大数模板——参考《入门经典》

代码：

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

const int maxn = 40000;
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,b;
    while(cin>>a>>b)
    {
        cout<<a+b<<endl;
        cout<<a-b<<endl;//a>=b时才可以
        cout<<a*b<<endl;
    }
    return 0;
}