Go: package main

import (
        "fmt"
        "bufio"
        "os"
)

func main() {
        var n int
        var a, b string
        sc := bufio.NewScanner(os.Stdin)
        fmt.Scanf("%d", &n)
        for i := 0; i < n; i++ {
                sc.Scan()
                a = sc.Text()
                sc.Scan()
                b = sc.Text()

                if len(a) > len(b) {
                        s := add(a, b)
                        if s[len(a)] > 0 {
                                if len(a) >= 80 {
                                        fmt.Printf("overflow")
                                } else {
                                        for i := 0; i <= len(a);i++ {
                                                fmt.Printf("%d", s[len(a)-i])
                                        }
                                }
                                fmt.Printf("\n")
                        } else {
                                if len(a)-1 >= 80 {
                                        fmt.Printf("overflow")
                                } else {
                                        for i := 0; i < len(a);i++ {
                                                fmt.Printf("%d", s[len(a)-i-1])
                                        }
                                }
                                fmt.Printf("\n")
                        }
                } else {
                        s := add(b, a)
                        if s[len(b)] > 0 {
                                if len(b) >= 80 {
                                        fmt.Printf("overflow")
                                } else {
                                        for i := 0; i <= len(b);i++ {
                                                fmt.Printf("%d", s[len(b)-i])
                                        }
                                }
                                fmt.Printf("\n")
                        } else {
                                if len(b)-1 >= 80 {
                                        fmt.Printf("overflow")
                                } else {
                                        for i := 0; i < len(b);i++ {
                                                fmt.Printf("%d", s[len(b)-i-1])
                                        }
                                }
                                fmt.Printf("\n")
                        }
                }
        }
}

// expects ln > lm
func add(n string, m string) (s [100]int) {
        ln := len(n)
        lm := len(m)
        var sum [100]int

        for i := 0; i < ln; i++ {
                var s, t int
                s = int(n[ln-1-i]) - int('0')

                if lm - 1 - i >= 0 {
                        t = int(m[lm-1-i]) - int('0')
                } else {
                        t = 0
                }

                sum[i+1] = (s + t + sum[i]) / 10
                sum[i] = (s + t + sum[i]) % 10
        }

        return sum
}

C++: #include <iostream>
#include <vector>
#include<string>
#include<sstream>
#define CAPACITY 9


namespace jisaku{//stoi?????¨?????????????????????????????????????????§??£?¨???????

	int stoi(std::string str){
		int ret;
		std::stringstream ss;
		ss << str;
		ss >> ret;
		return ret;
	}

     std::string to_string(int i){
		 std::string text;
		std::stringstream textline;
		textline<< i;
		textline >> text;
		return text;
	}

}




class OverDigit{

public:

	int num_a[CAPACITY];
	int num_b[CAPACITY];
	OverDigit(std::string str_a, std::string str_b);
	void ChangeDigit(std::string str, int num[]);
	void Add();
};

OverDigit::OverDigit(std::string str_a, std::string str_b){

	ChangeDigit(str_a, num_a);
	ChangeDigit(str_b, num_b);

}

void OverDigit::ChangeDigit(std::string str, int num[]){

	std::vector <int> num_str(CAPACITY, 0);
	int loc = CAPACITY, k = 8;
	int  str_rest,str_Max;

	str_rest = str.length() % CAPACITY;
	str_Max = str.length() / 9;

	for (int j = str.length() - CAPACITY; str_Max > 0; j = j - CAPACITY){
		num_str[k] = jisaku::stoi(str.substr(j, CAPACITY));
		loc = k;
		k--;
		str_Max--;

	}
	//?????????????????????????????????????????\??????

	if (str_rest != 0)num_str[loc - 1] = jisaku::stoi(str.substr(0, str_rest));

	for (int j = 0; j < CAPACITY; j++){
		num[j] = num_str[j];
	}
}
//80????????§????¶???????
void OverDigit::Add(){

	int num_ans[CAPACITY] = {};
	int fat, overflow = 0, count = 0;
	std::string text,t;


	for (int j = CAPACITY - 1; j >0; j--){
		if (num_a[j] + num_b[j] <= 999999999){
			num_ans[j] = num_a[j] + num_b[j];
		}
		else if (num_a[j] + num_b[j] > 999999999){
			fat = (num_a[j] + num_b[j]) - 1000000000;
			num_ans[j] = fat;
			num_a[j - 1]++;
		}

