高精度(大整数)运算重载运算符

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

struct BigInteger {
    static const int BASE = 100000000;
    static const int WIDTH = 8;
    vector<int> s;

    BigInteger(long long num = 0) { *this = num; } // 构造函数
    BigInteger operator = (long long num) { // 赋值运算符
        s.clear();
        do {
            s.push_back(num % BASE);
            num /= BASE;
        } while (num > 0);
        return *this;
    }
    BigInteger operator = (const string& str) { // 赋值运算符
        s.clear();
        int x, len = (str.length() - 1) / WIDTH + 1;
        for (int i = 0; i < len; i++) {
            int end = str.length() - i*WIDTH;
            int start = max(0, end - WIDTH);
            sscanf(str.substr(start, end-start).c_str(), "%d", &x);
            s.push_back(x);
        }
        return *this;
    }
    BigInteger operator + (const BigInteger& b) const {
        BigInteger c;
        c.s.clear();
        for (int i = 0, g = 0; ; i++) {
            if (g == 0 && i >= s.size() && i >= b.s.size()) break;
            int x = g;
            if (i < s.size()) x += s[i];
            if (i < b.s.size()) x += b.s[i];
            c.s.push_back(x % BASE);
            g = x / BASE;
        }
        return c;
    }
    BigInteger operator - (const BigInteger& b) const {
        BigInteger c;
        c.s.clear();
        for (int i = 0, g = 0; ; i++) {
            if (g == 0 && i >= s.size() && i >= b.s.size()) break;
            int x = g;            
            if (i < s.size()) x += s[i];
            if (i < b.s.size()) {                
                if (s[i] < b.s[i]) {
                    x += BASE;
                    g = -1;
                }
                x -= b.s[i];
            }
            c.s.push_back(x % BASE);
        }
        return c;
    }
    BigInteger operator += (const BigInteger& b) {
        *this = *this + b;
        return *this;
    }
    
    bool operator < (const BigInteger& b) const {
        if (s.size() != b.s.size())
            return s.size() < b.s.size();
        for (int i = s.size()-1; i >= 0; i--) //从低位开始比
            if (s[i] != b.s[i])
                return s[i] < b.s[i];
        return false; // 相等
    }
    bool operator > (const BigInteger& b) const { return b < *this; }
    bool operator <= (const BigInteger& b) const { return !(b < *this); }
    bool operator >= (const BigInteger& b) const { return !(*this < b); }
    bool operator != (const BigInteger& b) const { return (*this < b || b < *this); }
    bool operator == (const BigInteger& b) const { return (!(*this < b) && !(b < *this)); }
};

ostream& operator << (ostream &out, const BigInteger& x) {
    out << x.s.back();
    for (int i = x.s.size()-2; i >= 0; i--) {
        char buf[20];
        sprintf(buf, "%08d", x.s[i]);
        for (int j = 0; j < strlen(buf); j++)
            out << buf[j];
    }
    return out;
}

istream& operator >> (istream &in, BigInteger& x) {
    string s;
    if (!(in >> s)) return in;
    x = s;
    return in;
}

#include<set>
#include<map>
set<BigInteger> s;
map<BigInteger, int> m;

int main() {
    BigInteger y;
    BigInteger x = y;
    BigInteger z = 123;

    BigInteger a, b;
    cin >> a >> b;
    cout << a - b << "\n";
    //cout << BigInteger::BASE << "\n";
    return 0;
}