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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

From cppreference.com
< cpp‎ | utility
Defined in header <utility>
struct in_place_t {

    explicit in_place_t() = default;
};

inline constexpr std::in_place_t in_place{};
(since C++17)
template <class T> struct in_place_type_t {

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

inline constexpr std::in_place_type_t<T> in_place_type{};
(since C++17)
template <size_t I> struct in_place_index_t {

    explicit in_place_index_t() = default;
};
template <size_t I>

inline constexpr in_place_index_t<I> in_place_index{};
(since C++17)

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.

[edit] See also

(since C++17)
a wrapper that may or may not hold an object
(class template) [edit]
(since C++17)
a type-safe discriminated union
(class template) [edit]
(since C++17)
Objects that hold instances of any CopyConstructible type.
(class) [edit]