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<algorithm>
using namespace std;
int main(){
  int n;
  cin>>n;
  cin.ignore();
  while(n--){
    string a,b;
    getline(cin,a);
    int num[900] = {0},tab = 0,i = 0;
    int as = a.size()-1;
    int bs = b.size()-1;
    int nmi = min(a.size(),b.size());
    int nma = max(a.size(),b.size());
    for(int x; i<nma; i++){
      if(i<nmi){x = tab+a[as-i]+b[bs-i]-'0'*2;}
      else if(i>as){x = tab+b[bs-i]-'0';}
      else if(i>bs){x = tab+a[as-i]-'0';}
      tab = x/10;
      num[i] = x%10;
    }
    if(tab){num[nma] = tab;i++;}
    if(i>80){cout <<"overflow"<<endl;}
    else{
      for(int x=i-1; x>=0; x--){
	cout <<num[x];
      }
      cout <<endl;
    }
  }
  return 0;
}

C++: #include <iostream>
#include <string>
#include <algorithm>
using namespace std;
int main(){
	int n,m;
	string aindex, bindex, ans;
	cin >> m;
	n = 0;
	string a,b;
	for(int j = 0; j < m; j++){
		//cin >> a >> b;
		if(a.size()>80||b.size()>80){
			cout << "overflow" << endl;
		}else{
			for(int i = 0; i< max(a.size(),b.size()); i++){
				if(a.size()-i > 0){
					aindex = a[a.size()-i-1];
				}else{
					aindex = "0";
				}
				if(b.size()-i > 0){
					bindex = b[b.size()-i-1];
				}else{
					bindex = "0";
				}
				n = stoi(aindex) + stoi(bindex) + (n-n%10==10);
				aindex = to_string(n%10);
				ans = aindex + ans;
			}
			if(ans.size()>80){
				cout << "overflow" << endl;
			}else{
				cout << ans << endl;
			}	
		}
	}
}

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

int a[81] {0};
int b[81] {0};
int sum[55][81] {0};

int trans(char);

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

    for (int i = 0; i < n; i++)
    {
        string s1, s2;
        cin >> s1 >> s2;
        int l1 = s1.length(), l2 = s2.length();
        for (int j = l1 - 1; j >= 0; j--)
        {
            a[l1-1-j] = trans(s1[j]);
        }
        for (int j = l2 - 1; j >= 0; j--)
        {
            b[l2-1-j] = trans(s2[j]);
        }

        int ll = max(l1, l2);
        for (int j = 0; j < ll; j++)
            sum[i][j] = a[j] + b[j];

        for (int j = 0; j < ll; j++)
        {
            sum[i][j+1] += sum[i][j] / 10;
            sum[i][j] %= 10;
        }


        for (int j = 0; j < 81; j++)
        {
            a[j] = 0;
            b[j] = 0;
        }

    }

    for (int i = 0; i < n; i++)
    {
        if (sum[i][80] > 0)
            cout << "overflow" << endl;
        else
        {
            int j = 80;
            while (sum[i][j] <= 0)
                j--;
            while (j >= 0)
            {
                cout << sum[i][j];
                j--;
            }
            cout << endl;
        }
    }

    return 0;
}

int trans(char ch)
{
    switch (ch)
    {
    case '0':
        return 0;
    case '1':
        return 1;
    case '2':
        return 2;
    case '3':
        return 3;
    case '4':
        return 4;
    case '5':
        return 5;
    case '6':
        return 6;
    case '7':
        return 7;
    case '8':
        return 8;
    case '9':
        return 9;

    }
}