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 <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";
						}

		}

		int count = 0;
		for (int l = 0; l < text.length(); l++){
			if (count == 0 && text[l] == '0'){
			}
			else {
				std::cout << text[l];
				count++;
			}
		}
	}

};


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

#define BUF_SIZE 256

int main(void) {
	char buf[BUF_SIZE];
	char num1[100];
	char num2[100];
	char sum[100];
	char str[100];
	int n;
	int i, j, k;
	
	fgets(buf, sizeof(buf), stdin);
	sscanf(buf, "%d", &n);
	
	for (i=0; i<n; i++) {
		for (j=0; j<100; j++) {
			num1[j] = num2[j] = sum[j] = 0;
		}
		
		// first integer
		fgets(buf, sizeof(buf), stdin);
		sscanf(buf, "%s", &str);
		if (strlen(str) > 80) {
			printf("overflow\n");
			continue;
		}
		
		for (j=0; j<strlen(str); j++)  ;
		for (k=0; j>0; k++) {
			j--;
			num1[k] = str[j] - '0';
		}
		
		// second integer
		fgets(buf, sizeof(buf), stdin);
		sscanf(buf, "%s", &str);
		if (strlen(str) > 80) {
			printf("overflow\n");
			continue;
		}
		
		for (j=0; j<strlen(str); j++)  ;
		for (k=0; j>0; k++) {
			j--;
			num2[k] = str[j] - '0';
		}
		
		// sum
		for (j=0; j<80; j++) {
			sum[j] += num1[j]+num2[j];
			if (sum[j] >= 10) {
				sum[j+1]++;
				sum[j] = sum[j]%10;
			}
		}
		
		if (sum[80] != 0) {
			printf("overflow\n");
		}
		else {
			for (j=79; sum[j]==0 && j>=1; j--)  ;
			for (; j>=0; j--) {
				printf("%d", sum[j]);
			}
			printf("\n");
		}
	}
	
	return 0;
}

C++: #include <iostream>
#include <string.h>
#include <cstdio>
#include <cstring>
#include <algorithm>
using namespace std;
int main(){
  int n, maxlen;
  char a[100], b[100];

  cin >> n;
  for(int ii = 0 ; ii < n ; ii++){
    string ans = "";
    scanf("%s%s", a, b);
    
    maxlen = max(strlen(a), strlen(b));

    reverse(a, a+strlen(a));
    reverse(b, b+strlen(b));
    
    for(int i = strlen(a) ; i < 100 ; i++){
      a[i] = '0';
    }
    for(int i = strlen(b) ; i < 100 ; i++){
      b[i] = '0';
    }

    int tmp, tmp2 = 0;
    bool flag = 0;

    for(int i = 0 ; i < maxlen ; i++){
      tmp = (a[i] - '0') + (b[i] - '0') + tmp2;
      
      if(i == maxlen - 1){
	//cout << "OK" << endl;
	//tmp = (a[i] - '0') + (b[i] - '0') + tmp2;
	if(tmp >= 10){
	  tmp2 = 1;
	  tmp -= 10;
	  ans += tmp + '0';
	  ans += '1';
	}
	else ans += tmp + '0';
      }

      else if(tmp >= 10){
	tmp2 = 1;
	tmp -= 10;
	ans += tmp + '0';
      }
      else if(tmp <= 9){
	ans += tmp + '0';
	tmp2 = 0;
      }
    }

    reverse(ans.begin(), ans.end());
   
    if(ans.size() >= 80) cout << "overflow" << endl;
    else cout << ans << endl;
  }
  return 0;
}