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 <stdio.h>
#include <string.h>

class overflow {
public:
	overflow() {};
};

class BigNum {
public:
	BigNum() {} ;

	BigNum(int n) {
		size = n;
		c = new char [size];
		memset(c, 0, size);
	};

	BigNum(const char* buf) {
		keta = strlen(buf);
		if (buf[keta-1] == '\n') {
			keta --;
		}
		if (keta > 80) {
			throw overflow();
		}
		size = keta;
		c = new char [size];
		int k = 0;
		for (int i = keta - 1; i >= 0; i--) {
			c[k++] = buf[i] - '0';
		}
	};

	BigNum(BigNum& b) {
		size = b.size;
		keta = b.keta;
		c = new char [size];
		memcpy(c, b.c, size);
	};

	BigNum& operator=(const BigNum& b) {
		size = b.size;
		keta = b.keta;
		c = new char [size];
		memcpy(c, b.c, size);

		return *this;
	};

	~BigNum() {
		if (c) {
			delete [] c;
		}
	};

	BigNum add(BigNum& n) {
		BigNum* a;
		BigNum* b;

		if (keta > n.keta) {
			a = this;
			b = &n;
		} else {
			a = &n;
			b = this;
		}

		BigNum r(a->keta + 1);

		int h;
		int m = 0;
		int i;
		for (i = 0; i < a->keta; i++) {
			if (i <= b->keta) {
				h = a->c[i] + b->c[i] + m;
				m = h / 10;
				r.c[i] = h % 10;
			} else {
				h = a->c[i] + m;
				m = h / 10;
				r.c[i] = h % 10;
			}
		}
		if (m > 0) {
			r.c[i] = m;
			r.keta = i+1;
		} else {
			r.keta = i;
		}
		
		if (r.keta > 80) {
			throw overflow();
		}

		return r;
	};

	void print(void) const {
		for (int i = keta - 1; i >= 0; i --) {
			printf("%d", c[i]);
		}
		printf("\n");
	};

	char* c = nullptr;
	int keta = 0;

private:
	size_t size = 0;
};

int
main(void)
{
	char line[100];
	int n;

	if (fgets(line, sizeof line, stdin) == NULL) {
		return 1;
	}
	if (sscanf(line, "%d", &n) != 1) {
		return 1;
	}

	try {
		for (int i = 0; i < n; i++) {
			if (fgets(line, sizeof line, stdin) == NULL) {
				return 1;
			}
			BigNum a(line);

			if (fgets(line, sizeof line, stdin) == NULL) {
				return 1;
			}
			BigNum b(line);

			BigNum r;
			r = a.add(b);

			r.print();
		}
	}
	catch (overflow) {
		printf("overflow\n");
	}

	return 0;
}

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

string add(string a, string b) {
  while(a.size() < b.size()) a.insert(0, "0");
  while(b.size() < a.size()) b.insert(0, "0");
  int n = a.size();
  string res(n, ' ');
  for(int i = n-1, d = 0; i >= 0; --i) {
    int p = a[i]-'0';
    int q = b[i]-'0';
    int r = p+q+d;
    d = r/10;
    res[i] = r%10 + '0';
    if(i == 0 && d != 0) res.insert(0, string(1,d+'0'));
  }
  return res;
}

int main() {
  int Tc;
  cin >> Tc;
  while(Tc--) {
    string a, b;
    cin >> a >> b;
    string tmp = add(a,b);
    if(tmp.size() > 80) cout << "overflow" << endl;
    else cout << tmp << endl;
  }
  return 0;
}

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

using namespace std;


int main(){
	char buf1[101],buf2[101];
	bool overflowFLG;
	int n,length1,length2,kuriagari=0,tmp,output[101];
	scanf("%d",&n);

	for(int loop=0;loop<n;loop++){
		length1 = length2 = 0;
		scanf("%s",buf1);
		scanf("%s",buf2);
		for(int i=0; buf1[i] !='\0';i++)length1++;
		for(int i=0; buf2[i] !='\0';i++)length2++;

