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
                    
                                        
                    
                    
                                                            
                    | Defined in header  <utility> | ||
| struct in_place_t {     explicit in_place_t() = default; | (since C++17) | |
| template <class T> struct in_place_type_t {     explicit in_place_type_t() = default; | (since C++17) | |
| template <size_t I> struct in_place_index_t {     explicit in_place_index_t() = default; | (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) | 
| (since C++17) | a type-safe discriminated union (class template) | 
| (since C++17) | Objects that hold instances of any CopyConstructibletype.(class) | 


