求大神帮我改改c++程序，总是停止运行！

Question

#include
using namespace std;
class Fraction
{
private:
int num,den;
void normalize();
int gcf(int a,int b);
int lcm(int a,int b);
public:
Fraction() {set(0,1);cout<<"construct"<<endl;}
Fraction(int n,int d){set(n,d);cout<<"construct"<<endl;}
Fraction(Fraction &src);
void set(int n,int d)
{ num=n;den=d;normalize();}
int get_num() {return num;}
int get_den() {return den;}
Fraction add(Fraction other);
Fraction mult(Fraction other);
};
Fraction::Fraction(Fraction &src){
cout<<"Now calling copy constructor."<<endl;
num=src.num;
den=src.den;
}

void Fraction::normalize(){
if(den==0||num==0){
num=0;
den=0;
}
if(den<0){
num*=-1;
den*=-1;
}
int n=gcf(num,den);
num=num/n;
den=den/n;
}
int Fraction::gcf(int a,int b){
if(b==0)
return abs(a);
else
return gcf(b,a%b);
}
int Fraction::lcm(int a,int b){
int n=gcf(a,b);
return a/n*b;
}
Fraction Fraction::add(Fraction other){
Fraction fract;
int lcd=lcm(den,other.den);
int quot1=lcd/den;
int quot2=lcd/other.den;
fract.set(num*quot1+other.num*quot2,lcd);
return fract;
}
Fraction Fraction::mult(Fraction other)
{
Fraction fract;
fract.set(num*other.num,den*other.den);
return fract;
}

int main()
{
Fraction f1(3,4);
Fraction f2(f1);
Fraction f3=f1.add(f2);
cout<<"The value is";
cout<<f3.get_num()<<"/";
cout<<f3.get_den()<<endl;
system("PAUSE");
return 0;
}

Answer 1

void Fraction::normalize(){
if(den==0||num==0){
num=0; //这里把分子分母设置成了0，下面出现除0错误
den=0;
}
if(den<0){
num*=-1;
den*=-1;
}
int n=gcf(num,den);
num=num/n; //这里出现除0错误
den=den/n;
}