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<bits/stdc++.h>
#define rep(i,n) for(int i=0;i<n;++i)
#define FOR(i,a,b) for(int i=a;i<=b;++i)
#define LL long long
using namespace std;
static const LL INF = 1LL<<61LL;
typedef pair<int,int> PII;

string a,b;
string ans;
int M;
char t[]={'0','1','2','3','4','5','6','7','8','9'};

void add(int n,int m,int c,string s){
    int k;
    if(n-1<0&&m-1<0){
        if(c!=0)s+=t[c];
        for(int i=s.size()-1;i>=0;--i){
            ans+=s[i];
        }
        return ;
    }
    if(n-1>=0&&m-1>=0)k=a[n-1]-'0'+b[m-1]-'0'+c;
    else if(n-1>=0)k=a[n-1]-'0'+c;
    else if(m-1>=0)k=b[m-1]-'0'+c;
    if(k>=10){
        k-=10;
        c=1;
    }
    add(n-1,m-1,c,s+t[k]);
}

/*bool MOD1(string num){
    bool res=false;
    LL k=num.size();
    LL mod;
    int i=-1;
    LL n=0;
    while(k){
        n=(n*10)+num[i+1]-'0';
        mod=n%M;
        n=mod;
        k--;
        i++;
    }
    if(mod==0)return true;
    return false;
}
bool MOD2(string num){
    bool res=false;
    LL k=num.size();
    LL mod;
    int i=-1;
    LL n=0;
    while(k){
        n=(n*10)+num[i+1]-'0';
        mod=n%5;
        n=mod;
        k--;
        i++;
    }
    if(mod==0)return true;
    return false;
}
bool MOD3(string num){
    bool res=false;
    LL k=num.size();
    LL mod;
    int i=-1;
    LL n=0;
    while(k){
        n=(n*10)+num[i+1]-'0';
        mod=n%7;
        n=mod;
        k--;
        i++;
    }
    if(mod==0)return true;
    return false;
}
bool MOD4(string num){
    bool res=false;
    LL k=num.size();
    LL mod;
    int i=-1;
    LL n=0;
    while(k){
        n=(n*10)+num[i+1]-'0';
        mod=n%11;
        n=mod;
        k--;
        i++;
    }
    if(mod==0)return true;
    return false;
}*/
int main(){
    int n;
    cin>>n;
    rep(i,n){
    cin>>a>>b;
    //cin>>M;
        add(a.size(),b.size(),0,"");
        //if(!MOD1(ans)&&!MOD2(ans)&&!MOD3(ans)&&!MOD4(ans))cout<<ans<<endl;
        //else cout<<"king is angry"<<endl;
        if(ans.size()<=80&&a.size()<=80&&b.size()<=80)cout<<ans<<endl;
        else cout<<"overflow"<<endl;
        ans.clear();
    }
}

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

class OverDigit{

public:

	int num_a[9];
	int num_b[9];
	OverDigit(string str_a, string str_b);
	void ChangeDigit(string str,int num[]);
	void Add();
};

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

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

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

	vector <int> num_str(9, 0);
	int loc = 9, k = 8;
	int str_Max, str_rest;

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


	for (int j = str.length() - 9; str_Max > 0; j = j-9){
		num_str[k] = stoi(str.substr(j, 9));
		loc = k;
		k--;
		str_Max--;
	}
	//?????????????????????????????????????????\??????
	num_str[loc - 1] = stoi(str.substr(0, str_rest));

	for (int j = 0; j < 9; j++){
	num[j]=num_str[j];
	}
}
//80????????§????¶???????
void OverDigit::Add(){

	int num_ans[9] = {};
	int fat,overflow=0,count=0;

	for (int j = 8; 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 && j!=1){
			fat = (num_a[j] + num_b[j]) - 1000000000;
			num_ans[j] = fat;
			num_a[j - 1]++;		
		}
		else if (num_a[j] + num_b[j] > 999999999 && j ==1 ){
			overflow = 1;
		}
	
	}

	if (overflow == 1){
		cout << "overflow" << endl;
	}
	else {
		for (int j = 0; j < 9; j++){
			if (num_ans[j] != 0){
				count++;
				cout << num_ans[j];

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

		}cout << endl;
	}

};

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

int main(){

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

	cin >> N;
	cin.ignore();

	for (int i = 0; i < N; i++){
		getline(cin, str_a);
		getline(cin, str_b);
		OverDigit OverDigit_a(str_a,str_b);//???????????§??????
		OverDigit_a.Add();
	}

	return 0;
}