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 <string>
using namespace std;

string reverse(const string& str) {
	string ret;
	string::const_reverse_iterator it = str.rbegin();
	for (; it != str.rend(); it++) {
		ret.push_back(*it);
	}
	return ret;
}

int main()
{
	int N;
	cin >> N;
	for (int i = 0; i < N; i++) {
		string S1;
		string S2;
		int overflow = 0;
		cin >> S1 >> S2;
		if (S1.size() > 80 || S2.size() > 80) {
			cout << "overflow";
			overflow++;
		}
		S1 = reverse(S1);
		S2 = reverse(S2);
		S1.resize(80, '0');
		S2.resize(80, '0');
		int a = 0;
		int d = 0;
		for (int i = 0; i < 80; i++) {
			S1[i] += S2[i] - '0';
			if (S1[i] > '9') {
				if (i + 1 > 79) {
					cout << "overflow";
					d++;
				}
				S1[i + 1] += 1;
				S1[i] -= 10;
			}
			if (S1[i] != '0')
				a = i;
		}
		S1 = reverse(S1);
		if (d == 0)
			for (int i = 79 - a; i < 80; i++) {
				cout << S1[i];
			}
		cout << endl;
	}
	return 0;
}

C++: #include <iostream>
#include <algorithm>
#include <vector>
#include <cstdio>
#include <string>
#include <cmath>
#include <cfloat>
#include <map>
using namespace std;
int number(char c){
	if(c>='0'&&c<='9')
	return (c-'0');
	else
		return 0;
}
int main(){
	int n;
	cin>>n;
	
	while(n>0){
		string a;
		string b;
	int k=0;
	int c=0;
	string s="";
	
		cin>>a;
		cin>>b;
		
	
	for(int i=a.length()-1,j=b.length()-1;i>=0||j>=0;i--,j--){
		int d;
		int n1=0,n2=0;
		if(i>=0)
			n1=number(a[i]);
		if(j>=0)
			n2=number(b[j]);
		d=n1+n2+k;
		k=d/10;
		d=d%10;
		c++;
		s=to_string(d)+s;
	}
	if(c>=80&&k>0)
		cout<<"overflow"<<endl;
		else{
			if(k>0){
		s=to_string(k)+s;
			}
		cout<<s<<endl;
		}
		n--;
	}
}

C++: #include <stdio.h>
#include <string.h>

int main(){
	int i,j,k,n,tmp,flag,length;
	int yosan[20];
	char str[81],str2[256];

	scanf("%d",&n);

	for(i = 0;i < n;i++){
		memset(yosan,0,sizeof(yosan));
		flag = 0;
		for(j = 0;j < 2;j++){
			scanf("%s",str2);
			length = strlen(str2);
			if(flag && length >= 81){
				flag = 1;
				continue;
			}
			memset(str,'\0',sizeof(str));
			for(k = 0;k < 80;k++){
				if(k >= 80-length){
					str[k] = str2[k-80+length];
				}else{
					str[k] = '0';
				}
			}
			//printf("%s\n",str);
			tmp = 0;
			for(k = 0;k < 80;k++){
				tmp *= 10;
				tmp += (str[k] - '0');
				if(k % 4 == 3){
					yosan[k/4] += tmp;
					tmp = 0;
				}
			}
			for(k = 29;k > 0;k--){
				if(yosan[k] >= 10000){
					yosan[k-1]++;
					yosan[k] %= 10000;
				}
			}
			if(yosan[0] >= 10000){
				flag = 1;
				//break;
			}
		}
		if(!flag){
			tmp = 0;
			while(yosan[tmp] == 0){
				tmp++;
			}
			printf("%d",yosan[tmp]);
			tmp++;
			for(j = tmp;j < 20;j++){
				printf("%04d",yosan[j]);
			}
			printf("\n");
		}else{
			printf("overflow\n");
		}
	}

	return 0;
}

C++: using namespace std;

void sum80(int* a, int* b);
int get80(string str, int* num);

int main(void){
	string a80, b80;
	int a[81], b[81];
	int i, N;
	cin>>N;
	i=0;
	while(i<N){
		cin>>a80;
		cin>>b80;
		if(get80(a80, a)<0 || get80(b80, b)<0){
			cout<<"overflow"<<endl;
		}else{
			sum80(a, b);
		}
		i++;
	}
	return 0;
}

int get80(string str, int* num){
	int d=str.length();
	if(d<=80){
		for(int i=0; i<d; i++){
			num[i]=str[d-1-i]-'0';
		}
		for(int i=d;i<81; i++){
			num[i]=0;
		}
		return d;
	}else{
		return -1;
	}

}

void sum80(int* a, int* b){
	int s[81], amari;
	amari=0;
	for(int i=0; i<81; i++){
		s[i]=(a[i]+b[i]+amari)%10;
		if(a[i]+b[i]+amari>=10){
			amari=1;
		}else{
			amari=0;
		}
	}
	if(s[80]!=0){
		cout<<"overflow"<<endl;
	}else{
		bool flag=false;
		for(int i=79; i>=0; i--){
			if(flag==true || s[i]!=0){
				flag=true;
				cout<<s[i];
			}
		}
		cout<<endl;
	}
}

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;
}