高精度计算bign类

#include<cstdio>
#include<cstring>
#include<iostream>
using namespace std;

const int maxn = 200;
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
{
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 - (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;
}

int main() {
bign a;
cin >> a;
a += "123456789123456789000000000";
cout << a*2 << endl;
return 0;
}