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

FIRST PROBLEM:

To make your program compile, just use an lvalue reference to const as the second parameter of your operator << (both in the friend-declaration and in the definition of that function):

template < class U, unsigned int M >
std::ostream& operator<< ( std::ostream &out, Vector< U, M > const& cVector )
//                                                           ^^^^^

The reason why your program won't compile is that your overload of operator << accepts an lvalue reference to non-const as its second argument, and lvalue references to non-const cannot bind to rvalues.

Since the result of operator + between two instances of Vector is a temporary, and a temporary is an rvalue, the compiler can't invoke your operator <<, and is therefore unable to resolve the call.

SECOND PROBLEM:

Once you fixed the above issue, you'll have to solve a second one: your Vector class template does not provide a const version of operator [], so your rewritten operator <<, which now accepts a reference to a const vector, won't be able to access the vector's elements.

template < class T, unsigned int N >
class Vector
{
    // ...

    T& operator[] ( const unsigned int& );

    T const& operator[] ( const unsigned int& ) const; // <== ADD THIS!

    // ...
};

And of course the corresponding definition:

template < class T, unsigned int N >
T const& Vector< T, N >::operator[] ( const unsigned int &index ) const
{
    return _values[ index ];
}

Answer 2

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.