main.cpp

#include <iostream>
#include "some_header.h"

int main()
{
	inf_int x1(19);
	inf_int x2("125");
	inf_int x3;
	inf_int x4(-97);
	inf_int x5("-35");

	x2.deletFirstDigit();
	x3.setValue(42);
	x1.add(x2);
	x1.changeSign();
	x1.print();
	x3.concatenate(x2);

	x4 = x3;

	x3++;

	x5 = x3 + x2; // The return value isnt 4129 because we did x4 = x3. Meaning that x4 is not -97 anymore xD
	return 0;
}

some_class.cpp

#include "some_header.h"
#include <iostream>

inf_int::inf_int(int n)
{
	int nDigit = GetNumberOfDigits(n);
	thesign = GetSign(n);

	int strLen = thesign ? nDigit : nDigit + 1;
	digit = new char[strLen + 1];
	sprintf(digit, "%d", n);
	digit[strLen] = '\0'; // NULL terminator
	length = nDigit;
}

inf_int::inf_int(const char * s)
{
	digit = new char[strlen(s) + 1];
	strcpy(digit, s);
	digit[strlen(s)] = '\0'; // NULL terminator
	length = strlen(s);
	thesign = GetSign(s);
}

inf_int::inf_int()
{
	digit = new char[2];
	strcpy(digit, "0");
	digit[1] = '\0';
	length = 1;
	thesign = false;
}

inf_int::inf_int(const inf_int & x)
{
	*this = x; // calls assignment op
}

inf_int::~inf_int()
{
	delete[] digit;
}

inf_int & inf_int::operator=(const inf_int & rhs)
{
	int n = atoi(rhs.digit);
	int nDigit = GetNumberOfDigits(n);
	thesign = GetSign(n);

	int strLen = thesign ? nDigit : nDigit + 1;
	digit = new char[strLen + 1];
	sprintf(digit, "%d", n);
	digit[strLen] = '\0'; // NULL terminator
	length = nDigit;
	return *this;
}

inf_int & inf_int::operator++(int)
{
	// TODO: insert return statement here
	int current = atoi(digit);
	current += 1;

	inf_int tmp(current);
	*this = tmp; // calls assignment op
	return *this;
}

inf_int & inf_int::operator+(const inf_int & rhs)
{
	// TODO: insert return statement here
	int current = atoi(rhs.digit);
	current += atoi(digit);

	inf_int tmp(current);
	*this = tmp; // calls assignment op

	return *this;
}

void inf_int::deletFirstDigit()
{
	for (int i = 1; i < length; i++)
	{
		digit[i - 1] = digit[i];
	}

	char* newDigit = new char[length]; // (oldLength - 1) + 1 // NULL terminator
	strcpy(newDigit, digit);
	newDigit[length - 1] = '\0';

	inf_int tmp(newDigit);
	*this = tmp;
}

void inf_int::setValue(int n)
{
	int nDigit = GetNumberOfDigits(n);
	thesign = GetSign(n);

	int strLen = thesign ? nDigit : nDigit + 1;
	digit = new char[strLen + 1];
	sprintf(digit, "%d", n);
	digit[strLen] = '\0'; // NULL terminator
	length = nDigit;
}

void inf_int::add(const inf_int & x)
{
	*this = *this + x; // use + operator for this
}

void inf_int::changeSign()
{
	int n = atoi(digit) * -1;
	int nDigit = GetNumberOfDigits(atoi(digit));
	thesign = !thesign;

	int strLen = thesign ? nDigit : nDigit + 1;
	digit = new char[strLen + 1];
	sprintf(digit, "%d", n);
	digit[strLen] = '\0'; // NULL terminator
	length = nDigit;
}

void inf_int::print()
{
	std::cout << digit << std::endl;
}

void inf_int::concatenate(const inf_int & x)
{
	int newLen = x.length + length;
	char* newDigits = new char[newLen + 1];
	newDigits[0] = '\0'; // strncat needs NULL terminator

	strncat(newDigits, digit, length);
	strncat(newDigits, x.digit, x.length);
	setValue(atoi(newDigits));
	delete[] newDigits;
}

some_header.h

#pragma once

#define _CRT_SECURE_NO_WARNINGS

#include <cstdlib>

class inf_int {
private:
	char* digit;
	unsigned int length;
	bool thesign;
public:
	inf_int(int n); // integer constructor
	inf_int(const char* s); // string constructor
	inf_int(); // default constructor
	inf_int(const inf_int& x); // copy constructor
	~inf_int(); // destructor

	inf_int& operator=(const inf_int& rhs); // assignment overload
	inf_int& operator++(int rhs); // ++ overload
	inf_int& operator+(const inf_int& rhs); // += overload

	void deletFirstDigit();
	void setValue(int n);
	void add(const inf_int& x);
	void changeSign();
	void print();
	void concatenate(const inf_int& x);

private:
	int GetNumberOfDigits(int n)
	{
		int digits = 0;
		while (n != 0) { n /= 10; digits++; }
		return digits;
	}

	bool GetSign(int n)
	{
		return n > 0 ? true : false;
	}

	bool GetSign(const char* s)
	{
		return GetSign(atoi(s));
	}
};