Go: package main

import (
        "fmt"
        "bufio"
        "os"
)

func main() {
        var n int
        var a, b string
        sc := bufio.NewScanner(os.Stdin)
        fmt.Scanf("%d", &n)
        for i := 0; i < n; i++ {
                sc.Scan()
                a = sc.Text()
                sc.Scan()
                b = sc.Text()

                if len(a) > len(b) {
                        s := add(a, b)
                        if s[len(a)] > 0 {
                                if len(a) >= 80 {
                                        fmt.Printf("overflow")
                                } else {
                                        for i := 0; i <= len(a);i++ {
                                                fmt.Printf("%d", s[len(a)-i])
                                        }
                                }
                                fmt.Printf("\n")
                        } else {
                                if len(a)-1 >= 80 {
                                        fmt.Printf("overflow")
                                } else {
                                        for i := 0; i < len(a);i++ {
                                                fmt.Printf("%d", s[len(a)-i-1])
                                        }
                                }
                                fmt.Printf("\n")
                        }
                } else {
                        s := add(b, a)
                        if s[len(b)] > 0 {
                                if len(b) >= 80 {
                                        fmt.Printf("overflow")
                                } else {
                                        for i := 0; i <= len(b);i++ {
                                                fmt.Printf("%d", s[len(b)-i])
                                        }
                                }
                                fmt.Printf("\n")
                        } else {
                                if len(b)-1 >= 80 {
                                        fmt.Printf("overflow")
                                } else {
                                        for i := 0; i < len(b);i++ {
                                                fmt.Printf("%d", s[len(b)-i-1])
                                        }
                                }
                                fmt.Printf("\n")
                        }
                }
        }
}

// expects ln > lm
func add(n string, m string) (s [100]int) {
        ln := len(n)
        lm := len(m)
        var sum [100]int

        for i := 0; i < ln; i++ {
                var s, t int
                s = int(n[ln-1-i]) - int('0')

                if lm - 1 - i >= 0 {
                        t = int(m[lm-1-i]) - int('0')
                } else {
                        t = 0
                }

                sum[i+1] = (s + t + sum[i]) / 10
                sum[i] = (s + t + sum[i]) % 10
        }

        return sum
}

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

string reserve(string a, int len)
{
    string rev = a;
    for (int i=0;i<len;i++)
    {
        rev[i] = a[len-i-1];
    }
    return rev;
}

int main()
{
    string a,b,d;
    int num;
    int cons;
    d = "";
    int tmp;
    cin >> num;
    while(num--)
    {
        cin >> a >> b;
        a = reserve(a,a.size());

        b = reserve(b,b.size());
        int maxi = max(a.size(),b.size());
        int mini = min(a.size(),b.size());
        d.resize(maxi+1);
        a.resize(maxi+1,0);
        b.resize(maxi+1,0);
        int c[maxi+1];
        int i;
        for(i=0;i<=maxi;i++)
        {
            c[i] = 0;
        }


                for(int i=0 ;i <=maxi;i++)
                {
                    if(i<mini)
                        cons = 96;
                    else  if(i<maxi)
                        cons = 48;
                    else cons = 0;
                    tmp = a[i] + b[i] + c[i] - cons;
                    if (tmp > 9)
                    {
                        c[i+1] = 1;
                        tmp -= 10;

                    }
                    d[i] = tmp + 48 ;
                }


        if(c[maxi]==1)
            d.resize(maxi+1);
        else d.resize(maxi);

        if(d.size() > 80 || a.size() >80 || b.size()>80)
            cout << "overflow" << endl;
        else
        {
            d = reserve(d,d.size());
            cout << d << endl;
        }

    }
    return 0;
}

C++: #include <cstdio>
#include <cstdlib>
#include <iostream>
#include <string>
#include <cmath>
#include <algorithm>
#include <vector>
#include <queue>
#include <stack>

using namespace std;
typedef long long ll;

#define PI 3.141592653

int main(){
    string A,B;
    int n,Ak,Bk,ans[81];
    
    scanf("%d",&n);
    
    for(int i=0;i<n;i++){
        
        for(int j=0;j<80;j++){
            ans[j]=0;
        }
        
        cin >> A >> B;
        
        if(A==B && A=="0"){
            printf("0\n");
            continue;
        }
        
        Ak=(int)A.size();
        Bk=(int)B.size();
        
        for(int j=0;j<max(Ak,Bk);j++){
            if(j<min(Ak,Bk))
                ans[79-j]+=(A[Ak-1-j]-'0')+(B[Bk-1-j]-'0');
            else if(j<Ak)
                ans[79-j]+=A[Ak-1-j]-'0';
            else if(j<Bk)
                ans[79-j]+=B[Bk-1-j]-'0';
            
            if(ans[79-j]>=10){
                if(j==79){
                    break;
                }
                ans[79-j-1]++;
                ans[79-j]-=10;
            }
        }
        
        if(ans[79]>=10){
            puts("overflow");
        }else{
            bool f = false;
            for(int j=0;j<80;j++){
                if(ans[j]>0 || f){
                    printf("%1d",ans[j]);
                    f = true;
                }
            }
            printf("\n");
        }
    }
    
    return 0;
}

C++: #include <bits/stdc++.h>
using namespace std;

#define times(n, i) uptil(0, n, i)
#define upto(f, t, i) for(auto _##i = (t), i = decltype(_##i)(f) poi  i <= _##i poi  i++)
#define uptil(f, t, i) for(auto _##i = (t), i = decltype(_##i)(f) poi  i < _##i poi  i++)
#define downto(f, t, i) for(auto i = (f), _##i = decltype(i)(t) poi  i >= _##i poi  i--)
#define downtil(f, t, i) for(auto i = (f), _##i = decltype(i)(t) poi  i > _##i poi  i--)

#define unless(c) if(!(c))
#define until(c) while(!(c))
#define loop while(true)

#define poi ;
#define poipoi << "\n"

#define fuwafuwa(n, c) dokidoki((n) + (c) - 1, c)
#define dokidoki(n, c) ((n) / (c) * (c))

template<class T> bool read(T& t){
    return cin >> t poi
}
template<class T, class...U> bool read(T& t, U&... u){
    unless(cin >> t) return false poi
    return read(u...) poi
}

const int PYON = 80 poi

int main(){
    int N poi
    cin >> N poi
    times(N, i){
        string A, B poi
        cin >> A >> B poi
        if(A.size() > PYON || B.size() > PYON){
            cout << "overflow" poipoi poi continue poi
        }else if(A == "0" && B == "0"){
            cout << "0" poipoi poi continue poi
        }
        vector<int> a(PYON, 0), b(PYON, 0) poi
        upto(1, A.size(), i) a[PYON - i] = A[A.size() - i] - '0' poi
        upto(1, B.size(), i) b[PYON - i] = B[B.size() - i] - '0' poi
        int pyon = 0 poi
        vector<int> sum(PYON) poi
        downto(PYON - 1, 0, i){
            sum[i] = a[i] + b[i] + pyon poi
            pyon = sum[i] / 10 poi
            sum[i] %= 10 poi
        }
        if(pyon) cout << "overflow" poi
        else times(PYON, i) if((pyon = pyon || sum[i])) cout << sum[i] poi
        cout poipoi poi
    }
    return 0 poi
}