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++: #define scanf_s scanf
#define sscanf_s sscanf
#include <stdio.h>
#include <string>
#include <iostream>
#include <math.h>
using namespace std;
#define MAX 81
#define _MAX 100
//#define CONSTNUM 10000000
int main(void)
{
	int n, s = 0, x, y, r = 0, a, b;
	int sum_n[50][3] = { 0 };
	char sum[50][_MAX], astr[_MAX], bstr[_MAX], num[11] = { "0123456789" };
	char ca, cb;
	scanf_s("%d", &n);
	for (int i = 0; i < n; ++i) {
		scanf_s("%s %s", &astr, &bstr);
		for (x = 0; astr[x] != '\0'; ++x);
		for (y = 0; bstr[y] != '\0'; ++y);
		if (x >= 80 || y >= 80) {
			printf("overflow\n");
			continue;
		}
		for (int j = 0; j < x || j < y; ++j) {
			if (j < x && j < y) {
				ca = astr[x - j - 1]; cb = bstr[y - j - 1];
				sscanf_s(&ca, "%d", &a);
				sscanf_s(&cb, "%d", &b);
				if (x > y) sum[i][x - j] = num[(a + b + r) % 10];
				else sum[i][y - j] = num[(a + b + r) % 10];
				if (a + b + r >= 10) r = (a + b + r) / 10;
				else r = 0;
			}
			else {
				if (x > y) {
					if (r != 0) {
						ca = astr[x - j - 1];
						sscanf_s(&ca, "%d", &a);
						sum[i][x - j] = num[(a + r) % 10];
						if (a + r >= 10) r = (a + r) / 10;
						else r = 0;
					}
					else {
						sum[i][x - j] = astr[x - j - 1];
					}
				}
				else {
					if (r != 0) {
						cb = bstr[y - j - 1];
						sscanf_s(&cb, "%d", &b);
						sum[i][y - j] = num[(b + r) % 10];
						if (b + r >= 10) r = (b + r) / 10;
						else r = 0;
					}
					else {
						sum[i][y - j] = bstr[y - j - 1];
					}
				}
			}
		}
		if (y < 80 - r && x < 80 - r) {
			if (r == 1) 	printf("%d", r);
			for (int j = 1; j <= x || j <= y; ++j) {
				printf("%c", sum[i][j]);
			}
		}
		else {
			printf("overflow");
		}
		printf("\n");
		r = 0;
	}
}

C++: #include <iostream>
#include <string>
#include <algorithm>
using namespace std;
int main(){

  int n;
  cin>>n;
  for(int i=0;i<n;i++){
    string a,b,s;
    int how=0;
    cin>>a>>b;
    reverse(a.begin(),a.end());
    reverse(b.begin(),b.end());
    for(int j=0;j<a.size()||j<b.size();j++){
      if(j<a.size()) how+=a[j]-'0';
      if(j<b.size()) how+=b[j]-'0';
      s.push_back(how%10+'0');
      how/=10;
    }
    if(how!=0) s.push_back(how+'0');
    reverse(s.begin(),s.end());
    if(s.size()>80) cout<<"overflow"<<endl;
    else cout<<s<<endl;
  }
  return 0;

}

C++: #include<iostream>
#include<vector>
#include<string>
using namespace std;
vector<int> V,W,A;
int main(){
	int n,a,b;
	cin>>n;
	string str;
	char num[10]={'0','1','2','3','4','5','6','7','8','9'};
	for(int i=0;i<n;i++){
		cin>>str;
		for(int j=0;j<str.size();j++){
			for(int k=0;k<10;k++){
				if(str[j]==num[k]){
					V.push_back(k);
					break;
				}
			}
		}
		cin>>str;
		for(int j=0;j<str.size();j++){
			for(int k=0;k<10;k++){
				if(str[j]==num[k]){
					W.push_back(k);
					break;
				}
			}
		}
		int v=V.size(),w=W.size();
		int flag=0;
		while(v || w){
			int x=0;
			if(v==0){
				if(W[w-1]+flag>=10){
					A.push_back(W[w-1]+flag-10);
					W.pop_back();
					flag=1;
				}else{
					A.push_back(W[w-1]+flag);
					W.pop_back();
					flag=0;
				}
			}else if(w==0){
				if(V[v-1]+flag>=10){
					A.push_back(V[v-1]+flag-10);
					V.pop_back();
					flag=1;
				}else{
					A.push_back(V[v-1]+flag);
					V.pop_back();
					flag=0;
				}
			}else{
				if(V[v-1]+W[w-1]+flag>=10){
					A.push_back(V[v-1]+W[w-1]-10+flag);
					V.pop_back();
					W.pop_back();
					flag=1;
				}else{
					A.push_back(V[v-1]+W[w-1]+flag);
					V.pop_back();
					W.pop_back();
					flag=0;
				}
			}
			v=V.size();
			w=W.size();
		}
		if(flag)
		A.push_back(flag);
		for(int j=A.size()-1;j>=0;j--){
			cout<<A[j];
		}
		cout<<endl;
		V.clear();
		W.clear();
		A.clear();
	}
	return 0;
}