一个支持大数计算的类型 仅不支持除法

#include<iostream>
#include<cstdio>
#include<cstring>
#include<string>
using namespace std;
//一个完整的支持大数计算的类型
//仅不支持除法
int max(int a, int b) { return a > b ? a : b; }
const int maxn = 1002;
struct bign
{
      int len, s[maxn];
      bign()
      {
          memset(s, 0, sizeof(s));
          len = 1;
      }
      bign(int num)
      {
          *this = num;
      }
      bign(const char* num)
      {
          *this = num;
      }
      bign operator = (int num)
      {
          char s[maxn];
          sprintf(s, "%d", num);
          *this = s;
          return *this;
      }
      bign operator = (const char* num)
      {
          len = strlen(num);
          for(int i = 0; i < len; i++)
          s[i] = num[len-i-1] - '0';
          return *this;
      }
      string str() const
      {
          string res = "";
          for(int i = 0; i < len; i++)
          res = (char)(s[i] + '0') + res;
          if(res == "") res = "0";
          return res;
      }
      bign operator + (const bign& b) const                 //实现c+b的加法
      {
          bign c;
          c.len = 0;
          for(int i = 0, g = 0; g || i < max(len, b.len); i++)
          {
              int x = g;
              if(i < len) x += s[i];
              if(i < b.len) x += b.s[i];
              c.s[c.len++] = x % 10;
              g = x / 10;
          }
          return c;
      }
      void clean()
      {
          while(len > 1 && !s[len-1]) len--;
      }
      bign operator * (const bign& b)
      {
          bign c; c.len = len + b.len;
          for(int i = 0; i < len; i++)
          for(int j = 0; j < b.len; j++)
          c.s[i+j] += s[i] * b.s[j];
          for(int i = 0; i < c.len-1; i++)
          {
              c.s[i+1] += c.s[i] / 10;
              c.s[i] %= 10;
          }
          c.clean();
          return c;
      }
    /*   bign operator / (int b)
	  {
		  bign c;
		  int j=1,s=0,i;
		  for(i=c.len-1;i>=0;i--)
		  {
			  int k=0;
			  for(s=s*10+c.s[i];s>=b;s-=b) k++;
			  c.s[i]=k;
		  }
		  return c;
	  }   */
      bign operator - (const bign& b)
      {
          bign c; c.len = 0;
          for(int i = 0, g = 0; i < len; i++)
          {
              int x = s[i] - g;
              if(i < b.len) x -= b.s[i];
              if(x >= 0) g = 0;
              else
              {
                  g = 1;
                  x += 10;
              }
              c.s[c.len++] = x;
              }
              c.clean();
              return c;
       }
       bool operator < (const bign& b) const
       {
           if(len != b.len) return len < b.len;
           for(int i = len-1; i >= 0; i--)
           if(s[i] != b.s[i]) return s[i] < b.s[i];
           return false;
       }
       bool operator > (const bign& b) const
       {
           return b < *this;
       }
       bool operator <= (const bign& b)
       {
           return !(b > *this);
       }
       bool operator == (const bign& b)
       {
           return !(b < *this) && !(*this < b);
       }
       bign operator += (const bign& b)
       {
           *this = *this + b;
           return *this;
       }
};

istream& operator >> (istream &in, bign& x) {
  string s;
  in >> s;
  x = s.c_str();
  return in;
}

ostream& operator <<(ostream &out,const bign &x)
{
    out<<x.str();
    return out;
}