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<sstream>
#include<iomanip>
using namespace std;
class BigInt{
  unsigned int i[11];
public:
  BigInt(string str){
    if(str.size()>80){i[0]=1; return;}
    i[0]=0;
    for(int j=80;j>0;j-=8){
      if((int)str.size()>j){
	stringstream ss;
	ss << str.substr(str.size()-j,8);
	ss >> i[11-j/8];
      }else if((int)str.size()+8>j){
	stringstream ss;
	ss << str.substr(0,str.size()+8-j);
	ss >> i[11-j/8];
      }else{
	i[11-j/8]=0;
      }
    }
  }
  BigInt():i{0,0,0,0,0,0,0,0,0,0,0}
  {}
  void print(){
    if(i[0]>0)
      cout << "overflow\n";
    else{
      int j=1;
      for(;j<11;j++)
	if(i[j]>0 || j == 10)
	  break;
      cout << i[j++];
      for(;j<11;j++)
	cout << setfill('0') << setw(8) << right << i[j];
      cout << "\n";
    }
  }
  BigInt operator+(const BigInt& rhs){
    BigInt result = rhs;
    for(int j=10;j>0;j--){
      result.i[j] += i[j];
      if(result.i[j]>=100000000){
	result.i[j]-=100000000;
	result.i[j-1]++;
      }
    }
    return result;
  }
};

int main(){
  int n;
  cin >> n;
  for(int i=0;i<n;i++){
    string tmp;
    cin>>tmp;
    BigInt b(tmp);
    cin>>tmp;
    BigInt a(tmp);
    (a+b).print();
  }
}

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

using namespace std;

typedef long long ll;

const ll BASE = 10000;
const int LogB = 4;
const int MAX_DIGIT = 100;

struct Bigint {
	ll digit[MAX_DIGIT];
	int size;
	Bigint (int sz = 1, ll a = 0) : size(sz) {
		memset(digit, 0, sizeof(digit));
		digit[0] = a;
	}
};

const Bigint ZERO(1,0), ONE(1, 1);

bool operator < (Bigint x, Bigint y) {
	if (x.size != y.size) return x.size < y.size;
	for (int i=x.size-1; i>=0; i--) if (x.digit[i] != y.digit[i]) return x.digit[i] < y.digit[i];
	return false;
}

bool operator > (Bigint x, Bigint y) { return y < x; }
bool operator <= (Bigint x, Bigint y) { return !(y < x); }
bool operator >= (Bigint x, Bigint y) { return !(x < y); }
bool operator != (Bigint x, Bigint y) { return x < y || y < x; }
bool operator == (Bigint x, Bigint y) { return !(x < y) && !(y < x); }

Bigint normal(Bigint x) {
	ll c = 0;
	for (int i=0; i<x.size; i++) {
		while (x.digit[i] < 0)
			x.digit[i+1] -= 1, x.digit[i] += BASE;
		ll a = x.digit[i] + c;
		x.digit[i] = a % BASE;
		c = a / BASE;
	}
	for (; c>0; c /= BASE) x.digit[x.size++] = c % BASE;
	while (x.size > 1 && x.digit[x.size-1] == 0) x.size--;
	return x;
}

Bigint convert(ll a) { return normal(Bigint(1, a)); }

Bigint convert(const string& s) {
	Bigint x;
	int i = s.size() % LogB;
	if (i > 0) i -= LogB;
	for (; i<(int)s.size(); i += LogB) {
		ll a = 0;
		for (int j=0; j<LogB; j++) a = 10 * a + (i+j >= 0 ? s[i+j] - '0' : 0);
		x.digit[x.size++] = a;
	}
	reverse(x.digit, x.digit+x.size);
	return normal(x);
}

ostream &operator << (ostream& os, Bigint x) {
	os << x.digit[x.size-1];
	for (int i=x.size-2; i>=0; i--) os << setw(LogB) << setfill('0') << x.digit[i];
	return os;
}

istream &operator >> (istream& is, Bigint &x) {
	string s; is >> s;
	x = convert(s);
	return is;
}

Bigint operator + (Bigint x, Bigint y) {
	if (x.size < y.size) x.size = y.size;
	for (int i=0; i<y.size; i++) x.digit[i] += y.digit[i];
	return normal(x);
}

Bigint operator - (Bigint x, Bigint y) {
	assert(x >= y);
	for (int i=0; i<y.size; i++) x.digit[i] -= y.digit[i];
	return normal(x);
}

