模板类+运算符+朋友=未解决的外部

Question

I have a class called fraction, and I'm declaring some operators as friends. I declared the friend operators beforehand, as http://www.parashift.com/c++-faq-lite/templates.html#faq-35.16 told me to do, but it only fixed +, -, *, and /. << and >> still don't work.

template <class T> class Fraction;
template <class T> Fraction<T> operator+ (Fraction<T> const& left, Fraction<T> const& right);
template <class T> Fraction<T> operator- (Fraction<T> const& left, Fraction<T> const& right);
template <class T> Fraction<T> operator* (Fraction<T> const& left, Fraction<T> const& right);
template <class T> Fraction<T> operator/ (Fraction<T> const& left, Fraction<T> const& right);
template <class T> ostream& operator<< (const ostream& output, Fraction<T> const& value);
template <class T> istream& operator>> (const ostream& input, Fraction<T> const& value);

And the class:

template <class T>
class Fraction
{
 ...

 friend Fraction<T> operator+ <>(const Fraction<T>& left, const Fraction<T>& right);
 friend Fraction<T> operator- <>(const Fraction<T>& left, const Fraction<T>& right);
 friend Fraction<T> operator* <>(const Fraction<T>& left, const Fraction<T>& right);
 friend Fraction<T> operator/ <>(const Fraction<T>& left, const Fraction<T>& right);

 friend ostream& operator<< <>(const ostream& output, const Fraction<T> value);
 friend istream& operator>> <>(const istream& input, Fraction<T> value);
}

template <class T> ostream& operator<< <>(const ostream& output, const Fraction<T>& value)
{
 output << value.Numerator << '/' << value.Denominator;

 return output;
}
template <class T> istream& operator>> <>(const istream& input, Fraction<T>& value)
{
 T n, d, char seperator;

 cin >> n >> seperator >> d;

 value.SetNumerator(n);
 value.SetDenominator(d);

 return input;
}

Answer 1

As "dark_charlie" says, but remove the const .

I'd rather have made this as just a comment, but unfortunately StackOverflow does not yet allow me to comment (to the person feeling the urge to comment that I shouldn't make this comment: it's aimed at you ).

Answer 2

Replace

template <class T> istream& operator>> <>(const istream& input, Fraction<T>& value)  { ...

with

template <class T> istream& operator>> (const istream& input, Fraction<T>& value)  { ...

And analogically for the other operator. The problem is that this way you are creating a blank template specialization which yields a different symbol than the one being forward-declared as friend.

Another problem I see is this forward declaration:

template <class T> istream& operator>> (const ostream& input, Fraction<T> const& value);

You declare its first parameter as const ostream& , it should be const istream& . This definitely will cause a linker error.

Side note: The original answer should not fix the errors according to the FAQ. However, I'd give it a shot if the problem persisted.