Go: package main

import (
	"bufio"
	"fmt"
	"math/big"
	"os"
	"strconv"
)

var scanner = bufio.NewScanner(os.Stdin)

func nextString() string {
	scanner.Scan()
	return scanner.Text()
}

func nextInt() int {
	n, err := strconv.Atoi(nextString())
	if err != nil {
		fmt.Printf("strconv.Atoi failed: %v\n", err)
	}
	return n
}

func main() {
	scanner.Split(bufio.ScanWords)

	const maxCapacity = 512 * 1024
	buf := make([]byte, maxCapacity)
	scanner.Buffer(buf, maxCapacity)

	n := nextInt()
	maxint, _ := new(big.Int).SetString("100000000000000000000000000000000000000000000000000000000000000000000000000000000", 10)

	for i := 0; i < n; i++ {
		aStr := nextString()
		bStr := nextString()

		a, _ := new(big.Int).SetString(aStr, 10)
		b, _ := new(big.Int).SetString(bStr, 10)
		c := big.NewInt(0).Add(a, b)

		if c.Cmp(maxint) < 0 {
			fmt.Println(c)
		} else {
			fmt.Println("overflow")
		}
	}
}

C++: #include <iterator>
#include <typeinfo>
#include <list>
#include <map>     
#include <numeric>
#include <utility>
#include <queue>
#include <set>
#include <sstream>
#include <iomanip>
#include <iostream>
#include <stack>
#include <string>
#include <vector>
using namespace std;
#define ll long long
#define ld long double
#define rep(a,t) for(int a=0;a<t;++a) 
#define forever while(true)
#define Sort(a) sort(a.begin(),a.end())
#define Reverse(a) reverse(a.begin(),a.end())
#define pb push_back
#define int_maxvalue numeric_limits<int>::max()
ll mod = 1e9 + 7;
string Addition(string a,string b) {
	string ans = "";
	int bef = 0;
	int be1 = 0;
	for (int i = a.length() - 1; i >= 0; i--) {
		int added = stoi(a.substr(i, 1)) + stoi(b.substr(i, 1))+be1;

		if (added >= 10) {
			bef = 1;
			added -= 10;
		}
		else {
			bef = 0;
		}
		stringstream ss;
		ss << added;

		ans = ss.str()+ans;
		if (i == 0) 
		{
			break;
		}
		be1 = bef;
	}
	if (be1 == 1) {
		ans = "1" + ans;
	}
	return ans;
}
int main() {
	int n;
	cin >> n;
	rep(o1, n) {
		string a, b;
		cin >> a >> b;
		if (a.length() < b.length()) {
			swap(a, b);
		}
		string b1 = b;
		for (int i = 0; i < a.length() - b1.length(); ++i) {
			b = '0' + b;
		}
		string ans = Addition(a, b);
		if (ans.length() > 80) { cout << "overflow" << endl; }
		else {
			cout << ans << endl;
		}
	}
	return 0;
}

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

int main() {
  int n; cin >> n;
  for (int i = 0; i < n; i++) {
    string a, b;
    cin >> a >> b;
    vector<int> va, vb;
    for (int i = a.size() - 1; i >= 0; i--) va.push_back(a[i] - '0');
    for (int i = b.size() - 1; i >= 0; i--) vb.push_back(b[i] - '0');
    vector<int> vc;
    int rem = 0;
    for (int i = 0; i < va.size() || i < vb.size(); i++) {
      if (i < va.size()) rem += va[i];
      if (i < vb.size()) rem += vb[i];
      vc.push_back(rem % 10);
      rem = rem / 10;
    }
    if (rem) vc.push_back(rem);
    if (vc.size() > 80) {
      cout << "overflow" << endl;
    } else {
      for (int i = vc.size() - 1; i >= 0; i--) cout << vc[i];
      cout << endl;
    }
  }
}

