std::deque<T,Allocator>::emplace (3) - Linux Man Pages
template< class... Args > (since C++11)
iterator emplace( const_iterator pos, Args&&... args );
Inserts a new element into the container directly before pos. The element is constructed through std::allocator_traits::construct, which typically uses placement-new to construct the element in-place at a location provided by the container. The arguments args... are forwarded to the constructor as std::forward<Args>(args)....
All iterators, including the past-the-end iterator, are invalidated. References are invalidated too, unless pos == begin() or pos == end(), in which case they are not invalidated.
T (the container's element type) must meet the requirements of MoveAssignable, MoveInsertable and EmplaceConstructible.
Iterator pointing to the emplaced element.
Linear in the lesser of the distances between pos and either of the ends of the container.
If an exception is thrown other than by the copy constructor, move constructor, assignment operator, or move assignment operator of the value type, or if an exception is thrown while emplace is used to insert a single element at the either end, there are no effects (strong exception guarantee).
Otherwise, the effects are unspecified.
insert (public member function)