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
class S {
};
template<typename T>
class Wrapper {
private:
T object;
public:
Wrapper(T obj) : object(obj) { // implicit conversion from T to Wrapper<T>
}
friend void foo(Wrapper<T> const&) {
}
};
int main()
{
S s;
Wrapper<S> w(s);
foo(w); // OK: Wrapper<S> is a class associated with w
foo(s); // ERROR: Wrapper<S> is not associated with s
}