Overloading operator<<: cannot bind ‘std::basic_ostream’ lvalue to ‘std::basic_ostream&&’

Question

Searching for the title of this question gives me a number of people quoting the same error, but under different circumstances, and unfortunately the answers there provided are

Answer 1

Change this:

std::ostream& operator<< ( std::ostream&, Vector< U, M >& );

to this:

std::ostream& operator<< ( std::ostream&, const Vector< U, M >& );
//                                        ^^^^^

The compiler is telling you that C++ will not let you bind a temporary Vector such as u + v to a non-const Vector&.

And you don't modify that Vector, so it should be const to begin with.