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<string.h>

int n;
char a[256], b[256];
int temp[100], temp2[100];

int main()
{
        std::cin >> n;
	
	while( n-- ){
		memset( temp, 0, sizeof( temp ) );
		memset( temp2, 0, sizeof( temp2 ) );
		
		std::cin >> a >> b;
		
		int maxlen = std::max( strlen( a ), strlen( b ) );
		
		if( maxlen > 79 ){
			std::cout << "overflow" << std::endl;
		 	continue;
		}
		
		for( int i = 0; i < strlen( a ); i++ ){
			temp[strlen( a ) - i - 1] = a[i] - '0';
		}
		
		for( int i = 0; i < strlen( b ); i++ ){
			temp2[strlen( b ) - i - 1] = b[i] - '0';
		}
		
		for( int i = 0; i < maxlen; i++ ){
			temp[i+1] += ( temp[i] + temp2[i] ) / 10;
			temp[i] = ( temp[i] + temp2[i] ) % 10;
		}
		
		if( temp[maxlen] ){
		 	if( maxlen + 1 > 79 ){
				std::cout << "overflow" << std::endl;
				continue;
			}
			
			std::cout << temp[maxlen];
		}
		
		for( int i = maxlen - 1; i >= 0; i-- ){
		 	std::cout << temp[i];
		}
		std::cout << std::endl;
	}

 	return 0;
}

C++: #include<cstdio>
#include<cstring>
#include<iostream>

#define N(c) (c-'0')

using namespace std;
 /*
	N	:整数文字列ペアの数
	ar[3][100]	:文字配列

 */



int main(){
	int i, j, k, n, s;
	char  ar[3][100], c;

  scanf("%d",&n);

  while(n--){
	  scanf("%s %s",ar[0],ar[1]);

	  for(i=0; i<2; i++){
		  j=strlen(ar[i]);
		  for(k=0; k<j/2; k++){
			  c=ar[i][k]; ar[i][k]=ar[i][j-k-1]; ar[i][j-k-1]=c;
		  }
	  }

    if(strlen(ar[0])<strlen(ar[1])){
      strcpy(ar[2], ar[1]); strcpy(ar[1], ar[0]); strcpy(ar[0], ar[2]);
    }

    i=strlen(ar[0]);
    j=strlen(ar[1]);
    for(k=0,s=0; k<j; k++){
      s += N(ar[0][k]) + N(ar[1][k]);
      ar[2][k]=(s%10)+'0';
      s=s/10;
    }

    for( ; k<i; k++){
      if(ar[0][k]=='\0')  ar[2][k]=s+'0';
      else{
        s += N(ar[0][j]);
        ar[2][k]=(s%10)+'0';         
        s=s/10;
      }
    }

    if(0==s)  ar[2][k]='\0';
    else {
      ar[2][k]=s+'0';
      ar[2][k+1]='\0';
    }

// printf("%s\n%s\n%s\n",ar[0],ar[1],ar[2]);


	  i=2;
		  j=strlen(ar[i]);
		  for(k=0; k<j/2; k++){
			  c=ar[i][k]; ar[i][k]=ar[i][j-k-1]; ar[i][j-k-1]=c;
		  }

  if(80<strlen(ar[2]))  	printf("overflow\n");
  else printf("%s\n",ar[2]);
	}
    return 0;
}

C++: #include<iostream>
#include<string>
#include<algorithm>
using namespace std;
int main(){

  int n;
  cin >> n;
  for(int i=0;i<n;i++){
    string a,b,out,tmp;
    cin >> a;
    cin >> b;
    //int m=max(a.size(),b.size());
    if(a.size()<b.size()){
      tmp=a;
      a=b;
      b=tmp;
    }
    //cout << a << endl;
    int si=-a.size()+b.size();
    for(int j=a.size()-1;j>=0;j--){
      if(j+si>=0){
	//cout << si << endl;
	a[j]+=b[j+si]-'0';
	//cout << a[j] << endl;
      }else{
	a[j]+=0;
	//cout << "!" << endl;
      }
      if(a[j]>'9'&&j!=0){
	a[j-1]++;
	a[j]-=10;
      }else if(a[j]>'9'&&j==0){
	a='1'+a;
	a[1]-=10;
	//for(int i=a.size();i>0;i--)
	//  a[i]=a[i-1];
	//a[1]-=10;
	//a[0]='1';
      }
    }
    if(a[81]!='\0')
      cout << "overflow" << endl;
    else
      cout << a << endl;
  }

}

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=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++){
    vector<string> splittedStr1, splittedStr2;
    vector<int> splittedInt1, splittedInt2;
    string str1, str2;
    cin >> str1;
    
    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;

    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;
    cout << res << endl;
  }
}

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

int main () {
    int n;
    string a, b;
    
    cin >> n;
    for(int i=0;i<n;i++){
        int x[81]={0}, y[81]={0}, z[81]={0};
        
        cin >> a;
        cin >> b;
        
        reverse(a.begin(), a.end());
        reverse(b.begin(), b.end());
        
        for(int j=0;j<a.size();j++){
            x[j]=a[j]-'0';
        }
        for(int j=0;j<b.size();j++){
            y[j]=b[j]-'0';
        }
        int c=0;
        int r;
        for(int j=0;j<80;j++){
            r=x[j]+y[j]+c;
            if (r >= 10){
                z[j]=r%10;
                c=1;
            }else{
                z[j]=r;
                c=0;
            }
        }
        
        if(z[80]==1){
            cout << "overflow" <<endl;
        }else{
            bool flag = 0;
            for(int j=79;j>=0;j--){
                if(z[j]==0 && flag==0){
                    continue;
                }else{
                    flag=1;
                }
                cout<<z[j];
            }
            if(flag==0){
                cout<<0;
            }
            cout<<endl;
            
        }
    }
    return 0;
}