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 <algorithm>
#include <cstring>
using namespace std;

#define NUMBER_DIGITS 80

int main()
{
  int N;
  char str1[NUMBER_DIGITS + 1];
  char str2[NUMBER_DIGITS + 1];
  char str3[NUMBER_DIGITS / 2 + 1][3];
  char str4[NUMBER_DIGITS / 2 + 1][3];
  int n1[NUMBER_DIGITS / 2 + 1] = {};
  int n2[NUMBER_DIGITS / 2 + 1] = {};
  cin >> N;
  for(int i = 0; i < N; i++){
    cin >> str1;
    cin >> str2;
    int m1 = strlen(str1), m2 = strlen(str2);
    if(m1 > NUMBER_DIGITS || m2 > NUMBER_DIGITS)
      cout << "overflow" << endl;
    else if(atoi(str1) <= 2000000000 && atoi(str2) <= 2000000000 && atoi(str1) + atoi(str2) <= 2000000000){
      cout << atoi(str1) + atoi(str2) << endl;
    }else{
      for(int i = 2, j = NUMBER_DIGITS / 2; i <= m1 + 1; i += 2, j--){
	if(m1 % 2 == 1 && m1 - i < 0){
	  str3[j][0] = str1[0];
	  str3[j][1] == '\0';
	}else{
	  strncpy(str3[j], &str1[m1 - i], 2);
	  str3[j][2] == '\0';
	}
      }
      for(int i = 2, j = NUMBER_DIGITS / 2; i <= m2 + 1; i += 2, j--){
	if(m2 % 2 == 1 && m2 - i < 0){
	  str4[j][0] = str2[0];
	  str4[j][1] == '\0';
	}else{	  
	  strncpy(str4[j], &str2[m2 - i], 2);
	  str4[j][2] == '\0';
	}
      }
      for(int i = NUMBER_DIGITS / 2; i + ((m1 % 2 == 1) ? 1 : 0) >= NUMBER_DIGITS / 2 - m1 / 2; i--)
	if(str3[i][0] == '0' && str3[i][1] == '0')
	  n1[i] = 1000;
	else
	  n1[i] = atoi(str3[i]);
      for(int i = NUMBER_DIGITS / 2; i + ((m2 % 2 == 1) ? 1 : 0) >= NUMBER_DIGITS / 2 - m2 / 2; i--)
	if(str4[i][0] == '0' && str4[i][1] == '0')
	  n2[i] = 1000;
	else
	  n2[i] = atoi(str4[i]);
      int ans[NUMBER_DIGITS / 2 + 1] = {};
      int i = 0;
      int m = (m1 > m2) ? m1 : m2;
      for(int j = NUMBER_DIGITS / 2; NUMBER_DIGITS / 2 - 1 - m / 2 < ((m % 2) ? j + 1 : j); i++, j--)
	if(n1[j] + n2[j] >= 100)
	  if(n1[j] == 1000 && n2[j] < 10){
	    n1[j - 1] *= 10;
	    ans[i] = n2[j];
	  }else if(n2[j] == 1000 && n1[j] < 10){
	    n2[j - 1] *= 10;
	    ans[i] = n1[j];
	  }else if(n1[j] == 1000 && n2[j] == 1000){
	    if(n1[j - 1] % 10 != 0)
	      n1[j - 1] *= 10;
	    ans[i] = 1000;
	  }else if(n1[j] == 1000 && n2[j] >= 10){
	    ans[i] = n2[j];
	  }else if(n2[j] == 1000 && n1[j] >= 10){
	    ans[i] = n1[j];
	  }else if(n1[j] + n2[j] < 110){
	    n1[j - 1]++;
	    ans[i] = n1[j] + n2[j] + 900;
	  }else{
	    n1[j - 1]++;
	    ans[i] = n1[j] + n2[j] - 100;
	  }
	else
	  ans[i] = n1[j] + n2[j];
      int k = i;
      for(; i > 0; i--)
	if((i == k) && (ans[i - 1] == 0))
	  ;
	else if(ans[i - 1] == 1000)
	  cout << "00";
	else if(ans[i - 1] > 1000)
	  cout << "0" << ans[i - 1] % 1000;
	else
	  cout << ans[i - 1];
      cout << endl;
    }
  }
}

C++: #include <iostream>
#include <string>
#include <algorithm>
using namespace std;
int main(){
	int n;
	string aindex, bindex, ans;
	cin >> n;
	n = 0;
	string a,b;
	while(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];
				}else{
					aindex = "0";
				}
				if(b.size()-i > 0){
					bindex = b[b.size()-i];
				}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 <cstring>
int func()
{
	char i1[81]="",i2[81]="",res[81]="";
	bool cflag=false;
	std::string tmp;
	std::cin >> tmp;
	if(tmp.size()>80){
		std::cin >> tmp;
		return -1;
	}
	strcpy(i1,tmp.c_str());
	std::cin >> tmp;
	if(tmp.size()>80)
		return -1;
	strcpy(i2,tmp.c_str());
	for(int j=0;j<80;j++){
		char c='0';
		if(cflag)
			c+=1;
		cflag=false;
		if(strlen(i1)>j){
			c+=i1[strlen(i1)-j-1]-'0';
		}
		if(strlen(i2)>j){
			c+=i2[strlen(i2)-j-1]-'0';
		}
		if(c > '9'){
			cflag=true;
			c-=10;
		}
		res[79-j]=c;
	}

	if(cflag)
		return -1;
	char *p = res;
	while(*p++ == '0');
	std::cout << (p-1) << std::endl;

	return 0;
}
int main(void){
	int n;
	std::cin >> n;
	for(int i=0; i<n; i++)
		if(func()==-1)
			std::cout << "overflow" << std::endl;

}

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

int main(void)
{
int N,j,k=0;
cin >> N;
char strA[300],strB[300];
while(1){
k++;
if(k==N+1){
break;
}
cin >> strA >> strB;
if(cin.eof()){
    break;
}
int ans[81]={0};
ans[0]=11111;
if(strlen(strA)>80||strlen(strB)>80){
cout << "overflow" << endl;
continue;
}
int a[81]={0},b[81]={0};
j=strlen(strA)-1;
for(int i=80 ; j>=0 ; i--){
a[i]=(int)strA[j]-48;
j--;
}
j=strlen(strB)-1;
for(int i=80 ; j>=0 ; i--){
b[i]=(int)strB[j]-48;
j--;
}
for(int i=80 ; i!=0 ; i--){
ans[i]=ans[i]+a[i]+b[i];
if(ans[i]>=10){
ans[i]=ans[i]-10;
ans[i-1]++;
}
}
if(ans[0]!=11111){
cout << "overflow" << endl;
continue;
}
for(int i=1 ; i<81 ; i++){
if(ans[i]!=0){
j=i;
break;
}
}
char c[3];
for(int i=j ; i<81 ; i++){
printf("%c",ans[i]+48);
}
cout << '\n';
}
return 0;
}