std::in_place

From cppreference.com
< cpp‎ | utility‎ | optional
Defined in header <optional>
constexpr in_place_t in_place{};
(since C++17)

std::in_place is a constant of type std::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)