I am writing the implementation for a BigInt class using overloaded addition and subtraction operators. Everything compiles well, but when I run the program, it creates an infinite loop. What is causing this and how do I fix it?

My program

#include  BigInt BigInt::operator+(const BigInt& rhs)const { if(this->sign == ZERO) { return rhs; } else if(rhs.sign == ZERO) { return *this; } else if(this->sign == rhs.sign) { // Make copies of both *this and rhs to be manipulated // Also Create the BigInt object 'sum' BigInt augend(*this); BigInt addend(rhs); BigInt sum; if(augend.digits.size() > addend.digits.size()) { addend.digits.resize(augend.digits.size(), '0'); } else if(augend.digits.size() < addend.digits.size()) { augend.digits.resize(addend.digits.size(), '0'); } for(uint i = 0; i < augend.digits.size(); i++) { augend.digits[i] -= '0'; addend.digits[i] -= '0'; } int carry = 0; for(uint i = 0; i < augend.digits.size(); i++) { int digit = augend.digits[i] + addend.digits[i] + carry; carry = 0; if(digit >= 10) { carry = digit / 10; digit = digit % 10; } sum.digits.push_back(digit + '0'); } return sum; } else { BigInt minu; BigInt subtra; BigInt diff; if(*this < rhs) { minu = rhs; subtra = *this; minu.sign = POSITIVE; subtra.sign = POSITIVE; subtra.digits.resize(minu.digits.size(), '0'); diff.sign = rhs.sign; } else { minu = *this; subtra = rhs; minu.sign = POSITIVE; subtra.sign = POSITIVE; subtra.digits.resize(minu.digits.size(), '0'); diff.sign = this->sign; } for (uint i = 0; i < minu.digits.size(); i++) { minu.digits[i] -= '0'; subtra.digits[i] -= '0'; } int borrow = 0; for(uint i = 0; i < minu.digits.size(); i++) { int digit = borrow + minu.digits[i] - subtra.digits[i]; borrow = 0; if(digit < 0) { borrow--; digit = digit + 10; } diff.digits.push_back(digit + '0'); } return diff; } } BigInt BigInt::operator-(const BigInt& rhs)const { BigInt addend(rhs); if(addend.sign == NEGATIVE) { addend.sign = POSITIVE; } else if(addend.sign == POSITIVE) { addend.sign = NEGATIVE; } return *this + addend; } 

The header file for the class

#ifndef LAB46_H #define LAB46_H #include  #include  #include  using namespace std; typedef enum {NEGATIVE, ZERO, POSITIVE} Sign; bool isInt(string s); class BigInt {  friend ostream& operator<<( ostream& output, const BigInt& );  friend istream& operator>>( istream& input, BigInt& ); public:  BigInt();                   // constructor; digits = 0  BigInt( int num );              // constructor; digits = num  BigInt( const string str );          // constructor; digits = str  BigInt( const BigInt& other );        // copy constructor  bool  operator==( const BigInt& rhs ) const; // Equality  bool  operator< ( const BigInt& rhs ) const; // Less Than  BigInt operator+ ( const BigInt& rhs ) const; // Addition  BigInt operator- ( const BigInt& rhs ) const; // Subtraction private:  Sign sign;                  // Sign of #  deque<char> digits;              // Deque of digits of # }; #endif 

The main function to test the program

#include  using namespace std; int main() {  BigInt a, b;  while (cin >> a >> b)  {   cout << a << " + " << b << " = " << a + b << endl;   cout << a << " - " << b << " = " << a - b << endl;   cout << b << " - " << a << " = " << b - a << endl;  }  cout << endl;  cout << "Fibonacci Sequence" << endl;  BigInt i(1), end(51), first(1), second(1);  while (i < end)  {   cout << "Fib(" << i << ") = " << first << endl;   second = first + second;   first = second - first;   i = i + 1;  }  return EXIT_SUCCESS; }