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>
template<typename T>
bool Variant<Types...>::is() const
{
return this->getDiscriminator() ==
VariantChoice<T, Types...>::Discriminator;
}