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 <iostream>
#include <string>
#include <algorithm>
using namespace std;

int main(){
	cin.tie(0);
	ios::sync_with_stdio(false);
	
	int n;
	cin >> n;
	
	while(n-- > 0){
	
	string str1;
	cin >> str1;
	
	string str2;
	cin >> str2;
	
	reverse(str1.begin(), str1.end());
	reverse(str2.begin(), str2.end());
	
	int check;
	if(str1.size() > str2.size()){
		check = 1;
	}
	else if(str1.size() == str2.size()){
		check = 2;
	}
	else {
		check = 3;
	}
	
	int ans[100];
	bool next = false;
	int short_ = min(str1.size(), str2.size());
	int long_ = max(str1.size(), str2.size());
	for(int i = 0; i < short_; ++i){
		int num = 0;
		num += (str1[i] - '0') + (str2[i] - '0');
		if(next){
			num++;
		}
		ans[i] = num;
		next = false;	
		if(num >= 10){
			num -= 10;
			ans[i] -= 10;
			next = true;
		}
		//cout << num << endl;
	}
	/*for(int i = 0; i < min(str1.size(), str2.size()); ++i){
		cout << ans[i];
	}
	cout << endl;*/
	
	if(check == 1){
		for(int i = str2.size(); i < str1.size(); ++i){
			int num = str1[i] - '0';
			if(next){
				num++;
			}
			ans[i] = num;
			next = false;	
			if(num >= 10){
				num -= 10;
				ans[i] -= 10;
				next = true;
			}
		}
		if(next){
			ans[str1.size()] = 1;
			long_ ++;
		}
	}	
	
	if(check == 3){
		for(int i = str1.size(); i < str2.size(); ++i){
			int num = str2[i] - '0';
			if(next){
				num++;
			}
			ans[i] = num;
			next = false;	
			if(num >= 10){
				num -= 10;
				ans[i] -= 10;
				next = true;
			}
		}
		if(next){
			ans[str2.size()] = 1;
			long_ ++;
		}
	}

	for(int i = long_ - 1; i >= 0; --i){
		cout << ans[i];
	}/*
	for(int i = 0; i < min(str1.size(), str2.size()) + 1; ++i){
		cout << ans[i];
	}*/

	cout << endl;
	//reverse(ans.begin(), ans.end());
	//cout << ans << endl;
	}
		
	return 0;
}

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

int main(){
	string a,b;
	vector<int> sum;
	int al, bl, sl;
	int n, up, num;
	cin >> n;
	for(int i=0;i<n;++i){
		cin >> a >> b;
		al = a.size()-1;
		bl = b.size()-1;
		up = 0;
		while(al >= 0 || bl >= 0){
			num = up;
			if(al>=0) num += a[al] - 48;
			if(bl>=0) num += b[bl] - 48;
			if(num > 9){
				sum.push_back(num - 10);
				up = 1;
			}else{
				sum.push_back(num);
				up = 0;
			}
			al -= 1;
			bl -= 1;
		}
		sum.push_back(up);
		sl = sum.size();
		if(sl >= 80){
			cout << "overflow" << endl;
		}else{
			for(int j=sl-1;j>=0;--j){
				cout << sum[j];
			}
			cout << endl;
			sum.clear();
		}
	}
	return 0;
}

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


using namespace std;

//?????????10?????????????????????
vector<string> splitstring(string str, int n){
  vector<string> tmp;
  for(int i=str.length();i>0;i-=n){
    // cout << "i: " << i << ", i-n:" << i-n << ", i-1:" << i-1 << " ";
    if(i-n > 0){
      // cout << "str.substr: " << str.substr(i-n, n) << endl;
      tmp.push_back(str.substr(i-n, n));
    }
    else{
      // cout << "str.substr: " << str.substr(0, i) << endl;
      tmp.push_back(str.substr(0, i));
    }
  }
  
  // for(int i=0;i<str.length();i+=n){
    // cout << "str.substr: " << str.substr(i, n) << endl;
    // tmp.push_back(str.substr(i, n));
  // }

  return tmp;
}


int main(){
  int numDatasets;
  cin >> numDatasets;

  for(int i=0;i<numDatasets;i++){
    bool flag = false;
    vector<string> splittedStr1, splittedStr2;
    vector<int> splittedInt1, splittedInt2;
    string str1, str2;
    cin >> str1;

    //80????¶?????????´???
    if(str1.length() > 80){
      flag = true;
    }
    
    
    splittedStr1 = splitstring(str1, 7);
    for(int i=0;i<splittedStr1.size();i++){
      splittedInt1.push_back(std::stoi(splittedStr1[i]));
      // cout << splittedInt1[i] << endl;
    }

    cin >> str2;

    //80????¶?????????´???
    if(str2.length() > 80){
      flag = true;
    }
      
    splittedStr2 = splitstring(str2, 7);
    for(int i=0;i<splittedStr2.size();i++){
      splittedInt2.push_back(std::stoi(splittedStr2[i]));
      // cout << splittedInt2[i] << endl;
    }

    vector<int> calced(max(splittedInt1.size(), splittedInt2.size()), 0);
    
    for(int i=0;i<calced.size();i++){
      if(i < splittedInt1.size()){
	calced[i] += splittedInt1[i];
      }
      if(i < splittedInt2.size()){
	calced[i] += splittedInt2[i];
      }
    }

    //debug
    for(int i=0;i<calced.size();i++){
      // cout << i << ": " <<  calced[i] << endl;
    }

    // cout << "---" << endl;

    
    //??°???????????????
    vector<int> calcedRes(calced.size()+1, 0);
    for(int i=0;i<calced.size();i++){
      int tmp = (calced[i]+calcedRes[i])/(int)pow(10.0, 7);
      calcedRes[i] = (calced[i]+calcedRes[i]) - tmp*pow(10.0, 7);
      calcedRes[i+1] = tmp;
      // cout << "calcedRes[" << i  << "]: " << calcedRes[i] << ", tmp: " << tmp << endl;
    }
      
      
      
      // calcedRes[calced.size()-i-2] += tmp;
      // calced[calced.size()-i-1] -= tmp*pow(10.0, 7);

      // int tmp = calced[i-1]/(int)pow(10.0, 7);
      // calced[i-1] -= tmp;
      // calced[i] += tmp; 

    // cout << "---" << endl;

    //int to str;
    string res = "";
    for(int i=calcedRes.size()-1;i>-1;i--){
      string str = to_string(calcedRes[i]);
      // cout << i << ":" << str << endl;
      if(i==calcedRes.size()-1){
	
	if(calcedRes[i]==0){
	  continue;
	}
      }

      while(str.length()<7){
	if(i==calcedRes.size()-1 || i==calcedRes.size()-2){
	  break;
	}
	str += "0";
      }


      
      res += str;
    }

    // cout << "result: " << res << endl;
    if(res.length() > 80){
      flag = true;
    }
    
    if(flag == false){
      cout << res << endl;
    }else{
      cout << "overflow" << endl;
    }
    
  }
}