[問題] friend function

[ricky770927 (ricky)]

當我寫
class ModInt
{
        friend ostream& operator << (ostream& os, const ModInt& n);
private:
        int _number;
        static  unsigned _modulus;
};
ostream& operator<<(ostream &os, const ModInt &n )
{
        os<<n._number;
        return os;
}
為何他說n不能access到_number呢?
我明明有宣告friend呀?

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推 ric2k1:上面的 code compile OK 呀! What's the error message?