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... Types>
Variant<Types...>::Variant(Variant const& source) {
if (!source.empty()) {
source.visit([&](auto const& value) {
*this = value;
});
}
}