Question

why does
void operator<<(ostream out, Test &t);
return an error whereas
void operator<<(ostream &out, Test &t);
does not ?

Answer 1

Because you cannot copy streams, you have to pass them per reference.

Note that the canonical form of operator<< is this:

std::ostream& operator<<(std::ostream& out, const Test &t)
{
   // write t into out
   return out;
}

returning the stream is important so that you can string output together:

std::cout << Test() << '\n';