Go: package main

import (
	"bufio"
	"fmt"
	"math/big"
	"os"
	"strconv"
)

var scanner = bufio.NewScanner(os.Stdin)

func nextString() string {
	scanner.Scan()
	return scanner.Text()
}

func nextInt() int {
	n, err := strconv.Atoi(nextString())
	if err != nil {
		fmt.Printf("strconv.Atoi failed: %v\n", err)
	}
	return n
}

func main() {
	scanner.Split(bufio.ScanWords)

	const maxCapacity = 512 * 1024
	buf := make([]byte, maxCapacity)
	scanner.Buffer(buf, maxCapacity)

	n := nextInt()
	maxint, _ := new(big.Int).SetString("100000000000000000000000000000000000000000000000000000000000000000000000000000000", 10)

	for i := 0; i < n; i++ {
		aStr := nextString()
		bStr := nextString()

		a, _ := new(big.Int).SetString(aStr, 10)
		b, _ := new(big.Int).SetString(bStr, 10)
		c := big.NewInt(0).Add(a, b)

		if c.Cmp(maxint) < 0 {
			fmt.Println(c)
		} else {
			fmt.Println("overflow")
		}
	}
}

C++: #include<iostream>
#include<string>
#include<iomanip>
#include<cmath>
using namespace std;

int main(){

	int n;

	string a, b;
	char N1[80], N2[80];
	char sum[80], temp[82];
	string ans;

	cin >> n;

	for( int i=0;i<n;i++ ){

		cin >> a;
		
		cin >> b;

		if( a.length() > 80 ){
			cout << "overflow" << endl;
			continue;
		}

		if( b.length() > 80 ){
			cout << "overflow" << endl;
			continue;
		}

		for( int i=0;i<80;i++ ){
			N1[i] = -1; N2[i] = -1; sum[i] = 0;
		}

		for( int i=1;i<=a.length();i++ )
			N1[i] = a[a.length()-i] - '0';
		for( int i=1;i<=b.length();i++)
			N2[i] = b[b.length()-i] - '0';

		int over = 0;
		
		for( int i=0;i<80;i++ ){


			if( N1[i] == -1 && N2[i] == -1 ){
				if( over >= 10 ){
					sum[i] = over / 10;
					over = 0;
				}else
					sum[i] = -1;
				continue;
			}

			if( N1[i] != -1 )
				sum[i] += N1[i];
			if( N2[i] != -1 )
				sum[i] += N2[i];

			sum[i] += over / 10;

			over = floor((double)sum[i]/10)*10;

			sum[i] -= over;
			
		}

		if(over > 0)cout << "overflow" << endl;
		else{
			int j=0;
			for( int i=1;i<80;i++ ){
				if( sum[80-i] >= 0 ){
					temp[j] = sum[80-i] + '0';
					j++;
					continue;
				}
				
			}
			temp[j] = '\0';

			ans = temp;

			


			cout << ans << endl;
		}

	}
	return 0;
}

C++: #include<iostream>
#include<string>
using namespace std;
int main(){
    int n,k=0;
    for(cin>>n;n--;){
        string a,b;
        cin >> a;
        cin >> b;
        if(a.size() < b.size()) swap(a,b);//????????????a>b?????????
        while(b.size() < a.size()) b += '0';//????????????a==b?????????
        for(int i=a.size()-1;i>=0;i--){//???????????????????????????
            a[i] += b[i] + k;
            if(a[i] > '9'){
                a[i]-=10;
                k = 1;
            }
            else k = 0;
        }
        if(k)   a+="1";//??°??????????????????????????????????¶?????????¨???
        if(a.size() > 80)   cout << "overflow" << endl;
        else    cout << a << endl;
    }
    return 0;
}

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 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<cctype>
#include<cstring>
using namespace std;
int main(){
  int o,a,sl,ss,ansl[82],anss[82];
  cin >> o;
  char n[100],n2[100],nl[82],ns[82],c[80];
  cin.getline(c,80); 
  for(int i=0;i<o;i++){
    cin.getline(n,100);
    cin.getline(n2,100);
    if(strlen(n)>80||strlen(n2)>80){
      cout << "overflow" << endl;
      continue;
    }
    a = 0;
    if( strlen(n)>strlen(n2) ){
      sl = strlen(n);     
      ss = strlen(n2);
      for(int i=sl-1;i>=0;i--){
	nl[a] = n[i];
	a++;
      }
      a = 0;
      for(int i=ss-1;i>=0;i--){
	ns[a] = n2[i];
	a++;
      }
    }
    else{
      sl = strlen(n2);
      ss = strlen(n);
      for(int i=sl-1;i>=0;i--){
	nl[a] = n2[i];
	a++;
      }
      a = 0;
      for(int i=ss-1;i>=0;i--){
	ns[a] = n[i];
	a++;
      }
    }
    for(int i=0;i<sl;i++) ansl[i] = nl[i]-'0';
    for(int i=0;i<ss;i++) anss[i] = ns[i]-'0';
    for(int i=0;i<ss;i++) ansl[i] += anss[i];
    for(int i=0;i<sl;i++){
      if(ansl[i]>=10){
	ansl[i] -= 10;
	if(i+1<sl) ansl[i+1] += 1;
	else{
	  ansl[i+1] = 1;
	  sl += 1;
	}
      }
    }
    if(sl>80) cout << "overflow";
    else  for(int i=sl-1;i>=0;i--) cout << ansl[i];
    cout << endl;
  }
  return 0;
}