BIGINT

#include <iostream>
#include <string>
using namespace std;
class BIGINT
{
public:
	friend istream& operator >> (istream& in, BIGINT &BI);
	friend ostream& operator << (ostream& out, BIGINT BI);
	BIGINT operator +(BIGINT BI);
	BIGINT operator -(BIGINT BI);
	BIGINT operator *(BIGINT BI);
	BIGINT operator /(BIGINT BI);
private:
	string bigint;
};
istream& operator >> (istream& in, BIGINT &BI)
{
	cout << "Nhap so nguyen bat ky: ";
	in >> BI.bigint;
	return in;
}
ostream& operator << (ostream& out, BIGINT BI)
{
	out << "So BIGINT: ";
	out << BI.bigint;
	return out;
}
BIGINT BIGINT::operator +(BIGINT BI)
{
	int r = 0;
	BIGINT KQ;
	KQ.bigint = "";
	string s = "0";
	std::string::iterator it1, it2;
	it1 = bigint.end() - 1;
	it2 = BI.bigint.end() - 1;
	while (bigint.size() > BI.bigint.size())
	{
		BI.bigint.insert(0, s);
	}
	while (bigint.size() < BI.bigint.size())
	{
		bigint.insert(0, s);
	}
	while (1)
	{
		s = *(it1) + *(it2) - 48 + r;
		r = (s[0] - 48) / 10;
		s = (s[0] - 48) % 10 + 48;
		KQ.bigint.insert(0, s);
		if (it1 == bigint.begin() || it2 == BI.bigint.begin()) break;
		it1--;
		it2--;
	}
	if (r == 1)
	{
		s = "1";
		KQ.bigint.insert(0, s);
	}
	return KQ;
}