问奇怪的“未定义符号”错误？

EN

Undefined symbols:
  "_main", referenced from:
      start in crt1.10.6.o
ld: symbol(s) not found
collect2: ld returned 1 exit status

  #include <stdlib.h>
  #include <iostream>

  class Fraction {
      private:
          int num, den;
      public:
          void set(int n, int d) {num=n; den=d; normalize();}
          int get_num(){return num;}
          int get_den(){return den;}
      private:
          void normalize();

          int gcf(int a, int b);
          int lcm(int a, int b);
  };

  void Fraction::normalize() {
      if (den == 0 || num == 0) {
          num = 0;
          den = 1;
      }

      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 (a % b == 0)
          return abs(b);
      else return gcf(b, a % b);
  }
  int Fraction::lcm(int a, int b) {
      return(a / gcf(a, b)) * b;
  }