std::queue<T,Container>::emplace
From cppreference.com
                    
                                        
                    
                    
                                                            
                    
|   template< class... Args > void emplace( Args&&... args );  | 
 (since C++11)  (until C++17)  | 
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|   template< class... Args > decltype(auto) emplace( Args&&... args );  | 
(since C++17) | |
Pushes a new element to the end of the queue. The element is constructed in-place, i.e. no copy or move operations are performed. The constructor of the element is called with exactly the same arguments as supplied to the function.
Effectively calls c.emplace_back(std::forward<Args>(args)...);
Parameters
| args | - | arguments to forward to the constructor of the element | 
Return value
| (none) | (until C++17) | 
| The value or reference, if any, returned by the above call to Container::emplace_back. | (since C++17) | 
Complexity
Identical to the complexity of Container::emplace_back.
Defect reports
The following behavior-changing defect reports were applied retroactively to previously published C++ standards.
| DR | Applied to | Behavior as published | Correct behavior | 
|---|---|---|---|
| LWG 2783 | C++17 |  emplace returned reference, breaking compatibility with pre-C++17 containers
 | 
 returns decltype(auto)
 | 
See also
|    inserts element at the end  (public member function)  | |
|    removes the first element   (public member function)  |