		if(length1 >=81 || length2 >= 81)printf("overflow\n");
		else{
			overflowFLG=false;
			if(length1>length2){
				for(int k=0;length1-1-k>=0;k++){
					tmp = kuriagari;
					if(length2-1-k>=0){
						tmp = (buf1[length1-1-k]-'0')+(buf2[length2-1-k]-'0');
						if(tmp>=10){
							tmp %= 10;
							kuriagari = 1;
							if(k==79)overflowFLG=true;
						}else{
							kuriagari = 0;
						}
						output[length1-1-k] = tmp;
					}else{
						if(kuriagari == 0){
							output[length1-1-k] = buf1[length1-1-k]-'0';
						}else{
							output[length1-1-k] = buf1[length1-1-k]-'0' + kuriagari;
							if(output[length1-1-k]>=10){
								output[length1-1-k] %= 10;
								kuriagari = 1;
								if(k==79)overflowFLG=true;
							}else{
								kuriagari = 0;
							}
						}
					}
				}
				if(overflowFLG){
					printf("overflow\n");
				}else{
					if(kuriagari == 1)printf("1");
					for(int k=0;k<=length1-1;k++){
						printf("%d",output[k]);
					}
					printf("\n");
				}
			}else{
				for(int k=0;length2-1-k>=0;k++){
					tmp = kuriagari;
					if(length1-1-k>=0){
						tmp = (buf2[length2-1-k]-'0')+(buf1[length1-1-k]-'0');
						if(tmp>=10){
							tmp %= 10;
							kuriagari = 1;
							if(k==79)overflowFLG=true;
						}else{
							kuriagari = 0;
						}
						output[length2-1-k] = tmp;
					}else{
						if(kuriagari == 0){
							output[length2-1-k] = buf1[length2-1-k]-'0';
						}else{
							output[length2-1-k] = buf1[length2-1-k]-'0' + kuriagari;
							if(output[length2-1-k]>=10){
								output[length2-1-k] %= 10;
								kuriagari = 1;
								if(k==79)overflowFLG=true;
							}else{
								kuriagari = 0;
							}
						}
					}
				}
				if(overflowFLG){
					printf("overflow\n");
				}else{
					if(kuriagari == 1)printf("1");
					for(int k=0;k<=length2-1;k++){
						printf("%d",output[k]);
					}
					printf("\n");
				}
			}
		}
	}

    return 0;
}

C++: #include <iostream>
#define MAX 79

#include <string>

using namespace std;

int main()
{
	int n;
	cin >> n;
	for (int counter = 0; counter < n; counter++){
		string a, b;
		cin >> a >> b;
		if (a.length() > MAX || b.length() > MAX){
			cout << "overflow" << endl;
			continue;
		}
		int inta[MAX + 2], intb[MAX + 2], sum[MAX + 2];
		fill(inta, inta + MAX + 2, 0);
		fill(intb, intb + MAX + 2, 0);
		fill(sum, sum + MAX + 2, 0);
		for (int i = 0; i < (signed)a.length(); i++){
			inta[i] = a[a.length() - i - 1] - '0';
		}
		for (int i = 0; i < (signed)b.length(); i++){
			intb[i] = b[b.length() - i - 1] - '0';
		}

		int over = 0;
		for (int i = 0; i < MAX + 2; i++){
			sum[i] = inta[i] + intb[i] + over;
			if (sum[i] >= 10){
				sum[i] -= 10;
				over = 1;
			}
			else{
				over = 0;
			}
		}

		for (int i = MAX + 1;; i--){
			if (sum[i] != 0){
				if (i >= MAX){
					cout << "overflow" << endl;
					break;
				}
				for (int j = i; j >= 0; j--){
					cout << sum[j];
				}
				cout << endl;
				break;
			}
		}
	}
	return 0;
}

C++: #include<cstdio>
#include<iostream>
#include<vector>
#include<algorithm>
using namespace std;
int main() {
	int n;
	string a,b;
	cin >> n;
	while(n--) {
		cin >> a >> b;
		string ret = "";
		reverse(a.begin(),a.end());
		reverse(b.begin(),b.end());
		int a_len = a.size(),b_len = b.size();
		int num = max(a_len,b_len),carry = 0,v;
		char tmp[1];
		for(int i = 0; i < num; i++) {
			if(a_len <= i || b_len <= i) {
				if(b_len <= i) v = carry + a[i] - '0';
				else v = carry + b[i] - '0';
			}
			else v = carry + a[i] - '0' + b[i] - '0';
			carry = v / 10;
			sprintf(tmp,"%d",v % 10);
			ret += tmp;
		}
		reverse(ret.begin(),ret.end());
		cout << (ret.size() > 80 ? "overflow" : ret) << endl;
	}
	return 0;
}