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 <vector>
#include<string>
#include<sstream>
#define CAPACITY 9


namespace jisaku{//stoi?????¨?????????????????????????????????????????§??£?¨???????

	int stoi(std::string str){
		int ret;
		std::stringstream ss;
		ss << str;
		ss >> ret;
		return ret;
	}

     std::string to_string(int i){
		 std::string text;
		std::stringstream textline;
		textline<< i;
		textline >> text;
		return text;
	}

}




class OverDigit{

public:

	int num_a[CAPACITY];
	int num_b[CAPACITY];
	OverDigit(std::string str_a, std::string str_b);
	void ChangeDigit(std::string str, int num[]);
	void Add();
};

OverDigit::OverDigit(std::string str_a, std::string str_b){

	ChangeDigit(str_a, num_a);
	ChangeDigit(str_b, num_b);

}

void OverDigit::ChangeDigit(std::string str, int num[]){

	std::vector <int> num_str(CAPACITY, 0);
	int loc = CAPACITY, k = 8;
	int  str_rest,str_Max;

	str_rest = str.length() % CAPACITY;
	str_Max = str.length() / 9;

	for (int j = str.length() - CAPACITY; str_Max > 0; j = j - CAPACITY){
		num_str[k] = jisaku::stoi(str.substr(j, CAPACITY));
		loc = k;
		k--;
		str_Max--;

	}
	//?????????????????????????????????????????\??????

	if (str_rest != 0)num_str[loc - 1] = jisaku::stoi(str.substr(0, str_rest));

	for (int j = 0; j < CAPACITY; j++){
		num[j] = num_str[j];
	}
}
//80????????§????¶???????
void OverDigit::Add(){

	int num_ans[CAPACITY] = {};
	int fat, overflow = 0, count = 0;
	std::string text;


	for (int j = CAPACITY - 1; j >0; j--){
		if (num_a[j] + num_b[j] <= 999999999){
			num_ans[j] = num_a[j] + num_b[j];
		}
		else if (num_a[j] + num_b[j] > 999999999){
			fat = (num_a[j] + num_b[j]) - 1000000000;
			num_ans[j] = fat;
			num_a[j - 1]++;
		}

	}

	if (num_a[0] + num_b[0] > 99999999){
		overflow = 1;
	}
	else{
		num_ans[0] = num_a[0] + num_b[0];


	}

	if (overflow == 1){
		std::cout << "overflow" << std::endl;
	}
	else {
		for (int j = 0; j < CAPACITY; j++){
			if (num_ans[j] != 0){
				count++;
				text+=jisaku::to_string(num_ans[j]);
			}
			else if (num_ans[j] == 0 && count >= 1){
				for (int l = 0; l < 8; l++){
					text+="00000000";
				}

			}

		}std::cout <<text<< std::endl;
	}

};


int main(){


	//vector???????????¨??????80???????¨????
	int N;
	std::string str_a, str_b;

	std::cin >> N;
	std::cin.ignore();

	for (int i = 0; i < N; i++){
		getline(std::cin, str_a);
		getline(std::cin, str_b);
		if (str_a == "0" && str_b == "0"){std::cout << '0' << std::endl;
		}
		else if (str_a.length() >= 81 || str_b.length() >= 81){
		
			std::cout << "overflow" << std::endl;
		}
		else{

			OverDigit OverDigit_a(str_a, str_b);//???????????§??????
			OverDigit_a.Add();
		}
	}
	return 0;
}

C++: #include<iostream>
#include<string>
#include<algorithm>
using namespace std;
int main(){
  int n;
  cin >>n;
  while(n--){
    string a,b,c;
    cin >>a>>b;
    int s_a=a.size(),s_b=b.size();
    int s_max = max(s_a,s_b),s_min = min(s_a,s_b);
    int x=0, y=0;
    for(int i=0; i<s_max; i++){
      if(i<s_min) x = a[s_a-i-1]+b[s_b-i-1]-'0'*2+y;
      else if(i<s_a) x = a[s_a-i-1]-'0'+y;
      else if(i<s_b) x = b[s_b-i-1]-'0'+y;
      y = x/10;
      c+=(char)('0'+x%10);
    }
    if(y>0) c+='1';
    if(c.size()>80) cout <<"overflow";
    else for(int i=c.size()-1; i>=0; i--) cout <<c[i];
    cout <<endl;
  }
  return 0;
}

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

int main() {
    int num; cin >> num;
    for(int k = 0; k < num; k++) {
        string s1, s2, ret;
        vector<int> v1, v2;
        cin >> s1 >> s2;
        if(s2.length() > s1.length()) swap(s1,s2);

        for(int i=0; i<s1.length(); i++) v1.push_back((int)s1[i] - 48);
        for(int i=0; i<s2.length(); i++) v2.push_back((int)s2[i] - 48);
        reverse(v1.begin(), v1.end());
        reverse(v2.begin(), v2.end());

        int n = 0;
        for(int i=0; i<v2.size(); i++) {
            ret += to_string((v1[i] + v2[i] + n) % 10);
            if(v1[i] + v2[i] + n >= 10) n = 1;
            else n = 0;
        }
        for(int i=v2.size(); i<v1.size(); i++) {
            ret += to_string((v1[i] + n) % 10);
            if(v1[i] + n >= 10) n = 1;
            else n = 0;
        }

        if(v1.size() == v2.size() && v1[v1.size() - 1] + v2[v2.size() - 1] + n >= 10) ret += "1";
        else if(v1[v1.size() - 1] + n == 10) ret += "1";

        reverse(ret.begin(), ret.end());
        if(ret.length() > 80) cout << "overflow" << endl;
        else cout << ret << endl;
    }
    return 0;
}