std::in_place,std::in_place_type,std::in_place_index,std::in_place_t, (3) Linux Manual Page

std::in_place,std::in_place_type,std::in_place_index,std::in_place_t, – std::in_place,std::in_place_type,std::in_place_index,std::in_place_t,

Synopsis

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
(C++17)  (class template)
variant  a type-safe discriminated union
(C++17)  (class template)
any      Objects that hold instances of any CopyConstructible type.
(C++17)  (class)