bignum library in C++ - bignum.h

//a bignum library written in C++, it can (in theory) calculate with 4 billion digit long numbers
//this is mostly for demonstration purposes, if you need performance, use something like GNU arbitrary precision library
//Implemented by: http://www.youtube.com/user/thecplusplusguy
//this might be an older version, I have to check
#ifndef BIGNUM_H
#define BIGNUM_H
#include <iostream>
#include <cmath>
#include <cstring>

class bignum{
    int* num;
    int size;
    bool minus;
    void remainder();
    public:
    bignum();
    bignum(int);
    bignum(long);
    bignum(const bignum& b);
    bignum(const char* c);
    bignum(std::string& c);
    ~bignum();
    int getSize() const;
    int* getNum() const;
    int getNumSize() const;
    int numLength(int a);

    void allocate(int s);

    friend std::ostream& operator<<(std::ostream& os,const bignum& b);
    friend bignum& operator++(const bignum& bn);
    friend bignum& operator--(const bignum& bn);

    //compare

    bool operator==(const bignum& b) const;
    bool operator==(int n) const;
    bool operator==(const char* c) const;

    bool operator>(const bignum& b) const;
    bool operator>(int n) const;
    bool operator>(const char* c) const;

    bool operator<(const bignum& b) const;
    bool operator<(int n) const;
    bool operator<(const char* c) const;

    bool operator<=(const bignum& b) const;
    bool operator<=(int n) const;
    bool operator<=(const char* c) const;

    bool operator>=(const bignum& b) const;
    bool operator>=(int n) const;
    bool operator>=(const char* c) const;

    bool operator!=(const bignum& b) const;
    bool operator!=(int n) const;
    bool operator!=(const char* c) const;


    //operations

    bignum& operator=(int num);
    bignum& operator=(const char* c);
    bignum& operator=(const bignum& b);

    bignum operator+(int num);
    bignum operator+(const bignum& bn);
    bignum operator+(const char* c);

    bignum operator-(int num);
    bignum operator-(const bignum& bn);
    bignum operator-(const char* c);


    bignum operator*(int num);
    bignum operator*(const bignum& bn);
    bignum operator*(const char* c);


    bignum operator/(int num);
    bignum operator/(const bignum& bn);
    bignum operator/(const char* c);

    bignum operator%(int num);
    bignum operator%(const bignum& bn);
    bignum operator%(const char* c);

    bignum& operator/=(int num);
    bignum& operator/=(const bignum& bn);
    bignum& operator/=(const char* c);


    bignum& operator+=(int num);
    bignum& operator+=(const bignum& bn);
    bignum& operator+=(const char* c);

    bignum& operator-=(int num);
    bignum& operator-=(const bignum& bn);
    bignum& operator-=(const char* c);

    bignum& operator*=(int num);
    bignum& operator*=(const bignum& bn);
    bignum& operator*=(const char* c);

    bignum& operator%=(int num);
    bignum& operator%=(const bignum& bn);
    bignum& operator%=(const char* c);

    bignum& operator++();
    bignum& operator--();
};
#endif