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 <stdio.h>
#include <string.h>

int max(int a, int b) {
  return a > b ? a : b;
}

int main(void) {
  int i, ans = 0, j, n;
  char a[110], b[110];
  scanf("%d", &n);
  for(i = 0; i < n; ++i) {
    scanf("%s\n%s", a, b);
    if(strlen(a) > 80 || strlen(b) > 80) {
      printf("overflow\n");
      continue;
    }
    for(j = 0; j < strlen(a); ++j) a[j] -= '0' - 1;
    for(j = 0; j < strlen(b); ++j) b[j] -= '0' - 1;
    /*for(j = 0; j < strlen(a); ++j) printf("%d", (int)a[j] - 1);
    printf("\n");
    for(j = 0; j < strlen(b); ++j) printf("%d", (int)b[j] - 1);
    printf("\n");*/
    if(strlen(a) < strlen(b)) {
      char t[110];
      for(j = 0; j <= strlen(a); ++j) t[j] = a[j];
      for(j = 0; j <= strlen(b); ++j) a[j] = b[j];
      for(j = 0; j <= strlen(t); ++j) b[j] = t[j];
    }
    char t[110];
    int st = 0;
    for(j = 0; j <= strlen(b); ++j) t[j] = b[j];
    for(j = 0; j <= strlen(a); ++j) b[j] = 1;
    for(j = strlen(a) - strlen(t); j <= strlen(a); ++j) b[j] = t[j - strlen(a) + strlen(t)];
    /*for(j = 0; j < strlen(a); ++j) printf("%d", (int)a[j] - 1);
    printf("\n");
    for(j = 0; j < strlen(a); ++j) printf("%d", (int)b[j] - 1);
    printf("\n");*/
    for(j = strlen(a) - 1; j >= 0; --j) {
      a[j] = a[j] + b[j] - 1;
      if(a[j] >= 11) {
        if(j) {
          a[j - 1]++;
          a[j] -= 10;
        } else {
          st = 1;
          a[j] -= 10;
        }
      }
    }
    if(st) {
      if(strlen(a) == 80) {
        printf("overflow\n");
        continue;
      } else printf("1");
    }
    for(j = 0; j < strlen(a); ++j) printf("%d", (int)a[j] - 1);
    printf("\n");
  }
  return 0;
}

C++: #include <iostream>
#include <cstdio>
#include <cstdlib>
#include <string>

using namespace std;

int main()
{
	int N; cin >> N;

	for(int i=0; i<N; i++){
		int a[80], b[80], c[80];
		string s, t, m;
		cin >> s >> t;
		if((s.length() > 80)||(t.length()>80)){
			cout << "overflow" << endl;
		}
		else{
			for(int j=0; j<80; j++){ a[j]=0; b[j]=0; c[j]=0;}
			for(int j=s.length()-1, k=0; j>=0; j--, k++){
				m=s.substr(k,1);
				if(m=="0"){ a[j]=0;}
				else if(m=="1"){ a[j]=1;}
				else if(m=="2"){ a[j]=2;}
				else if(m=="3"){ a[j]=3;}
				else if(m=="4"){ a[j]=4;}
				else if(m=="5"){ a[j]=5;}
				else if(m=="6"){ a[j]=6;}
				else if(m=="7"){ a[j]=7;}
				else if(m=="8"){ a[j]=8;}
				else{ a[j]=9;}
			}
			for(int j=t.length()-1, k=0; j>=0; j--,k++){
				m=t.substr(k,1);
				if(m=="0"){ b[j]=0;}
				else if(m=="1"){ b[j]=1;}
				else if(m=="2"){ b[j]=2;}
				else if(m=="3"){ b[j]=3;}
				else if(m=="4"){ b[j]=4;}
				else if(m=="5"){ b[j]=5;}
				else if(m=="6"){ b[j]=6;}
				else if(m=="7"){ b[j]=7;}
				else if(m=="8"){ b[j]=8;}
				else{ b[j]=9;}
			}

			int d=0;
			bool flag=false;
			for(int j=0; j<80; j++){
				c[j] = (a[j]+b[j]+d)%10;
				d = (a[j]+b[j]+d)/10;
				if(j==79 && d>0){ flag=true;}
			}
		
			if(flag){cout << "overflow";}	
			else{
					flag = false;
					for(int j=79; j>=0; j--){
						if(flag){ cout << c[j];}
						else if(c[j] != 0){ cout << c[j]; flag=true;}
					}
			}
			cout << endl;
		}
	}

	return 0;
}

C++: #include <iostream>
#include <vector>
#include<string>
#include<sstream>
#define CAPACITY 9


namespace jisaku{//stoi?????¨?????????????????????????????????????????§??£?¨???????

	int stoi(std::string str){
		int ret;
		std::stringstream ss;
		ss << str;
		ss >> ret;
		return ret;
	}

     std::string to_string(int i){
		 std::string text;
		std::stringstream textline;
		textline<< i;
		textline >> text;
		return text;
	}

}




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_rest,str_Max;

	str_rest = str.length() % CAPACITY;
	str_Max = str.length() / 9;

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

	}
	//?????????????????????????????????????????\??????

	if (str_rest != 0)num_str[loc - 1] = jisaku::stoi(str.substr(0, str_rest));

	for (int j = 0; j < CAPACITY; j++){
		num[j] = num_str[j];
	}
}
//80????????§????¶???????
void OverDigit::Add(){

	int num_ans[CAPACITY] = {};
	int fat, overflow = 0, count = 0;
	std::string text,t;


	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++;
				t = jisaku::to_string(num_ans[j]);
				if (t.length() != 9){
					for (int l = 0; l < (9 - t.length()); l++){
						text += "0";
					}
				}
				text += t;


			}
			else if (num_ans[j] == 0 && count >= 1){
					text+="000000000";
						}

		}std::cout <<text<< std::endl;
	}

};


int main(){

	
	//vector???????????¨??????80???????¨????
	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);
		if (str_a == "0" && str_b == "0"){std::cout << '0' << std::endl;
		}
		else if (str_a.length() >= 81 || str_b.length() >= 81){
		
			std::cout << "overflow" << std::endl;
		}
		else{

			OverDigit OverDigit_a(str_a, str_b);//???????????§??????
			OverDigit_a.Add();
		}
	}
	return 0;
}

C++: #include <iostream>
#include <string>
#include <cstdlib>
#include <sstream>
#include <math.h>
using namespace std;

int main()
{
	int n;
	cin >> n;
	for (int kk = 0; kk < n; ++kk)
	{
		int A[1001] = {},B[1001] = {},C[1001] = {},keta = 0;
		string in1,in2;
		cin >> in1 >> in2;
		if(in1.size() > 80 || in2.size() > 80)
		{
			cout << "overflow" << endl;
			continue;
		}
		for (int i = 0; i < in1.size(); ++i)
			A[in1.size()-i] = (int)(in1[i]-'0');
		for (int i = 0; i < in2.size(); ++i)
			B[in2.size()-i] = (int)(in2[i]-'0');
		bool flag = 1;
		for (int i = 1; i <= 1000; ++i)
		{
			C[i] += A[i]+B[i];
			if(C[i] >= 10)
			{
				C[i+1] += C[i]/10;
				C[i] -= C[i]/10*10;
			}
			if(C[i] != 0)
				keta = i;
		}
		if(keta > 80)
		{
			cout << "overflow" << endl;
			continue;
		}
		for (int i = keta; i > 0; --i)
		{
			cout << C[i];
		}
		cout << endl;
	}
}