std::vector<T,Allocator>::emplace_back (3) - Linux Manuals
template< class... Args > (since C++11)
void emplace_back( Args&&... args ); (until C++17)
template< class... Args > (since C++17)
reference emplace_back( Args&&... args );
Appends a new element to the end of the container. The element is constructed through std::allocator_traits::construct, which typically uses placement-new to construct the element in-place at the location provided by the container. The arguments args... are forwarded to the constructor as std::forward<Args>(args)....
If the new size() is greater than capacity() then all iterators and references (including the past-the-end iterator) are invalidated. Otherwise only the past-the-end iterator is invalidated.
T (the container's element type) must meet the requirements of MoveInsertable and EmplaceConstructible.
(none) (until C++17)
A reference to the inserted element. (since C++17)
If an exception is thrown, this function has no effect (strong exception guarantee). If T's move constructor is not noexcept and is not CopyInsertable into *this, vector will use the throwing move constructor. If it throws, the guarantee is waived and the effects are unspecified.
Since reallocation may take place, emplace_back requires the element type to be MoveInsertable for vectors.
The specialization std::vector<bool> did not have emplace_back() member until C++14.
The following code uses emplace_back to append an object of type President to a std::vector. It demonstrates how emplace_back forwards parameters to the President constructor and shows how using emplace_back avoids the extra copy or move operation required when using push_back.
// Run this code
push_back (public member function)