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 <cmath>
#include <cstdlib>
#include <cstring>

const int MAX_INPUT = 1000000;
const int ARRAY_SIZE = 11;
const int MAX_LEN = 8;
char inputA[MAX_INPUT];
char inputB[MAX_INPUT];

void toBigInt(char *str, long *r) {
    int len = strlen(str);
    int endIndex = ARRAY_SIZE - (len / MAX_LEN) - 1;

    {
        int offset = 0;
        for (int i = 0; i < ARRAY_SIZE; ++i) {
            char temp[MAX_LEN + 1] = { 0 };
            int readLen = ((len - offset) < MAX_LEN) ? len - offset : MAX_LEN;
            char* start = str + len - offset - readLen;

            strncpy(temp, start, readLen);
            r[ARRAY_SIZE - i- 1] = (long)atoi(temp);
            offset += readLen;
        }
    }
}

int main() {
    const char OVER[] = "overflow\n";
    int n = 0;
    scanf("%d", &n);

    for (int i = 0; i < n; ++i) {
        scanf("%s\n%s", &inputA, &inputB);
        long a[ARRAY_SIZE] = { 0 };
        long b[ARRAY_SIZE] = { 0 };
        long r[12] = { 0 };
        int lenA = 0;
        int lenB = 0;
        const int MAX_INPUT_LEN = 80;
        lenA = strlen(&inputA[0]);
        lenB = strlen(&inputB[0]);
        if ((MAX_INPUT_LEN < lenA) || (MAX_INPUT_LEN < lenB)) {
            printf(OVER);
            continue;
        }

        toBigInt(inputA, a);
        toBigInt(inputB, b);
        for (int i = 0; i < ARRAY_SIZE; ++i) {
            int curRIndex = 12 - i - 1;
            r[curRIndex] += a[ARRAY_SIZE - i - 1] + b[ARRAY_SIZE - i - 1];
            const int MAX = 99999999;
            if (MAX < r[curRIndex]) {
                r[curRIndex - 1] = r[curRIndex] / (MAX + 1);
                r[curRIndex] %= (MAX + 1);
            }
        }

        if ((9999999 < r[2]) || (0 < r[1])) {
        //if ((9999999 < r[2])) {
            printf(OVER);
            continue;
        }

        {
            bool started = false;
            bool noOutput = true;
            for (int i = 0; i < 12; ++i) {
                if (!started && (0 < r[i])) {
                    started = true;
                    printf("%d", r[i]);
                    noOutput = false;
                    continue;
                }

                if (started) {
                    long temp = (r[i] == 0) ? 1 : r[i];
                    for (int j = 0; temp * pow(10, j) < 1000000; ++j) {
                        printf("0");
                    }

                    printf("0");
                    printf("%d", r[i]);
                }
            }

            if (noOutput) {
                printf("0");
            }
        }
        printf("\n");
        for (int i = 0; i < MAX_INPUT; ++i) {
            inputA[i] = 0;
            inputB[i] = 0;
        }
    }
    return 0;
}

C++: #include<iostream>
#include<string>
#include<algorithm>
using namespace std;
int main(){
  int n;
  cin >> n;
  for(int i = 0;i < n;i++){
    string a,b,c; char tmp;
    cin >> a >> b;
    if(a.size() > 80 || b.size() > 80){
      cout << "overflow" << endl;
      continue;
    }
    
    if(a.size() < b.size()){swap(a,b);}
    for(int k = 0;k< a.size();k++)
      c[k] = '0';
     for(int k = 0;k < b.size();k++){
       c[k] = b[b.size()-1-k];
     }
     b.resize(a.size());
     for(int k = 0;k < a.size();k++)
       {b[k] = c[k]; c[k] = a[k];} 
     for(int k = 0;k < a.size();k++){
      	a[a.size()-k-1] = c[k];
     }
	
     tmp = '0';
     for(int k = 0;k < a.size();k++){
       c[k] = a[k] + b[k] + tmp -'0' -'0';
	  if(c[k] > '9'){ tmp = '1'; c[k] = c[k] - ':' + '0'; }
     } 
     if(a.size() < 80 ){if(tmp == '1'){cout << tmp;}
       for(int k = a.size()-1;k >=0;k--){
	 cout << c[k]; 
	  }
       cout << endl;
     } else {cout <<"overflow"<<endl; } 
     
  }
}

C++: #include<stdio.h>
#include<string.h>
#include<stdlib.h>
int main(){
	long int io[11],it[11],three[11];
	int n,a,b,c,xo,xt,x;
	char one[81],two[81],t[8];
	scanf("%d",&n);
	for(a=0;a<n;a++){
		scanf("%s",&one);
		scanf("%s",&two);
		xo=(strlen(one)-1)/8;
		xt=(strlen(two)-1)/8;
		if(xo<xt){x=xt;}
		else{x=xo;}
		for(b=0;b<11;b++){
			three[b]=0;
		}
		for(b=0;b<10;b++){
			for(c=0;c<8;c++){
				t[c]=one[c+b*8];
			}
			io[b]=atoi(t);
		}
		for(b=0;b<10;b++){
			for(c=0;c<8;c++){
				t[c]=two[c+b*8];
			}
			it[b]=atoi(t);
		}
		for(b=0;b<=x+1;b++){
			three[b]+=io[b]+it[b];
			three[b+1]+=three[b]/100000000;
			three[b]=three[b]%100000000;
		}
		if(three[x+1]!=0){x++;}
		if(three[10]==0){
			printf("%ld,%ld",three[x] ,x);
//			for(b=1;b<=x;b++){
//				printf("%08ld",three[x-b]);
//			}
			printf("\n");
		}
		else{printf("overflow\n");}
	}
	return 0;
}

C++: #include <cstdio>
#include <cstdlib>
#include <cmath>
#include <map>
#include <utility>
#include <set>
#include <iostream>
#include <string>
#include <vector>
#include <algorithm>
#include <sstream>
#include <queue>
#include <cstring>
#include <functional>

using namespace std;

#define REP(i,a,b) for((i)=(a);(i)<(int)(b);(i)++)
#define rep(i,n) REP(i,0,n)
// BEGIN CUT HERE
#define foreach(itr,c) for(typeof((c).begin()) itr=(c).begin();itr!=(c).end();itr++)
template<class T> inline string toString(T x){ostringstream sout;sout<<x;return sout.str();}
// END CUT HERE

int main(){
  int i, j, n;
  char tmp;

  scanf("%d",&n);
  rep(i,n){
    string str1, str2, res;
    cin >> str1;
    cin >> str2;

    vector<int> a(82, 0), b(82, 0);
    rep(j,str1.size()) a[j] = str1[(int)str1.size()-1-j] - '0';
    rep(j,str2.size()) b[j] = str2[(int)str2.size()-1-j] - '0';

    rep(j,81){
      int sum = a[j] + b[j];
      if(sum >= 10){ sum %= 10; a[j+1]++;}
      res += (sum + '0');
    }

    reverse(res.begin(), res.end());
    int f = 0;
    rep(j,res.size()){
      if(res[j] != '0') f = 1;
      if(f) cout << res[j];
    }

    cout << endl;
  }

  return 0;
}