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 <bits/stdc++.h>
using namespace std;
using uint = unsigned int;
using lint = long long int;
using ulint = unsigned long long int;
template<class T = int> using V = vector<T>;
template<class T = int> using VV = V< V<T> >;
template<class T, class U> void assign(V<T>& v, int n, const U& a) { v.assign(n, a); }
template<class T, class... Args> void assign(V<T>& v, int n, const Args&... args) { v.resize(n); for (auto&& e : v) assign(e, args...); }


string add(string& a, string& b) {
  int n = max(a.size(), b.size()) + 1;
  reverse(begin(a), end(a));
  reverse(begin(b), end(b));
  while (a.size() < n) a += '0';
  while (b.size() < n) b += '0';
  string c(n, 0);
  int d = 0;
  for (int i = 0; i < n; ++i) {
    int x = (a[i] - '0') + (b[i] - '0') + d;
    if (x < 10) {
      c[i] = '0' + x;
      d = 0;
    } else {
      c[i] = '0' + x - 10;
      d = 1;
    }
  }
  while (c.back() == '0') c.pop_back();
  reverse(begin(c), end(c));
  return c;
}

int main() {
  cin.tie(nullptr); ios_base::sync_with_stdio(false);
  int n; cin >> n;
  for (int _ = 0; _ < n; ++_) {
    string a, b; cin >> a >> b;
    auto c = add(a, b);
    if (c.size() > 80) cout << "overflow" << '\n';
    else cout << c << '\n';
  }
}

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 {
    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 <iomanip>
#include <vector>
#include <string>
#include <stack>
#include <set>
#include <cmath>
#include <algorithm>
#include <numeric>
using namespace std;

int stoi(char c){
	return c-'0';
}

char itos(int a){
	return a+'0';
}

int main(){
	int n;
	string s1,s2,ans;
	int lmin , lmax , sum , tmp , tmp2;
	bool bs1=true;
	cin >> n;
	for(int i=0; i<n ; ++i){
		cin >> s1 >> s2;
		reverse(s1.begin(),s1.end());
		reverse(s2.begin(),s2.end());
		lmin=min(s1.length(),s2.length());
		lmax=max(s1.length(),s2.length());
		if(s1.length()<s2.length()) bs1=false;
		for(int j=0;j<lmin;++j){
			ans.resize(j+2 , '0');
			sum=stoi(s1[j])+stoi(s2[j]);
			if(sum>=10){
				ans[j+1]=itos(1);
				tmp=stoi(ans[j]);
				tmp+=sum%10;
				ans[j]=itos(tmp);
			} else {
				int tmp=stoi(ans[j]);
				tmp+=sum;
				if(tmp>=10){
					ans[j+1]=itos(1);
					tmp2=stoi(ans[j]);
					tmp2=tmp%10;
					ans[j]=itos(tmp2);
				} else {
					ans[j]=itos(tmp);
				}
			}
			
		}
		for(int k=lmin; k<lmax ; ++k){
			ans.resize(k+2, '0');
			if(bs1){
				sum=stoi(ans[k])+stoi(s1[k]);
				if(sum>=10){
				ans[k+1]=itos(1); 
				tmp=sum%10;
				ans[k]=itos(tmp);
				} else {
					ans[k]=itos(sum);
				}
			} else {
				sum=stoi(s2[k]);
				sum+=stoi(ans[k]);
				if(sum>=10){
				ans[k+1]=itos(1);
				tmp=sum%10;
				ans[k]=itos(tmp);
				} else {
				ans[k]=itos(sum);
				}
			}
		}
		reverse(ans.begin(),ans.end());
		if(ans[0]=='0') ans.erase(0,1);
		if(ans.length()>80|| s1.length()>80 || s2.length()>80) {cout << "overflow" << endl;}
		else {
			
			cout << ans << endl;
			ans.erase();
		}
	}
	return 0; 
}