std::in_place
From cppreference.com
                    
                                        
                    
                    
                                                            
                    |   Defined in header  
<optional>
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|   constexpr in_place_t in_place{}; 
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(since C++14) | |
std::in_place is a constant of type std::in_place_t that is used to disambiguate the overloads of constructors and member functions of that take arguments (possibly a parameter pack) for in-place construction of some value.
[edit] See also
|    (C++14) 
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   disambiguation tag type for in-place construction of optional types   (class)  | 
|    (C++14) 
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   a wrapper that may or may not hold an object   (class template)  |