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.h>

using namespace std;

int main(void){
	int n;

	cin >> n;
	for(int i=0;i<n;i++){
		int len,mlen=0;
		char val[2][81]={0},tmp[81],crr=0;

		for(int j=0;j<2;j++){
			cin >> tmp;
			len = strlen(tmp);
			mlen = mlen>len?mlen:len;
			for(int k=0;k<len;k++)
				val[j][len-1-k]=tmp[k]-0x30;
		}

		for(int j=0;j<mlen;j++){
			val[0][j]+=val[1][j]+crr;
			crr=val[0][j]/10;
			val[0][j]%=10;

			if(j==mlen-1&&crr)
				mlen++;
		}

		if(!val[0][80]){
			for(int j=0;j<mlen;j++)
				tmp[j]=val[0][mlen-1-j]+0x30;
			tmp[mlen]='\0';
		}
		else
			strcpy(tmp,"overflow");

		cout << tmp << endl;
	}

	return 0;
}

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

int main(void)
{
	char num1[1000], num2[1000];
	int n, num1_k, num2_k, rec, nnum1, nnum2, k_up, bnum1, bnum2;
	int f1, f2;
	scanf("%d", &n);
	fflush(stdin);
	for(f1 = 0; f1 < n; f1++)
	{
		//初期化
		k_up = 0;
		//入力
		scanf("%s", num1);
		scanf("%s", num2);
		//それぞれの桁数を求める
		num1_k = strlen(num1);
		num2_k = strlen(num2);
		bnum1 = num1_k - 1;
		bnum2 = num2_k - 1;
		//桁数で低いほうの桁数分だけ回す
		rec = (num1_k > num2_k) ? num2_k : num1_k;
		for(f2 = rec - 1; f2 >= 0; f2--)
		{
			//それぞれ現在桁の数を割り出す
			nnum1 = num1[bnum1] - '0';
			nnum2 = num2[bnum2] - '0';
			//桁数の大きいほうへ入れる
			if(num1_k > num2_k)
			{
				num1[bnum1] = ((nnum1 + nnum2) % 10 + k_up) + '0';
			}
			else
			{
				num2[bnum2] = ((nnum1 + nnum2) % 10 + k_up) + '0';
			}
			//桁上がりを求める
			k_up = (nnum1 + nnum2) / 10;
			bnum1--;
			bnum2--;
		}
		//残りの部分を求める
		rec = (bnum1 > bnum2) ? bnum1 : bnum2;
		for(f2 = rec; f2 >= 0; f2--)
		{	
			//桁数の大きいほうへ入れる
			if(num1_k > num2_k)
			{
				nnum1 = num1[f2] - '0';
				num1[f2] = ((k_up + nnum1) % 10) + '0';
				k_up = (k_up + nnum1) / 10;
			}
			else
			{
				nnum2 = num2[f2] - '0';
				num2[f2] = ((k_up + nnum2) % 10) + '0';
				k_up = (k_up + nnum2) / 10;
			}			
		}
		if(num1_k > 80 || num2_k > 80)
		{
			printf("overflow\n");
		}
		else
		{
			if(k_up == 1)
			{
				if(num1_k + 1 > 80 || num2_k + 1 > 80)
				{
					printf("overflow\n");
				}
				else
				{
					printf("%d", k_up);
				}
			}
			if(num1_k > num2_k)
			{
				printf("%s\n", num1);
			}
			else
			{
				printf("%s\n", num2);
			}
		}
	}
	return 0;
}

C++: #include <cstdio>
#include <cmath>
#include <cstdlib>
#include <cstring>

const int MAX_INPUT = 1000000;
const int ARRAY_SIZE = 20;
const int MAX_LEN = 8;
char inputA[MAX_INPUT];
char inputB[MAX_INPUT];

void toBigInt(char *str, long *r) {
    int len = strlen(str);
    int endIndex = ARRAY_SIZE - (len / MAX_LEN) - 1;

