BigInt implementation

//Trần Việt Anh - 7/5/2019 - 10:04PM
#include<iostream>
#include<cstring>

using namespace std;

class BigInt {
    string value;
    int sign;
public:
    BigInt();
    BigInt(const char*);
    BigInt(int );
    BigInt(const BigInt& );

    friend ostream& operator<< (ostream& , const BigInt& );

    BigInt& operator=(int );
    BigInt& operator=(const char*);
    BigInt& operator=(const BigInt& );

    friend BigInt operator+(const BigInt& , const BigInt & );
    friend BigInt operator+(const BigInt& , int );
    friend BigInt operator+(int , const BigInt &);

    friend BigInt operator-(const BigInt& , const BigInt & );
    friend BigInt operator-(const BigInt& , int );
    friend BigInt operator-(int , const BigInt &);

    BigInt& operator+=(int );
    BigInt& operator+=(const BigInt& );

    BigInt& operator-=(int );
    BigInt& operator-=(const BigInt& );
};

ostream& operator<<(ostream& os, const BigInt& num) {
    if (num.sign == 0) cout << "-";
    int startIndex = 0;
    while (num.value[startIndex] == '0' && startIndex < num.value.length() - 1) startIndex++;
    for (int i = startIndex; i < num.value.length(); i++) cout << num.value[i];
    return os;
}

BigInt::BigInt() {
    this -> value = "0";
    this -> sign = 1;
}

BigInt::BigInt(const char *num) {
    this -> value = ""; int startIndex;
    if (num[0] == '-') {
        this -> sign = 0; startIndex = 1;
    }
    else {
        this -> sign = 1; startIndex = 0;
    }
    for (int i = startIndex; i < strlen(num); i++)
        this -> value += num[i];
}

BigInt::BigInt(int num) {
    if (num < 0) {
        this -> sign = 0;
        num *= -1;
    }
    else this -> sign = 1;
    this -> value = "";
    while (num != 0) {
        int value = num % 10;
        this -> value = char(value + 48) + this -> value;
        num /= 10;
    }
}

BigInt::BigInt(const BigInt& other) {
    this -> value = other.value;
    this -> sign = other.sign;
}

BigInt& BigInt::operator=(int num) {
    if (num < 0) {
        this -> sign = 0;
        num *= -1;
    }
    else this -> sign = 1;
    this -> value = "";
    while (num != 0) {
        int value = num % 10;
        this -> value = char(value + 48) + this -> value;
        num /= 10;
    }
    return *this;
}

BigInt& BigInt::operator=(const BigInt& num) {
    this -> value = num.value;
    this -> sign = num.sign;
    return *this;
}

BigInt& BigInt::operator=(const char *num) {
    this -> value = ""; int startIndex;
    if (num[0] == '-') {
        this -> sign = 0; startIndex = 1;
    }
    else {
        this -> sign = 1; startIndex = 0;
    }
    for (int i = startIndex; i < strlen(num); i++)
        this -> value += num[i];
    return *this;
}

BigInt operator+(const BigInt& num, const BigInt& other)
{
    BigInt result;
    if (num.sign == 1 && other.sign == 0) {
        BigInt newNum(other); newNum.sign = 1;
        return num - newNum;
    }
    if (num.sign == 0 && other.sign == 1) {
        BigInt newNum(num); newNum.sign = 1;
        return other - newNum;
    }
    if (num.sign == 0 && other.sign == 0) result.sign = 0;
    else result.sign = 1;
    string num1 = num.value, num2 = other.value;
    while (num1.length() < num2.length()) num1 = "0" + num1;
    while (num2.length() < num1.length()) num2 = "0" + num2;
    result.value = num1;
    int remember = 0;
    for (int i = num1.length() - 1; i >= 0; i--) {
        int value = num1[i] - 48 + num2[i] - 48 + remember;
        result.value[i] = value % 10 + 48;
        remember = value / 10;
    }
    if (remember != 0) result.value = "1" + result.value;
    return result;
}

BigInt operator+(const BigInt& num, int other) {
    return num + BigInt(other);
}

BigInt operator+(int num, const BigInt& other) {
    return BigInt(num) + other;
}

bool isBiggerThanOrEqual(string s1, string s2) {
    if (s1.length() > s2.length()) return true;
    else if (s1.length() < s2.length()) return false;
    for (int i = 0; i < s1.length(); i++) {
        if (s1[i] < s2[i]) return false;
        else if (s1[i] > s2[i]) return true;
    }
    return true;
}

BigInt operator-(const BigInt& num, const BigInt& other)
{
    BigInt result;
    if (num.sign == 0 && other.sign == 1) {
        BigInt newNum(num); newNum.sign = 1;
        result = newNum + other;
        result.sign = 0;
        return result;
    }
    if (num.sign == 1 && other.sign == 0) {
        BigInt newNum(other); newNum.sign = 1;
        return num + newNum;
    }
    if (num.sign == 0 && other.sign == 0) {
        BigInt newNum1(num); newNum1.sign = 1;
        BigInt newNum2(other); newNum2.sign = 1;
        BigInt result(newNum1 + newNum2);
        result.sign = 0;
        return result;
    }
    string num1 = num.value, num2 = other.value;
    if (!isBiggerThanOrEqual(num1, num2)) {
        string mid = num1; num1 = num2; num2 = mid;
        result.sign = 0;
    }
    else result.sign = 1;
    while (num1.length() < num2.length()) num1 = "0" + num1;
    while (num2.length() < num1.length()) num2 = "0" + num2;
    result.value = num1;
    int remember = 0;
    for (int i = num1.length() - 1; i >= 0; i--) {
        int value = (num1[i] - 48) - (num2[i] - 48) + remember;
        if (value < 0) {
            result.value[i] = value + 10 + 48;
            remember = -1;
        }
        else {
            result.value[i] = value + 48;
            remember = 0;
        }
    }
    return result;
}

BigInt operator-(const BigInt& num, int other)
{
    return num - BigInt(other);
}

BigInt operator-(int num, const BigInt& other)
{
    return BigInt(num) - other;
}

BigInt& BigInt::operator+=(int num)
{
    BigInt result(*this + BigInt(num));
    *this = result;
    return *this;
}

BigInt& BigInt::operator+=(const BigInt& num)
{
    BigInt result(*this + num);
    *this = result;
    return *this;
}

BigInt& BigInt::operator-=(int num)
{
    BigInt result(*this - BigInt(num));
    *this = result;
    return *this;
}

BigInt& BigInt::operator-=(const BigInt& num)
{
    BigInt result(*this - num);
    *this = result;
    return *this;
}

int main() {
    cout << BigInt() << endl;
}