Language: C++

#include <iostream>
#include <vector>
#include<string>
#include<sstream>

#define CAPACITY 9

 
namespace stoi{
 
    int stoi(std::string str){
        int ret;
        std:: stringstream ss;
        ss << str;
        ss >> ret;
        return ret;
    }
 
}
class OverDigit
{
public:
    int num_a[CAPACITY];
    int num_b[CAPACITY];
    OverDigit(std::string str_a, std::string str_b);
    void ChangeDigit(std::string str,int num[]);
    void Add();
};
 
OverDigit::OverDigit(std::string str_a,std::string str_b)
{
    ChangeDigit(str_a, num_a); 
    ChangeDigit(str_b, num_b);
}

void OverDigit::ChangeDigit(std::string str, int num[])
{
td::vector <int> num_str(CAPACITY, 0);
    int loc = CAPACITY, k = 8;
    int str_Max, str_rest;

    str_Max = str.length() / CAPACITY;//
    str_rest = str.length() % CAPACITY;


	for (int j = str.length() - CAPACITY; str_Max > 0; j = j-CAPACITY)
	{
        num_str[0] = stoi::stoi(str.substr(j, CAPACITY));
        loc = k;
        k--;
        str_Max--;
    }

    if(str_rest!=0)
	{
		num_str[loc - 1] = stoi::stoi(str.substr(0, str_rest));
	}
 
    for (int j = 0; j < CAPACITY; j++)
	{
		num[j]=num_str[j];
    }
}

void OverDigit::Add()
{
    int num_ans[CAPACITY] = {};
    int fat,overflow=0,count=0;
 
    for (int j = CAPACITY-1; j >0; j--)
	{
        if (num_a[j] + num_b[j]<=999999999)
		{
            num_ans[j] = num_a[j] + num_b[j];
        }
        else
		 if (num_a[j] + num_b[j] > 999999999)
		{
            fat = (num_a[j] + num_b[j]) - 1000000000;
            num_ans[j] = fat;
            num_a[j - 1]++;     
        }
    }

    if (num_a[0] + num_b[0] > 99999999)
	{
        overflow = 1;
    }
    else
	{
		 num_ans[0]= num_a[0] + num_b[0];
    }
 
    if (overflow == 1)
	{
        std::cout << "overflow" << std::endl;
    }
    else
	{
        for (int j = 0; j < CAPACITY; j++)
		{
            if (num_ans[j] != 0)
			{
                count++;
                std::cout << num_ans[j];
            }
            else
			 if (num_ans[j] == 0 && count >= 1)
			{
                std::cout << "000000000";
            }
        }
		std::cout << std::endl;
    }
}
 
 
int main(){
    int N;
    std::string str_a, str_b;
 
    std::cin >> N;
    std::cin.ignore();
 
   for (int i = 0; i < N; i++){
        getline(std::cin, str_a);
        getline(std::cin, str_b);
        OverDigit OverDigit_a(str_a,str_b);
       OverDigit_a.Add();
   }
 
    return 0;
}

Compiler output:
a.cc: In member function 'void OverDigit::ChangeDigit(std::string, int*)':
a.cc:38:1: error: 'td' has not been declared
   38 | td::vector <int> num_str(CAPACITY, 0);
      | ^~
a.cc:38:13: error: expected primary-expression before 'int'
   38 | td::vector <int> num_str(CAPACITY, 0);
      |             ^~~
a.cc:48:9: error: 'num_str' was not declared in this scope; did you mean 'num_a'?
   48 |         num_str[0] = stoi::stoi(str.substr(j, CAPACITY));
      |         ^~~~~~~
      |         num_a
a.cc:56:17: error: 'num_str' was not declared in this scope; did you mean 'num_a'?
   56 |                 num_str[loc - 1] = stoi::stoi(str.substr(0, str_rest));
      |                 ^~~~~~~
      |                 num_a
a.cc:61:24: error: 'num_str' was not declared in this scope; did you mean 'num_a'?
   61 |                 num[j]=num_str[j];
      |                        ^~~~~~~
      |                        num_a