Go: package main

import (
        "fmt"
        "bufio"
        "os"
)

func main() {
        var n int
        var a, b string
        sc := bufio.NewScanner(os.Stdin)
        fmt.Scanf("%d", &n)
        for i := 0; i < n; i++ {
                sc.Scan()
                a = sc.Text()
                sc.Scan()
                b = sc.Text()

                if len(a) > len(b) {
                        s := add(a, b)
                        if s[len(a)] > 0 {
                                if len(a) >= 80 {
                                        fmt.Printf("overflow")
                                } else {
                                        for i := 0; i <= len(a);i++ {
                                                fmt.Printf("%d", s[len(a)-i])
                                        }
                                }
                                fmt.Printf("\n")
                        } else {
                                if len(a)-1 >= 80 {
                                        fmt.Printf("overflow")
                                } else {
                                        for i := 0; i < len(a);i++ {
                                                fmt.Printf("%d", s[len(a)-i-1])
                                        }
                                }
                                fmt.Printf("\n")
                        }
                } else {
                        s := add(b, a)
                        if s[len(b)] > 0 {
                                if len(b) >= 80 {
                                        fmt.Printf("overflow")
                                } else {
                                        for i := 0; i <= len(b);i++ {
                                                fmt.Printf("%d", s[len(b)-i])
                                        }
                                }
                                fmt.Printf("\n")
                        } else {
                                if len(b)-1 >= 80 {
                                        fmt.Printf("overflow")
                                } else {
                                        for i := 0; i < len(b);i++ {
                                                fmt.Printf("%d", s[len(b)-i-1])
                                        }
                                }
                                fmt.Printf("\n")
                        }
                }
        }
}

// expects ln > lm
func add(n string, m string) (s [100]int) {
        ln := len(n)
        lm := len(m)
        var sum [100]int

        for i := 0; i < ln; i++ {
                var s, t int
                s = int(n[ln-1-i]) - int('0')

                if lm - 1 - i >= 0 {
                        t = int(m[lm-1-i]) - int('0')
                } else {
                        t = 0
                }

                sum[i+1] = (s + t + sum[i]) / 10
                sum[i] = (s + t + sum[i]) % 10
        }

        return sum
}

C++: #include <iostream>
#include <vector>
using namespace std;

int main() {
  int n; cin >> n;
  for (int i = 0; i < n; i++) {
    string a, b;
    cin >> a >> b;
    vector<int> va, vb;
    for (int i = a.size() - 1; i >= 0; i--) va.push_back(a[i] - '0');
    for (int i = b.size() - 1; i >= 0; i--) vb.push_back(b[i] - '0');
    vector<int> vc;
    int rem = 0;
    for (int i = 0; i < va.size() || i < vb.size(); i++) {
      if (i < va.size()) rem += va[i];
      if (i < vb.size()) rem += vb[i];
      vc.push_back(rem % 10);
      rem = rem / 10;
    }
    if (rem) vc.push_back(rem);
    if (vc.size() > 80) {
      cout << "overflow" << endl;
    } else {
      for (int i = vc.size() - 1; i >= 0; i--) cout << vc[i];
      cout << endl;
    }
  }
}

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[])
{
	//std::vector <int> num_str(CAPACITY, 0);
	int num_str[CAPACITY*2];
	for (int j = 0; j < CAPACITY*2; j++) { num_str[j]=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[k] = 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;
}

C++: #include<iostream>
#include<algorithm>
#include<string>


#define MAX 1000


using namespace std;


int main()
{
	int n = 0;
	string in[2];
	char out[MAX] = {NULL};

	cin >> n;

	for (int i = 0; i < n; i++){
		cin >> in[0] >> in[1];

		for (int i = 0; i < MAX; i++){
			out[i] = '\0';
		}

		int count = in[0].size() - 1;

		for (int i = 0; i < in[0].size(); i++){
			out[count--] += in[0][i];
		}

		count = in[0].size();
		int count2 = in[1].size() - 1;

		count = max(count , count2) - 1;

		for (int i = 0; i < in[1].size(); i++){
			if (out[i] != '\0'){
				out[i] -= '0';
			}
			out[i] += in[1][count2--];
			count--;
		}
		
		int i = 0;
		for (i = 0; out[i] != '\0'; i++){
			;
		}

		bool flag = false;

		/*
		for (i--; i >= 0; i--){
			if (out[i] > '9'){
				out[i] -= 10;
				if (i != 0){
					out[i - 1] += 1;
				}else{
					flag = true;
				}
			}
		}
		*/

		for (int j = 0; j < i; j++){
			if (out[j] > '9'){
				out[j] -= 10;
				out[j + 1] += 1;
			}
		}

		if (out[i] > '\0' && out[i] < '0'){
			out[i] += '0';
		}
		
		for (i = 0; i < out[i] != '\0'; i++){
			;
		}

		for (int j = 0; j < i / 2; j++){
			char tmp = out[j];
			out[j] = out[i - j - 1];
			out[i - j - 1] = tmp;
		}

		if (i >= 80 || (flag == true && i >= 79)){
			cout << "overflow" << endl;
		}else{
			if (flag){
				//cout << "1";
			}
			cout << out << endl;
		}
	}

	return 0;
}