    {
        int offset = 0;
        for (int i = 0; i < ARRAY_SIZE; ++i) {
            char temp[MAX_LEN + 1] = { 0 };
            int readLen = ((len - offset) < MAX_LEN) ? len - offset : MAX_LEN;
            char* start = str + len - offset - readLen;

            strncpy(temp, start, readLen);
            r[ARRAY_SIZE - i- 1] = (long)atoi(temp);
            offset += readLen;
        }
    }
}

int main() {
    const char OVER[] = "overflow\n";
    int n = 0;
    scanf("%d", &n);

    for (int i = 0; i < n; ++i) {
        scanf("%s", &inputA);
        scanf("%s", &inputB);
        long a[ARRAY_SIZE] = { 0 };
        long b[ARRAY_SIZE] = { 0 };
        long r[12] = { 0 };
        int lenA = 0;
        int lenB = 0;
        const int MAX_INPUT_LEN = 80;
        lenA = strlen(&inputA[0]);
        lenB = strlen(&inputB[0]);
        if ((MAX_INPUT_LEN < lenA) || (MAX_INPUT_LEN < lenB)) {
            printf(OVER);
            continue;
        }

        toBigInt(inputA, a);
        toBigInt(inputB, b);
        for (int i = 0; i < ARRAY_SIZE; ++i) {
            int curRIndex = 12 - i - 1;
            r[curRIndex] += a[ARRAY_SIZE - i - 1] + b[ARRAY_SIZE - i - 1];
            const int MAX = 99999999;
            if (MAX < r[curRIndex]) {
                r[curRIndex - 1] = r[curRIndex] / (MAX + 1);
                r[curRIndex] %= (MAX + 1);
            }
        }

        if ((9999999 < r[2]) || (0 < r[1])) {
            printf(OVER);
            continue;
        }

        {
            bool started = false;
            bool noOutput = true;
            for (int i = 0; i < 12; ++i) {
                if (!started && (0 < r[i])) {
                    started = true;
                    printf("%d", r[i]);
                    noOutput = false;
                    continue;
                }

                if (started) {
                    long temp = (r[i] == 0) ? 1 : r[i];
                    bool didFor = false;
                    for (int j = 0; temp * pow(10, j) < 1000000; ++j) {
                        printf("0");
                        didFor = true;
                    }
                    if (didFor) {
                        printf("0");
                    }
                    printf("%d", r[i]);
                }
            }

            if (noOutput) {
                printf("0");
            }
        }
        printf("\n");
        for (int i = 0; i < MAX_INPUT; ++i) {
            inputA[i] = 0;
            inputB[i] = 0;
        }
    }
    return 0;
}

C++: #include <iostream>
#include <algorithm>
#include <vector>
#include <cstdio>
#include <string>
#include <cmath>
#include <cfloat>
#include <map>
using namespace std;
int number(char c){
	if(c>='0'&&c<='9')
	return (c-'0');
	else
		return 0;
}
int main(){
	int n;
	cin>>n;
	
	while(n>0){
		string a;
		string b;
	int k=0;
	int c=0;
	string s="";
	
		cin>>a;
		cin>>b;
		
	
	for(int i=a.length()-1,j=b.length()-1;i>=0||j>=0;i--,j--){
		int d;
		int n1=0,n2=0;
		if(i>=0)
			n1=number(a[i]);
		if(j>=0)
			n2=number(b[j]);
		d=n1+n2+k;
		k=d/10;
		d=d%10;
		c++;
		s=to_string(d)+s;
	}
	if(c>=80&&k>0)
		cout<<"overflow"<<endl;
		else{
			if(k>0){
		s=to_string(k)+s;
			}
		cout<<s<<endl;
		}
		n--;
	}
}

C++: #include <cstdio>
#include <iostream>
#include <vector>
#include <cstring>
using namespace std;

int main(){
	int n;
	cin >> n;
	while( n-- > 0 ){
		string num1, num2;
		cin >> num1 >> num2;
		char sum[101];
		int degit = 0;
		for( int i = 0; i < 101; i++ ) sum[i] = '0';
		for( int i = 0; i < num1.size(); i++ ){
			sum[100-i] = num1[num1.size()-i-1];
		}
		int d;
		for( int i = 0; i < num1.size() || i < num2.size() || degit != 0; i++ ){
			int now;
			if( i < num2.size() ){
				now = (int)sum[100-i]-(int)('0') + (int)num2[num2.size()-i-1]-(int)('0') + degit;
			}else{
				now = (int)sum[100-i]-(int)('0') + degit;
			}
			sum[100-i] = (char)((now%10)+(int)'0');
			if( now >= 10 ) degit = 1;
			else degit = 0;
			d = i;
		}
		if( d < 80 ){
			for( int i = 100-d; i < 101; i++ ) cout << sum[i];
			cout << endl;
		}else{
			cout << "overflow" << endl;
		}
	}
}