std::experimental::in_place

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Defined in header <experimental/optional>
constexpr in_place_t in_place{};
(library fundamentals TS)

std::experimental::in_place is a constant of type std::experimental::in_place_t that is 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

(since C++17)
disambiguation tag type for in-place construction of optional types
(class)
(since C++17)
a wrapper that may or may not hold an object
(class template)