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<stdio.h>
#include <algorithm>
#include<string.h>
#include <utility>
using namespace std;
using namespace std;
int main(){
	int m;
	string in,in2;
	int a[90],b[90],ans[90];
	int f;
	int a_len,b_len,m_len;
	cin>>m;
	while(m--){
		for(int i=0;i<90;i++){
			a[i]=b[i]=ans[i]=0;
		}
		cin>>in;
		cin>>in2;
		a_len=in.size();
		b_len=in2.size();
		for(int i=0;i<a_len;i++){
			a[a_len-1-i]=in[i]-'0';
		}
		for(int i=0;i<b_len;i++){
			b[b_len-1-i]=in[i]-'0';
		}
		m_len=max(a_len,b_len);
		if(m_len>80){
			cout<<"overflow"<<endl;
			continue;
		}
		f=0;
		for(int i=0;i<m_len+1;i++){
			ans[i]=(a[i]+b[i]+f)%10;
			f=(a[i]+b[i]+f)/10;
		}
		if(ans[m_len]!=0){
			//if(m_len>79){
				//cout<<"overflow"<<endl;
				//continue;
			//}
			//cout<<ans[m_len];
		}
		
		for(int i=0;i<m_len;i++){
			//cout<<ans[m_len-1-i];
		}cout<<endl;
		
		
	}
	return 0;
}

C++: //0015
#include <bits/stdc++.h>
using namespace std;

int main(){
	long n;
	cin>>n;
	while(n--){
		string a,b,ans;
		long tmp = 0,i;
		cin>>a>>b;
		while(a.size() != b.size()){
		if(a.size() > b.size()){
			while(a.size() != b.size()){
				b = '0' + b;
			}
		}

		else if(a.size() < b.size()){
			while(a.size() != b.size()){
				a = '0' + a;
			}
		}
		}

		reverse(a.begin(),a.end());
		reverse(b.begin(),b.end());

		for(i = 0;i < a.size()||i < b.size();i++){
			tmp = (int)(a[i] - '0') + (int)(b[i] - '0') + (int)tmp;
			ans = ((char)((tmp % 10)+'0')) + ans;

			tmp /= 10;
		}

		if((tmp + '0') != (char)'0')
			ans = (char)tmp + ans;

		if(ans.size() > 80)
			cout<<"overflow"<<endl;
		else
			cout<<ans<<endl;
	}
	return 0;
}

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

using namespace std;

int main(){
	int n,l1,l2,i;
	string a, b;
	char ans[80];
	int ia,ib,sum;
	cin>>n;
	while(n--){
		cin>>a>>b;
		if(a.length()>80 || b.length()>80){
			cout<<"overflow"<<endl;
			continue;
		}
		if(a.length()==80 || b.length()==80){
			if((int)a[0]-'0'+(int)b[0]-'0'>9){
				cout<<"overflow"<<endl;
				continue;
			}
		}
		if(a.length()<b.length()){
			swap(a,b);
		}
		l1=a.length();
		l2=b.length();
		for(i=0;i<80;i++)ans[i]=0;
		for(i=1;i<l2+1;i++){
			ia=(int)a[l1-i]-(int)'0';
			ib=(int)b[l2-i]-(int)'0';
			sum=ia+ib;
			ans[80-i]+=sum+'0';
		}
		for(i=l2+1;i<l1+1;i++){
			ans[80-i]=a[l1-i];
		}
		for(i=79;i>=0;i--){
			if(ans[i]-'0'>9 && ans[i]!=0){
				ans[i]=ans[i]-10;
				if(ans[i-1]==0)ans[i-1]+=49;
				else ans[i-1]+=1;
			}
		}
		for(i=0;i<80;i++){
			if(ans[i]!=0){
				cout<<(int)ans[i]-'0';
			}
		}
		cout<<endl;
	}
	while(1);
	return 0;
}

