++ pre-fix operator overload is not working with << - c++

Question

Bit of a new C++ dev here. I have a class Rational used to represent rational numbers and allow the user to perform arithmetic and relational operations on them. I overloaded all the operators and they're all working properly except when I try to combine the ++/-- pre-fix operators (e.g., --Rational) or the +/- unary operators with the << output operator, as in:
Rational num(7, 2); // initializes rational to 7/2 (fraction)
cout << --num; // should change val to 5/2 (decrements by one)
// error: no match for 'operator<<'(operand types are 'std::ostream' {aka 'std::basic_ostream<char>'} and 'Rational')
or:
Rational num(1, 10);
cout << -num; // should change to -1/10
// error: no match for 'operator<<'(operand types are 'std::ostream' {aka 'std::basic_ostream<char>'} and 'Rational')
Which I don't understand why, because I've overloaded all of these operators to return a Rational object, and I've overloaded the << operator to accept a Rational object as well. Interestingly enough, when I try this:
Rational num(5, 3);
cout << num;
It works as intended. So can someone tell me what's going on here?
Relevant code for the class:
class Rational
{
private:
// Instance variables declarations
int numerator, denominator;
public:
// Constructors declarations
Rational(int numer_val, int denom_val = 1);
Rational();
// Unary operators declarations
friend Rational operator +(const Rational &num);
friend Rational operator -(const Rational &num);
friend Rational operator ++(Rational &num);
friend Rational operator --(Rational &num);
// I/O operators
friend ostream& operator <<(ostream &outs, Rational &num);
};
// Constructors definitions
Rational::Rational(int numer_val, int denom_val) : numerator(numer_val)
{
set_denominator(denom_val); // irrelevant
simplify(); // irrelevant
}
Rational::Rational() : Rational(0) { }
// Unary operators definitions
Rational operator +(const Rational &num)
{
return Rational(+num.numerator, num.denominator);
}
Rational operator -(const Rational &num)
{
return Rational(-num.numerator, num.denominator);
}
Rational operator ++(Rational &num)
{
num.numerator += num.denominator;
return num;
}
Rational operator --(Rational &num)
{
num.numerator -= num.denominator;
return num;
}
// I/O operators
ostream& operator <<(ostream &outs, Rational &num)
{
outs << to_string(num.numerator) + "/" + to_string(num.denominator);
return outs;
}

Answer 1

The second argument of your operator<< binds to an l-value reference:
ostream& operator <<(ostream &outs, Rational & num)
// ^
But all your operators return r-values. So for example, when you do:
cout << -num;
you are trying to bind an l-value reference to an r-value, which is not allowed.
You can fix this by changing operator<< to accept a const reference:
ostream& operator <<(ostream &outs, Rational const & num)
// ^^^^^^^