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<bits/stdc++.h>
#include<vector>
#include<list>
#include<stack>
#include<queue>
#include<algorithm>
using namespace std;


int main(){
    int t;
    scanf("%d",&t);
    while(t--){
        char c1[80],c2[80];
        scanf("%s",&c1);
        scanf("%s",&c2);
        string s1,s2;
        s1=c1, s2=c2;
        if(s1=="0" && s2=="0"){
            printf("0\n");
            continue;
        }
        reverse(s1.begin(),s1.end());
        reverse(s2.begin(),s2.end());
        int n=max(s1.size(),s2.size())+1;
        for(int i=s1.size();i<n;i++){
            s1+='0';
        }
        for(int i=s2.size();i<n;i++){
            s2+='0';
        }
        string res;
        int b=0;
        for(int i=0;i<max(s1.size(),s2.size());i++){
            int u=s1[i]-'0'+s2[i]-'0'+b;
            b=(u>=10); u%=10;
            res+=u+'0';
        }

        reverse(res.begin(),res.end());
        string res2;
        int j=0;
        while(res[j]=='0'){
            j++;
        }
        for(;j<res.size();j++){
            res2+=res[j];
        }

        if(res2.size()>80){
            printf("overflow\n");
        }else{
            printf("%s\n",res2.c_str());
        }
    }
    return 0;
}

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

using namespace std;
string a,b,sum;
void check(int z,int x);
int A,B;
int main(void){

    int n;
    cin>>n;
    while(n--){
        a="",b="",sum="";

        cin>>a>>b;
        //cout<<"a="<<a<<"b="<<b<<"sum="<<sum<<endl;
        if(a.length()>80||b.length()>80){
            sum="overflow";
            goto end;
        }
        A=a.length(),B=b.length();
        //cout<<A<<B<<endl;
        reverse(a.begin(),a.end());
        reverse(b.begin(),b.end());
        if(A<B)swap(A,B),swap(a,b);
        for(int i=0;i<=A-B;i++){
            b+="0";
        }
        check(0,0);
        //cout<<sum<<endl;
        if(sum.length()>80){
            sum="overflow";
            goto end;
        }
        reverse(sum.begin(),sum.end());
        end:;
        cout<<sum<<endl;
    }
    return 0;
}

void check(int z,int x){
    int i;


    if(a[z]==0)a[z]='0';
    if(b[z]==0)b[z]='0';
    //cout<<a[z]<<"."<<b[z]<<"."<<sum<<endl;
    //cout<<"--"<<z<<endl;
    if(z==81||(z>A&&z>B&&x==1)||(z>=A&&z>=B&&x==0)){
            x=0;
            return;
    }
    i=(int)a[z]-'0' + (int)b[z]-'0' + x,x=0;
    //cout<<"*"<<i<<endl;
    //cout<<"-"<<z<<endl;
    if(i>=10)x=1,i-=10;
    sum+=i+'0';
    //cout<<"+"<<sum<<endl;

    check(z+1,x);

}

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

int main() {
	int n;
	cin >> n;
	for (int x = 0;x < n;x++) {
		string a, b;
		cin >> a >> b;
		int za = a.size();
		int zb = b.size();
		vector<int>v;

		for (int i = 0;i < max(za, zb);i++) {
			if (i < min(za, zb)) {
				int c = za - i - 1;
				int d = zb - i - 1;
				v.push_back((a[za - i - 1] - '0') + (b[zb - i - 1] - '0'));
			}
			else {
				if (za < zb)v.push_back(b[zb - i - 1] - '0');
				else v.push_back((a[za - i - 1]) - '0');
			}
		}
		int z = v.size();
		for (int i = 0;i < v.size();i++) {
			if (i != z- 1 && v[i] >= 10) {
				v[i] -= 10;
				v[i + 1]++;
			}
		}
		if (v.size() >= 81)cout << "overflow";
		else 	for (int i =z - 1;i > -1;i--)cout << v[i];
		cout << endl;
	}
	return 0;
}

C++: #include <stdio.h>
#include <string.h>

int main(void)
{
  int N;
  scanf("%d", &N);

  int i,j,k;
  for(i=0; i<N; ++i){

    int digits[2][80];
    for(j=0; j<2; ++j){
      for(k=0; k<80; ++k){
        digits[j][k] = 0;
      }
    }

    int overflow = 0; 

    for(j=0; j<2; ++j){
      char digit[256];
      scanf("%256s", digit);
      getchar();
      
      if(overflow || strlen(digit)>80){
        overflow = 1;
      }else{
        for(k=0; k<strlen(digit); ++k){
          digits[j][k] = digit[strlen(digit)-1-k]-'0';
        }
      }
    }

    if(!overflow){
      int sum[81];
      for(j=0; j<81; ++j){
        sum[j] = 0;
      }

      for(j=0; j<80; ++j){
        for(k=0; k<2; ++k){
          sum[j] += digits[k][j];
        }
        sum[j+1] += sum[j]/10;
        sum[j] %= 10;
      }

      if(sum[80]>0){
        overflow = 1;
      }else{
        int available = 0;
        for(j=80; j>=0; --j){
          if(available || sum[j]!=0 || j==0){
            available = 1;
            printf("%d", sum[j]);
          }
        }
        printf("\n");
      }
    }

    if(overflow){
      printf("overflow\n");
    }
  }

  return 0;
}

C++: #include<cstdio>
#include<cstring>
#include<cstdlib>
#include<algorithm>
#include<string>
#include<iostream>
using namespace std;
int chrtoint(char a){
int i;
i=a-48;
 return i;
return 0;
}

int main(){
char n1[80];
char n2[80];
int rs[80];
int c1, c2;

int i,j,m, cmax, sum,testcase;
scanf("%d",&testcase);
while(testcase--){
scanf("%s", &n1);
scanf("%s", &n2);
c1 = strlen(n1);
c2 = strlen(n2);

reverse(n1,n1+c1);
reverse(n2,n2+c2);

cmax = c1;
if(c1<c2){
cmax = c2;
}

m=0;
for(i=0; i< cmax; i++){
if(c1==c2 || (i < c1 && i < c2)){
sum = m+chrtoint(n1[i])+chrtoint(n2[i]);
}else if(i >=c1){
sum = m+chrtoint(n2[i]);
}else if(i >=c2){
sum = m+chrtoint(n1[i]);
}
if(sum>9){
m = 1;
rs[i] = sum-10;
}
else{
m=0;
rs[i] = sum;
}
}

if(m){
rs[i]=m;
i++;
}
if(i>80){
    printf("overflow\n");
    continue;
}
for(j=0; j < i; j++){
printf("%d", rs[i-j-1]);
}
}
return 0;
}

C++: n = STDIN.gets.to_i
while n > 0

	line = STDIN.gets
	line =~ /(\d+)/
	a = $1.to_i
	line = STDIN.gets
	line =~ /(\d+)/
	b = $1.to_i
	
	puts a+b

	n -= 1
end