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Why doesn't my default argument work?

bool maybe (93)
I'm experimenting with default arguments but they don't deem to be working for me. On line 43 I make a call to a function that's declared on line 9 and defined on line 105. When I run this program I get an error on line 43:

'rational::output' : function does not take 0 argument

and on line 106:

'void rational::output(std::ostream &)' : overloaded member function not found in 'rational'

What am I doing wrongly?

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#include <iostream>

using namespace std;

class rational
{
public:
	rational();
	void output(ostream& out) const;
	void set(short num, short den);

	short get_numerator() const;
	short get_denominator() const;
	rational add(rational number);
	rational sub(rational number);
	rational sub(short number);
	rational mul(rational number);
	rational mul(short number);
	rational div(rational number);
	rational div(short number);
	rational neg();

private:
	short numerator;
	short denominator;
};

int main()
{
	char loop = 'y';
	do
	{
		rational rational1, rational2;
		cout << "Input the numerator and denominator: ";
		short num, den;
		cin >> num >> den;
		rational1.set(num, den);
		cout << "Input the numerator and denominator: ";
		cin >> num >> den;
		rational2.set(num, den);

		//testing
		rational1.output();

		cout << "What would you like to do?\n"
			 << "1) add\n"
			 << "2) subtract\n"
			 << "3) multiply\n\n";
		char choice;
		cin >> choice;
		while(choice != '1' && choice != '2' && choice != '3')
		{
			cout << "Invalid choice. What would you like to do?\n"
				 << "1) add\n"
				 << "2) subtract\n"
				 << "3) multiply\n\n";
			cin >> choice;
		}

		rational result;

		
		if(choice == '1')
		{
			result = rational1.add(rational2);
			cout << "The result is: ";
			result.output(cout);
		}/*
		else if(choice == '2')
		{
			result = rational1.sub(rational2);
			cout << "The result is: ";
			result.output(cout);
		}
		else if(choice == '3')
		{
			result = rational1.mul(rational2);
			cout << "The result is: ";
			result.output(cout);
		}*/
		cout << endl;

		cout << "Would you like to run the program again? (y/n)? ";
		cin >> loop;
		loop = tolower(loop);
		while(loop != 'y' && loop != 'n')
		{
			cout << "Incorrect output. Would you like to run the program again? ";
			cin >> loop;
		}
	}while(loop == 'y');

	cout << "Press a key to exit: ";
	char exit;
	cin >> exit;

	return 0;
}

rational::rational() : numerator(0), denominator(1)
{
	//default constructor
}

void rational::output(ostream& out = cout)
{
	out << numerator << "/" << denominator;
}

void rational::set( short num, short den = 1)
{
	numerator = num;
	denominator = den;
}

short rational::get_denominator() const
{
	return denominator;
}

short rational::get_numerator() const
{
	return numerator;
}

rational rational::add(rational number)
{
	rational sum;
	sum.numerator = numerator*number.denominator+number.numerator*denominator;
	sum.denominator = denominator*number.denominator;
	return sum;
}

/*
rational rational::sub(rational number)
{
	numerator = numerator*number.denominator - denominator*number.numerator;
	denominator = denominator*number.denominator;
}

rational rational::sub(short integer)
{
	numerator = numerator - integer*denominator;
}

rational rational::mul(rational number)
{
	numerator = numerator * number.numerator;
	denominator = denominator * number.denominator;
}

rational rational::mul(short integer)
{
	numerator = integer * numerator;
}*/
helios (17574)
Put the default expression (the "= cout" part) in the declaration, not the definition. Otherwise, the compiler can't know that out is a default parameter.
bool maybe (93)
That helped. Thanks.
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