help overloading a c++ class operators

#include "iostream"
using namespace std;
#include "string"
#include "vector"


class Mint{
public:
	Mint();
	Mint(int);
	Mint(const char*);
	Mint operator+=(const Mint &rhs);
	bool operator<(const Mint&);
	bool operator>(const Mint&);
	void display();
private:
	unsigned char* num;
	int size;
};

#include "Mint.h"




Mint::Mint()
{
	num = new unsigned char[size= 1];
	num[0] = 0;
}
Mint::Mint(const char* s)
{
		num = new unsigned char[size= strlen(s)/2 + strlen(s)%2];
		if(strlen(s)%2 == 1)
		num[0] = s[0]-'0';
			unsigned int i;
			int j=strlen(s)%2;
			for(i=strlen(s)%2;i<strlen(s);i+=2)
			{
							int left = s[i] - '0';
							int right = s[i+1] - '0';
							num[j] = left << 4 ;
							num[j] |= right;
							j++;
			}

}
Mint Mint::operator+=(const Mint  &rhs){
		int i,carry=0,sum;
		for(i=this->size-1;i>=0;i--)
		{
			sum = ((num[i]  ^ rhs.num[i])  ^ carry);
			carry = ((num[i] & rhs.num[i]) | (num[i] & carry)) | (rhs.num[i] & carry);
			num[i] = sum;
		}
		return *this;
}
bool Mint::operator<(const Mint& rhs)
{
	unsigned int i;
	if (size  < rhs.size)
			return true;
	if (size == rhs.size)
	for(i=0;i<size;i++)
	{
		if(num[i] < rhs.num[i])
			return true;
	}
	return false;
}
bool Mint::operator>(const Mint& rhs)
{
	if (*this < rhs)
	 return false;
	return true;
}
void Mint::display()
{
	 int i;
	for (i=0;i<size ;i++)
		{
		int first_digit = (num[i] & '\xF0')>>4;
		int second_digit = (num[i] & '\x0F');
		if (i || first_digit)
			cout << first_digit;
		if (i || second_digit)
			cout << second_digit;
		}
}