C++: #include <iostream>
#include <string>
#include <sstream>
#include <deque>
class BigInt
{
  std::deque<char> m_digits;
public:
  BigInt(std::string str) {
    for(auto it = str.begin(); it != str.end(); ++it) {
      m_digits.push_back(*it - '0');
    }
  }

  BigInt(int num) {
    std::ostringstream oss;
    oss << num;
    BigInt(oss.str());
  }

  std::string str() const {
    if (m_digits.size() >= 80) {
      return "overflow";
    }
    std::string str;
    for (auto it = m_digits.begin(); it != m_digits.end(); ++it) {
      str += *it + '0';
    }
    return str;
  }

  const BigInt& operator +=(const BigInt& rhs) {
    std::deque<char> new_digits;
    auto lhs_rit = m_digits.rbegin();
    auto rhs_rit = rhs.m_digits.rbegin();

    char inc = 0;
    while (lhs_rit != m_digits.rend() &&
           rhs_rit != rhs.m_digits.rend()) {
      char temp = *lhs_rit + *rhs_rit;
      temp += inc;
      if (temp >= 10) {
        inc = temp / 10;
        temp -= inc * 10;
      }
      new_digits.push_front(temp);
      ++lhs_rit;
      ++rhs_rit;
    }

    if (lhs_rit != m_digits.rend()) {
      while (lhs_rit != m_digits.rend()) {
        char temp = *lhs_rit + inc;
        if (temp >= 10) {
          inc = temp / 10;
          temp -= inc * 10;
        } else {
          inc = 0;
        }
        new_digits.push_front(temp);
        ++lhs_rit;
      }
    }
    else if (rhs_rit != rhs.m_digits.rend()) {
      while (rhs_rit != rhs.m_digits.rend()) {
        char temp = *rhs_rit + inc;
        if (temp >= 10) {
          inc = temp / 10;
          temp -= inc * 10;
        } else {
          inc = 0;
        }
        new_digits.push_front(temp);
        ++rhs_rit;
      }
    }
    if (inc > 0) new_digits.push_front(inc);
    m_digits = new_digits;
  }
};

BigInt operator +(const BigInt& lhs, const BigInt& rhs)
{
  BigInt result(lhs);
  result += rhs;
  return result;
}

int main()
{
  int n;
  std::cin >> n;
  while (n--) {
    std::string num;
    std::cin >> num;
    BigInt num1(num);
    std::cin >> num;
    BigInt num2(num);
    
    BigInt num3 = num1 + num2;
    std::cout << num3.str() << std::endl;
  }
  return 0;
}

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

int main() {
  
  int n,i,j;
  cin >> n;

  for(i=0;i<n;i++){
    string str1,str2,temp;
    int kuriagari=0;
    cin >> str1 >> str2;
    
    if (str1.size()<str2.size()){
      temp = str1;
      str1 = str2;
      str2 = temp;
    }
    int goukei[str1.size()+1];
    
    for(j=0;j<str1.size()+1;j++){
      
      if (j==str1.size()){
	goukei[0] = kuriagari;
      }else if (j<str2.size()){
	goukei[str1.size()-j] = str2[str2.size()-1-j] + str1[str1.size()-1-j] + kuriagari -'0' - '0';
	if (goukei[str1.size()-j] >= 10) {
	  goukei[str1.size()-j] -= 10;
	  kuriagari = 1;
	}else{
	  kuriagari = 0;
	}
      }else{
	goukei[str1.size()-j] = str1[str1.size()-1-j] + kuriagari -'0';
	if (goukei[str1.size()-j] >= 10) {
	  goukei[str1.size()-j] -= 10;
	  kuriagari = 1;
	}else{
	  kuriagari = 0;
	}
      }
    }
    if ((str1.size()>81)||( (str1.size()==80)&&(goukei[0]==1) )){
      cout << "overflow" <<endl;
    }else{
      if (goukei[0]!=0)
	cout << goukei[0] ;
      for(j=0;j<str1.size();j++)
	cout << goukei[j+1];
      cout <<endl;
    }
  }
      
  return 0;
  
}