	}

	if (num_a[0] + num_b[0] > 99999999){
		overflow = 1;
	}
	else{
		num_ans[0] = num_a[0] + num_b[0];


	}

	if (overflow == 1){
		std::cout << "overflow" << std::endl;
	}
	else {
		for (int j = 0; j < CAPACITY; j++){
			if (num_ans[j] != 0){
				count++;
				t = jisaku::to_string(num_ans[j]);
				if (t.length() != 9){
					for (int l = 0; l < (9 - t.length()); l++){
						text += "0";
					}
				}
				text += t;


				std::cout << text<<std::endl;
			}
			else if (num_ans[j] == 0 && count >= 1){
					text+="000000000";
					std::cout << text<<std::endl;
			}

		}std::cout <<text<< std::endl;
	}

};


int main(){


	//vector???????????¨??????80???????¨????
	int N;
	std::string str_a, str_b;

	std::cin >> N;
	std::cin.ignore();

	for (int i = 0; i < N; i++){
		getline(std::cin, str_a);
		getline(std::cin, str_b);
		if (str_a == "0" && str_b == "0"){std::cout << '0' << std::endl;
		}
		else if (str_a.length() >= 81 || str_b.length() >= 81){
		
			std::cout << "overflow" << std::endl;
		}
		else{

			OverDigit OverDigit_a(str_a, str_b);//???????????§??????
			OverDigit_a.Add();
		}
	}
	return 0;
}

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

using namespace std;

int main(void)
{
	char a[256],b[256];
	int n,flag,count1,count2,count3,daisyo=0,miss=0;
	cin>>n;

	for (int i=0; i<n; i++) 
	{
		flag=0;
		daisyo=0;
		miss=0;

		cin>>a>>b;
		count1= strlen(a);
		count2= strlen(b);
		
		if (count1>80||count2>80) {
			flag=1;
		}
		count3=count1-count2;
		
		if(count2>count1)
		{
			count3*=-1;
			daisyo=1;
		}
		
		if(daisyo==0)
		{
			if(!(count1==count2))
			for(int j=0;j<=count2;j++)
			{
				b[count2-j+count3]=b[count2-j];
			}
			for (int j=0; j<count3; j++) {
				b[j]='0';
			}
			
			for (int j=1; j<=count1; j++) {
				a[count1-j]+=b[count1-j]-48;
				if (((int)a[count1-j])-48>9) 
				{
					if (j>=80) {
						flag=1;
						break;
					}
					a[count1-j]=(a[count1-j]-48)%10+48;
					if ((count1-j)==0) {
						miss=1;
					}
					else {
							a[count1-j-1]=a[count1-j-1]+1;

						
					}
				}
			}
		}
		
		else
		{
			for(int j=0;j<=count1;j++)
			{
				a[count1-j+count3]=a[count1-j];
			}
			for (int j=0; j<count3; j++) {
				a[j]='0';
			}
			
			for (int j=1; j<=count2; j++) {
				b[count2-j]+=a[count2-j]-48;
				if (((int)b[count2-j]-48)>9) 
				{
					if (j>=80) {
						flag=1;
						break;
					}
					b[count2-j]=(b[count2-j]-48)%10+48;
					if ((count2-j)==0) {
						miss=1;
					}
					else {
						b[count2-j-1]=b[count2-j-1]+1;
					}
				}
			}
		}
		
		if(flag==1)
		{
			cout << "overflow" << endl;
		}
		
		else 
		{
			if (miss==1) {
				cout << '1';
			}
			if(daisyo==0) 
			{
				for (int j=0; j<count1; j++) 
				{
					cout << (int)a[j]-48;
				}
			}
			
			else 
			{
				for (int j=0; j<count2; j++) 
				{
					cout << (int)b[j]-48;
				}
			}
			cout << endl;

		}
		
		
	}
		
		
	
	return (0);
}