Id返回1退出状态

Question

我收到Id返回1退出状态错误。   inc函数不应该得到任何论证者。   我试图把它作为朋友功能这次我收到这个错误“inc不是成员功能”

请告知

#include <iostream>
using namespace std;

class fraction
{
    long num;
    long den;

public:
    fraction();
    fraction(long, long);
    fraction(const fraction &);
    ~fraction();

    long getNum(void)const;
    long getDen(void)const;
    void print(void);
    fraction inc();

    friend fraction add(const fraction &f1, const fraction &f2);
    friend fraction sub(const fraction &f1, const fraction &f2);
    friend fraction mult(const fraction &f1, const fraction &f2);
    friend fraction div(const fraction &f1, const fraction &f2);
};
fraction::fraction()
{
    num = 0L;
    den = 1L;
}
fraction::fraction(long l_num, long l_den)
{
    num = l_num;
    den = l_den;
}
fraction::fraction(const fraction& f)
{
    num = f.num;
    den = f.den;
}
fraction:: ~fraction()
{
    cout << "Destructor" << endl;
}
long fraction::getNum(void) const
{
    return num;
}
long fraction::getDen(void) const
{
    return den;
}
void fraction::print(void)
{
    if (den == 0)
        den = 1L;
    cout << num << "/" << den << endl;
}
fraction fraction::inc()
{
    long num = (num)+(den);
    long den = (den);
    return fraction(num, den);
}
fraction add(fraction &f1, fraction &f2)
{
    long num = (f1.getNum() * f2.getDen()) + (f1.getDen() * f2.getNum());
    long den = (f1.getDen() * f2.getDen());
    return fraction(num, den);
}
fraction sub(fraction &f1, fraction &f2)
{
    long num = (f1.getNum() * f2.getDen()) - (f1.getDen() * f2.getNum());
    long den = (f1.getDen() * f2.getDen());
    return fraction(num, den);
}
fraction mult(fraction &f1, fraction &f2)
{
    long num = (f1.getNum() * f2.getNum());
    long den = (f1.getDen() * f2.getDen());
    return fraction(num, den);
}
fraction div(fraction &f1, fraction &f2)
{
    long num = (f1.getNum() * f2.getDen());
    long den = (f1.getDen() * f2.getNum());
    return fraction(num, den);
}

int main()
{
    fraction f1, f2(5L, 0L);
    fraction f3(f2);
    f1.print();
    f2.print();
    f3.print();

    f3 = add(f3, fraction(-7, 8));
    f1 = add(f2, f3);
    f1.print();

    f1 = sub(f2, f3);
    f1.print();

    f1 = mult(f2, f3);
    f1.print();

    f1.inc().inc().print();
    f1 = div(f2, f3);
    f1.print();

    return 0;
}