想请问这个程序哪里出了错？

Question

#include
using namespace std;

class Rational {
public:
void read();
void print();
void sum(Rational r);
void prod(Rational r);
void minus(Rational r);
void div(Rational r);
private:
int num;
int den;
void reduce()
{
int a,b;
for(int i=1;i<min(num,den);++i)
{
if(num%i==0&den%i==0)
{
a=num/i;
b=den/i;
}
}
num=a;
den=b;
}
};

void Rational:: read()
{
cin>>num>>den;
}

void Rational:: print()
{
if(num!=den)
cout<<num<<'/'<<den;
else
cout<<1;
}

void Rational:: sum(Rational r)
{
num=num*r.den+r.num*den;
den=den*r.den;
reduce();
print();
}

void Rational:: prod(Rational r)
{
num=num*r.num;
den=den*r.den;
reduce();
print();
}

void Rational:: minus(Rational r)
{
num=num*r.den-r.num*den;
den=den*r.den;
reduce();
print();
}

void Rational:: div(Rational r)
{
num=num*r.den;
den=den*r.num;
reduce();
print();
}

int main() {
Rational a, b;
a.read();
b.read();
a.sum(b).print(); cout << endl;//以下四行报错： invalid use of 'void'
a.prod(b).print(); cout << endl;
a.minus(b).print(); cout << endl;
a.div(b).print(); cout << endl;
return 0;
}

Answer 1

因为sum等函数返回void，所以不能 .print
有两个修改方式，任选其一

一个是
a.sum(b).print();
->
a.sum(b);
a.print();
别的类似

另一个是
Rational Rational:: sum(Rational r)
{
num=num*r.den+r.num*den;
den=den*r.den;
reduce();
print();
return *this;
}
别的类似