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<cstdio>
#include<algorithm>
#include<string.h>
//#include<string>

using namespace std;

#define MAX(x,y) ((x>y)? x:y)


int n;//データセットの数

char str[2][10000];//	入力される整数文字列
int n_str[2];//長さを収納

char answer[85];

FILE *fp=stdin;

int main()
{
	fscanf(fp,"%d",&n);
	for(int i=0;i<n;i++)
	{
		sta:
		fscanf(fp,"%s%s",&str[0],&str[1]);
		for(int j=0;j<2;j++)n_str[j]=strlen(str[j]);
		int max =0,min=1;
		if(n_str[1]>n_str[0])
		{
			max=1;
			min=0;
		}
		else
		{
		int s = n_str[max]-n_str[min];
		char cha[85];
		fill(cha,cha+80,'0');
		for(int j=n_str[min];j>=0;j--)
		{
			cha[j+s]=str[min][j];
		}
		strcpy(str[min],cha);
		fill(answer,answer+85,0);
		for(int j=n_str[max]-1;j>=0;j--)
		{
			answer[j]=str[0][j]+str[1][j]-'0';
			if(answer[j]>'9')//繰り上がり時の処理
			{
				answer[j]-=10;
				if(j==0)
				{
					if(n_str[max]>=80)
					{
						n_str[max]++;
					}

					else printf("1");
				}
				else str[0][j-1]++;
			}
		}
		if(n_str[max]>80)
		{
			printf("overflow\n");
		}
		else printf("%s\n",answer);
		}
	}
	fclose(fp);
	return 0;
}

C++: 
#include <stdio.h>
#include <string.h>

int main(void){
  int N;
  scanf("%d", &N);

  int i,j,k;
  for(i=0; i<N; ++i){
    int digits[3][81];
    for(j=0; j<3; ++j){
      for(k=0; k<81; ++k){
        digits[j][k] = 0;
      }
    }

    int overflow = 0;
    for(j=0; j<2; ++j){
      char digit[80];
      scanf("%s", digit);
      if(overflow || strlen(digit)>80){
        overflow = 1;
      }else{
        for(k=0; k<strlen(digit); ++k){
          digits[j][k] = digit[strlen(digit)-1-k]-'0';
        }
      }
    }
    if(overflow){
      printf("overflow\n");
    }else{
      for(j=0; j<80; ++j){
        digits[2][j] += (digits[0][j]+digits[1][j]);
        digits[2][j+1] += digits[2][j]/10;
        digits[2][j] %= 10;
      }

      if(digits[2][80]>0){
        printf("overflow\n");
      }else{
        int flag = 0;
        for(j=80; j>=0; --j){
          if(flag || digits[2][j]!=0){
            flag = 1;
            printf("%d", digits[2][j]);
          }
        }
        printf("\n");
      }
    }
  }
  
  return 0;
}

C++: #include <bits/stdc++.h>

//const static double	de_PI	= 3.14159265358979323846;
//const static int	de_MOD	= 1000000007;
//const static int	de_MAX	= 999999999;
//const static int	de_MIN = -999999999;

inline std::string Add_MultiplePrecision(const std::string A, const std::string B) {

	int a[100] = {};
	int b[100] = {};
	int c[200] = {};

	for (unsigned int i = 0; i < A.length(); i++) {
		a[i] = A[A.length() - 1 - i] - '0';
	}
	for (unsigned int i = 0; i < B.length(); i++) {
		b[i] = B[B.length() - 1 - i] - '0';
	}

	int carry = 0;
	for (unsigned int i = 0; i < std::max(A.length(), B.length()); i++) {
		c[i] = (a[i] + b[i] + carry) % 10;
		c[i + 1] = carry = (a[i] + b[i] + carry) / 10;
	}

	int st = 199;
	while (c[st] == 0) {
		st--;
	}

	std::string ans;
	for (int i = st; i >= 0; i--) {
		ans += std::to_string(c[i]);
	}

	if (ans == "") {
		ans = "0";
	}

	return ans;
}

int main(void) {

	//std::ifstream in("123.txt");	std::cin.rdbuf(in.rdbuf());

	int n = 0;
	std::cin >> n;
	std::vector<std::string> ans(n);

	for (int i = 0; i < n; i++) {
		std::string temp1, temp2, temp3;
		std::cin >> temp1 >> temp2;
		temp3 = Add_MultiplePrecision(temp1, temp2);
		if (temp3.length() >= 80) {
			ans[i] = "overflow";
		}
		else {
			ans[i] = temp3;
		}
	}

	for (int i = 0; i < n; i++) {
		std::cout << ans[i] << std::endl;
	}
}

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

int main()
{
  int N;
  char str1[81];
  char str2[81];
  char s1[81];
  char s2[81];
  cin >> N;
  for(int i = 0; i < N; i++){
    cin >> str1;
    cin >> str2;
    int m1 = strlen(str1), m2 = strlen(str2);
    if(m1 > 80 || m2 > 80)
      cout << "overflow" << endl;
    else if(atoi(str1) + atoi(str2) < 2147483647 && atoi(str1) + atoi(str2) > 0)
      cout << atoi(str1) + atoi(str2) << endl; 
    else{
      if(m1 > m2){
	for(int i = 0; i < m1 - m2 + 1; i++)
	  s1[i] = '0';
	s1[m1 - m2] = '\0';
	strcat(s1, str2);
	strcpy(s2, str1);
      }else if(m2 > m1){
	for(int i = 0; i < m2 - m1; i++)
	  s2[i] = '0';
	s2[m2 - m1] = '\0';
	strcat(s2, str1);
	strcpy(s1, str2);
      }else{
	strcpy(s1, str2);
	strcpy(s2, str1);
      }
      char ans[81];
      int n = 0;
      for(int i = ((m1 > m2) ? m1 : m2) - 1; i >= 0; i--, n++)
	if((s1[i] - '0') + (s2[i] - '0') >= 10){
	  s1[i - 1] = s1[i - 1] + 1;
	  ans[i] = (s1[i]- '0') + (s2[i] - '0') + 38;
	}else
	  ans[i] = (s1[i] - '0') + (s2[i] - '0') + 48;
      if(n > 80)
	cout << "overflow" << endl;
      else{
	if((s1[0] - '0') + (s2[0] - '0') >= 10)
	  cout << "1";
	cout << ans;
      }
    }
  }
}