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<cstdio>
#include<cstring>
using namespace std;
char ans[101];
char n1[101];
char n2[101];
int main(){
	int d;
	int h1,h2,limit;
	for(cin>>d;d>0;d--){
		for(int j=0;j<100;j++)ans[j]='0';
		scanf("%s",n1);
		scanf("%s",n2);
		h1=strlen(n1)-1;
		h2=strlen(n2)-1;
		limit=max(h1,h2);
		int i;
		bool flg=false;
		for( i=0;i<=limit;i++,h1--,h2--){
			flg=false;
			int n1_tmp = 0;
			int n2_tmp = 0;
			if(h1>=0){
				n1_tmp=n1[h1]-'0';
			}
			if(h2>=0){
				n2_tmp=n2[h2]-'0';
			}
			int sum = n1_tmp + n2_tmp;
			if(sum>=10){
				if(i==79){
					i++;
					flg=true;
					break;
				}
				ans[i+1]+=1;
				sum=sum%10;
				flg=true;
			}

			if(ans[i]-'0'+sum > 9){
				if(i==79){
					i++;
					flg=true;
					break;
				}
				ans[i]=ans[i]+(sum-10);
				ans[i+1]+=1;
				flg=true;
			}
			else ans[i]+=sum;
		}
		if(flg){
			i++;
		}
		ans[i]='\0';
		if(i>=80)cout<<"overflow"<<endl;
		else{
			for(--i;i>=0;i--){
				if(i>=0)cout<<ans[i];
			}
			cout<<endl;
		}
//		cout<<"size:"<<strlen(ans)<<endl;
	}
	return 0;
}

C++: #include <cstdio>
#include <cstdlib>
#include <iostream>
#include <algorithm>
#include <cstring>
#include <string.h>
#include <queue>
#include <stack>
#include <utility>
#include <queue>
#define mp make_pair
#define pii pair<int,int> 
#define ff first
#define ss second

using namespace std;

int t,temp1,temp2,tempc;
char a[1000],b[1000];
char temp[1000];
int c[1000];

int addition(int n1,int n2)
{
	//printf("%d,%d ",n1,n2);
	if(n1>=n2)
	{
		if(n2!=-1)
		{
			temp1=(int)a[n1]-48;
			temp2=(int)b[n2]-48;
			tempc=temp1+temp2;
			//printf("%d,%d,%d",temp1,temp2,tempc);
			if(tempc+c[n1+1]>=10)
			{
				tempc-=10;
				c[n1]+=1;
			}
			c[n1+1]+=tempc;
			//printf("%d,%d",c[n1+1],tempc);
			addition(n1-1,n2-1);
		}
		else if(n1!=-1)
		{
			tempc=(int)a[n1]-48;
			if(tempc+c[n1+1]>=10)
			{
				c[n1+1]-=10;
				c[n1]+=1;
			}
			c[n1+1]+=tempc;
			addition(n1-1,n2);
		}
	}
	else
	{
		for(int x=0;x<max(strlen(a),strlen(b))+5;x++)
		{
			temp[x]=a[x];
			a[x]=b[x];
			b[x]=temp[x];
		}
		addition(n2,n1);
	}
}

int main()
{
	scanf("%d",&t);
	while(t--)
	{
		scanf("%s",a);
		scanf("%s",b);
		for(int x=0;x<max(strlen(a),strlen(b))+5;x++)
		{
			c[x]=0;
		}
		if(strlen(a)>80||strlen(b)>80)
		{
			printf("overflow\n");
			continue;
		}
		else
		{
			addition(strlen(a)-1,strlen(b)-1);
			//printf("%d",(int)a[1]);
			//printf("%d,%d\n",strlen(a),strlen(b));
		}
		if(c[0]>0&&max(strlen(a),strlen(b))>=80)
		{
			printf("overflow\n");
		}
		else
		{
			for(int z=0;z<max(strlen(a),strlen(b))+1;z++)
			{
				if(c[z]==0&&z==0)
				continue;
				printf("%d",c[z]);
			}
			printf("\n");
		}
	}
	return 0;
}

C++: #include <iostream>
#include <cstring>
#include <cstdlib>
using namespace std;
const int DIGIT=9;

int main() {
  int n;
  char buf[81];
  int l, x[2], y, z;
  char buf_part[DIGIT+1];
  int a[2][9];
  int b[9];

  cin >> n;
  while (n--) {
    for (int i=0; i<2; i++) {
      cin >> buf;
      l = strlen(buf);
      x[i] = (l+DIGIT-1)/DIGIT;
      y = (l-1)%DIGIT+1;
      for (int j=0; j<9; j++) {
        a[i][j] = 0;
      }
      for (int j=0; j<x[i]; j++) {
        if (!j) {
          memcpy(buf_part, buf, y);
          buf_part[y] = '\0';
        }
        else {
          memcpy(buf_part, &buf[y+DIGIT*(j-1)], DIGIT);
          buf_part[DIGIT] = '\0';
        }
        a[i][x[i]-j-1] = atoi(buf_part);
      }
    }

    for (int i=0; i<9; i++) b[i] = 0;
    z = (x[0] > x[1]) ? x[0] : x[1];
    for (int i=0; i<z; i++) {
      b[i] = a[0][i] + a[1][i];
      if (b[i] >= 1000000000) {
        a[0][i+1]++;
        b[i] -= 1000000000;
      }
    }
    if (z<9 && a[0][z]) {
      b[z] = a[0][z]+a[1][z];
      ++z;
    }
    if (b[8] >= 100000000) {
      cout << "overflow" << endl;
    } else {
      cout << b[z-1];
      for (int i=z-2; i>=0; i--) {
        cout.fill('0');
        cout.width(DIGIT);
        cout << b[i];
      }
      cout << endl;
    }
  }
  return 0;
}