Посоветуйте функции для арифметики больших целых чисел

#pragma once
 
#include <iostream>
#include <string>
 
class BigInt
{
private:
    std::string value = "0";
    bool sign = false;
public:
    BigInt()=default;
    BigInt(const std::string&);
    BigInt(const char*);
 
    BigInt& operator=(const char*);
    BigInt& operator+=(const BigInt&);
    BigInt& operator-=(const BigInt&);
    BigInt& operator*=(const BigInt&);
    BigInt& operator/=(const BigInt&);
    BigInt& operator%=(const BigInt&);
    BigInt& operator^=(const int);
 
    const std::string& get_value() const;
    bool get_sign() const;
    
    friend std::ostream& operator<<(std::ostream&, const BigInt&);
 
    friend BigInt operator+(const BigInt&, const BigInt&);
    friend BigInt operator-(const BigInt&, const BigInt&);
    friend BigInt operator-(const BigInt&);
    friend BigInt operator*(const BigInt&, const BigInt&);
    friend BigInt operator/(const BigInt&, const BigInt&);
    friend BigInt operator%(const BigInt&, const BigInt&);
    friend BigInt operator^(const BigInt&, const int);
 
    friend bool operator>=(const BigInt&, const BigInt&);
    friend bool operator<=(const BigInt&, const BigInt&);
    friend bool operator==(const BigInt&, const BigInt&);
    friend bool operator!=(const BigInt&, const BigInt&);
    friend bool operator>(const BigInt&, const BigInt&);
    friend bool operator<(const BigInt&, const BigInt&);
    
    friend BigInt Abs(const BigInt&);
    friend BigInt Factorial(const int);
    friend BigInt GCD(const BigInt&, const BigInt&);
    friend BigInt LCM(const BigInt&, const BigInt&);
    friend BigInt ISqrt(const BigInt&);
    friend BigInt Fibonacci(const int);
};
 
inline BigInt Add(const BigInt& x, const BigInt& y){return x + y;}
inline BigInt Subtract(const BigInt& x, const BigInt& y){return x - y;}
inline BigInt Minus(const BigInt& x){return -x;}
inline BigInt Multiply(const BigInt& x, const BigInt& y){return x * y;}
inline BigInt Divide(const BigInt& x, const BigInt& y){return x / y;}
inline BigInt Remainder(const BigInt& x, const BigInt& y){return x % y;}
inline BigInt Pow(const BigInt& x, const int y){return x ^ y;}
 
inline bool EqualQ(const BigInt& x, const BigInt& y){return x == y;}
inline bool NotEqualQ(const BigInt& x, const BigInt& y){return x != y;}
inline bool GreaterQ(const BigInt& x, const BigInt& y){return x > y;}
inline bool LessQ(const BigInt& x, const BigInt& y){return x < y;}
inline bool GreaterEqualQ(const BigInt& x, const BigInt& y){return x >= y;}
inline bool LessEqualQ(const BigInt& x, const BigInt& y){return x <= y;}
 
BigInt Abs(const BigInt&);
BigInt Factorial(const int);
BigInt GCD(const BigInt&, const BigInt&);
BigInt LCM(const BigInt&, const BigInt&);
BigInt ISqrt(const BigInt&);
BigInt Fibonacci(const int);