The following code example is taken from the book
C++ Templates - The Complete Guide, 2nd Edition
by David Vandevoorde, Nicolai M. Josuttis, and Douglas Gregor,
Addison-Wesley, 2017
© Copyright David Vandevoorde, Nicolai M. Josuttis, Douglas Gregor 2017
template<typename T, typename Cont>
template<typename T2, typename Cont2>
Stack<T,Cont>&
Stack<T,Cont>::operator= (Stack<T2,Cont2> const& op2)
{
elems.clear(); // remove existing elements
elems.insert(elems.begin(), // insert at the beginning
op2.elems.begin(), // all elements from op2
op2.elems.end());
return *this;
}