std::in_place,std::in_place_type,std::in_place_index,std::in_place_t,std::in_place_type_t,std::in_place_index_t (3) - Linux Manuals

std::in_place,std::in_place_type,std::in_place_index,std::in_place_t,std::in_place_type_t,std::in_place_index_t: std::in_place,std::in_place_type,std::in_place_index,std::in_place_t,std::in_place_type_t,std::in_place_index_t


std::in_place,std::in_place_type,std::in_place_index,std::in_place_t,std::in_place_type_t,std::in_place_index_t - std::in_place,std::in_place_type,std::in_place_index,std::in_place_t,std::in_place_type_t,std::in_place_index_t


Defined in header <utility>
struct in_place_t {
explicit in_place_t() = default; (since C++17)
inline constexpr in_place_t in_place{};
template <class T> struct in_place_type_t {
explicit in_place_type_t() = default;
}; (since C++17)
template <class T>
inline constexpr in_place_type_t<T> in_place_type{};
template <std::size_t I> struct in_place_index_t {
explicit in_place_index_t() = default;
}; (since C++17)
template <std::size_t I>
inline constexpr in_place_index_t<I> in_place_index{};

std::in_place, std::in_place_type, and std::in_place_index are disambiguation tags that can be passed to the constructors of std::optional, std::variant, and std::any to indicate that the contained object should be constructed in-place, and (for the latter two) the type of the object to be constructed.
The corresponding type/type templates std::in_place_t, std::in_place_type_t and std::in_place_index_t can be used in the constructor's parameter list to match the intended tag.

See also

optional a wrapper that may or may not hold an object
         (class template)

variant a type-safe discriminated union
         (class template)

any Objects that hold instances of any CopyConstructible type.