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

struct in_place_t {
    explicit in_place_t() = default;
};
inline constexpr in_place_t in_place{};

template <class T> struct in_place_type_t {
    explicit in_place_type_t() = default;
};
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;
};
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