.

See also

in-place construction tag
(class template)
(since C++17)
a wrapper that may or may not hold an object
(class template)