C++: #include <iostream>
#include <vector>
#include<string>
#include<sstream>

#define CAPACITY 9

 
namespace stoi{
 
    int stoi(std::string str){
        int ret;
        std:: stringstream ss;
        ss << str;
        ss >> ret;
        return ret;
    }
 
}
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 num_str[CAPACITY*2];
	for (int j = 0; j < CAPACITY*2; j++) { num_str[j]=0; }
    int loc = CAPACITY, k = 8;
    int str_Max, str_rest;

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


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

    if(str_rest!=0)
	{
		num_str[loc - 1] = stoi::stoi(str.substr(0, str_rest));
	}
 
    for (int j = 0; j < CAPACITY; j++)
	{
		num[j]=num_str[j];
    }
}

void OverDigit::Add()
{
    int num_ans[CAPACITY] = {};
    int fat,overflow=0,count=0;
 
    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++;
                std::cout << num_ans[j];
            }
            else
			 if (num_ans[j] == 0 && count >= 1)
			{
                std::cout << "000000000";
            }
        }
		std::cout << std::endl;
    }
}
 
 
int main(){
    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);
        OverDigit OverDigit_a(str_a,str_b);
       OverDigit_a.Add();
   }
 
    return 0;
}

C++: #include<iostream>
#include<algorithm>
#include<string>


#define MAX 1000


using namespace std;


int main()
{
	int n = 0;
	string in[2];
	char out[MAX] = {NULL};

	cin >> n;

	for (int i = 0; i < n; i++){
		cin >> in[0] >> in[1];

		for (int i = 0; i < MAX; i++){
			out[i] = '\0';
		}

		int count = in[0].size() - 1;

		for (int i = 0; i < in[0].size(); i++){
			out[count--] += in[0][i];
		}

		count = in[0].size();
		int count2 = in[1].size() - 1;

		count = max(count , count2) - 1;

		for (int i = 0; i < in[1].size(); i++){
			if (out[i] != '\0'){
				out[i] -= '0';
			}
			out[i] += in[1][count2--];
			count--;
		}
		
		int i = 0;
		for (i = 0; out[i] != '\0'; i++){
			;
		}

		bool flag = false;

		/*
		for (i--; i >= 0; i--){
			if (out[i] > '9'){
				out[i] -= 10;
				if (i != 0){
					out[i - 1] += 1;
				}else{
					flag = true;
				}
			}
		}
		*/

		for (int j = 0; j < i; j++){
			if (out[j] > '9'){
				out[j] -= 10;
				out[j + 1] += 1;
			}
		}

		if (out[i] > '\0' && out[i] < '0'){
			out[i] += '0';
		}
		
		for (i = 0; i < out[i] != '\0'; i++){
			;
		}

		for (int j = 0; j < i / 2; j++){
			char tmp = out[j];
			out[j] = out[i - j - 1];
			out[i - j - 1] = tmp;
		}

		if (i >= 80 || (flag == true && i >= 79)){
			cout << "overflow" << endl;
		}else{
			if (flag){
				//cout << "1";
			}
			cout << out << endl;
		}
	}

	return 0;
}

C++: #include<bits/stdc++.h>
using namespace std;
 
signed main(){
int a;
cin>>a;
for(int b=0;b<a;b++){
    int goukei[81]{};
    char c[110]{},d[110]{};
    scanf("%s%s",c,d);
    int e,f;
    e=strlen(c);f=strlen(d);
    if(e>80||f>80)puts("overflow");
    e--;f--;
    for(int g=80;g!=0;g--,e--,f--){
	int s=goukei[g];
	if(e>-1)s+=c[e]-48;
	if(f>-1)s+=d[f]-48;
	goukei[g-1]=s/10;
	goukei[g]=s%10;
	}
	if(goukei[0])puts("overflow");
	else{
	    bool ss=false;
	    for(int m:goukei){
		if(m)ss=true;
		if(ss)printf("%d",m);
		}
		puts("");
	    }
    }
}