Bigint operator * (Bigint x, Bigint y) {
	Bigint z(x.size + y.size);
	for (int i=0; i< x.size; i++)
		for (int j=0; j<y.size; j++)
			z.digit[i+j] += x.digit[i] * y.digit[j];
	return normal(z);
}

Bigint operator * (Bigint x, ll a) {
	for (int i=0; i<x.size; i++)
		x.digit[i] *= a;
	return normal(x);
}

pair<Bigint, ll> divmod(Bigint x, ll a) {
	ll c = 0, t;
	for (int i=x.size-1; i>=0; i--) {
		t = BASE*c + x.digit[i];
		x.digit[i] = t / a;
		c = t % a;
	}
	return pair<Bigint, ll>(normal(x), c);
}

Bigint operator / (Bigint x, ll a) { return divmod(x, a).first; }

ll operator % (Bigint x, ll a) { return divmod(x, a).second; }

pair<Bigint, Bigint> divmod(Bigint x, Bigint y) {
	if (x.size < y.size) return pair<Bigint, Bigint>(ZERO, x);
	int F =BASE / (y.digit[y.size-1] + 1);
	x = x * F; y = y * F;
	Bigint z(x.size - y.size + 1);
	for (int k = z.size - 1, i = x.size-1; k >= 0; k--, i--) {
		z.digit[k] = (i+1 < x.size ? x.digit[i+1] : 0) * BASE + x.digit[i];
		z.digit[k] /= y.digit[y.size-1];
		Bigint t(k + y.size);
		for (int j=0; j < y.size; j++) t.digit[k+j] = z.digit[k] * y.digit[j];
		t = normal(t);
		while (x < t) {
			z.digit[k] -= 1;
			for (int j = 0;j<y.size; j++) t.digit[k+j] -= y.digit[j];
			t = normal(t);
		}
		x = x-t;
	}
	return pair<Bigint, Bigint>(normal(z), x/F);
}

Bigint operator / (Bigint x, Bigint y) { return divmod(x, y).first; }

Bigint operator % (Bigint x, Bigint y) { return divmod(x, y).second; }

int main() {
	int n;
	cin >> n;
	string s1, s2;
	
	for (int i=0; i<n; i++) {
		cin >> s1;
		cin >> s2;
		Bigint x = convert(s1), y = convert(s2);
		Bigint z = x + y;
		if (z.size > 80) cout << "overflow" << endl;
		else cout << z << endl;
	}
	
	return 0;
}

C++: #include<iostream>
#include<stdio.h>
#include <algorithm>
#include <utility>
using namespace std;
using namespace std;
int main(){
	
	int m;
	char in[2][10000];
	char ans[100000];
	int ie[2];
	int ien[2];
	bool max10;
	int Min;
	bool NMin;
	scanf("%d",&m);
	for(;m--;){
		
		for(int i=0;i<2;i++){
			scanf(" %s",&in[i]);
		}
		for(int i=0;i<2;i++){
			for(ie[i]=0;in[i][ie[i]]!='\0';ie[i]++){
				in[i][ie[i]]=in[i][ie[i]]-'0';
			}
		}
		ie[0]--;
		ie[1]--;
		Min=max(ie[0],ie[1]);
		if((Min>80)||(Min>79)||(int(in[0][0])+int(in[1][0])>9)){
			cout<<"overflow"<<endl;
		}else{	
			if(ie[0]>ie[1]){
				NMin=0;
			}else{
				NMin=1;
			}
			ien[0]=ie[0];
			ien[1]=ie[1];
			for(;min(ien[0],ien[1])+1;ien[0]--,ien[1]--){
				in[NMin][ien[NMin]]=in[0][ien[0]]+in[1][ien[1]];
				if(in[NMin][ien[NMin]+1]>9){
					in[NMin][ien[NMin]+1]-=10;
					in[NMin][ien[NMin]]  +=1;
				}
				
			}
			max10=0;
			if(in[NMin][ien[NMin]+1]>9){
				in[NMin][ien[NMin]+1]-=10;
				max10=1;
			}
			for(int i=0;i<2;i++){
				for(int j=0;j<ie[i]+1;j++){
					in[i][j]=in[i][j]+'0';
				}
			}
			if(max10) cout<<"1";
			cout<<in[NMin]<<endl;
		}
	}
	return 0;
}