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 <string>
using namespace std;

int main() {
  
  int n,i,j;
  cin >> n;

  for(i=0;i<n;i++){
    string str1,str2,temp;
    int kuriagari=0;
    cin >> str1 >> str2;
    
    if (str1.size()<str2.size()){
      temp = str1;
      str1 = str2;
      str2 = temp;
    }
    int goukei[str1.size()+1];
    
    for(j=0;j<str1.size()+1;j++){
      
      if (j==str1.size()){
	goukei[0] = kuriagari;
      }else if (j<str2.size()){
	goukei[str1.size()-j] = str2[str2.size()-1-j] + str1[str1.size()-1-j] + kuriagari -'0' - '0';
	if (goukei[str1.size()-j] >= 10) {
	  goukei[str1.size()-j] -= 10;
	  kuriagari = 1;
	}else{
	  kuriagari = 0;
	}
      }else{
	goukei[str1.size()-j] = str1[str1.size()-1-j] + kuriagari -'0';
	if (goukei[str1.size()-j] >= 10) {
	  goukei[str1.size()-j] -= 10;
	  kuriagari = 1;
	}else{
	  kuriagari = 0;
	}
      }
    }
    if ((str1.size()>81)||( (str1.size()==80)&&(goukei[0]==1) )){
      cout << "overflow" <<endl;
    }else{
      if (goukei[0]!=0)
	cout << goukei[0] ;
      for(j=0;j<str1.size();j++)
	cout << goukei[j+1];
      cout <<endl;
    }
  }
      
  return 0;
  
}

C++: #include <iostream>
#include <string>
#include <cstdlib>
#include <algorithm>

#define MAX_DIGIT 80
#define ERROR ("overflow")



int main(void){
	using namespace std;

	int N;
	cin >> N;

	vector< pair<string,string> > input;
	input.reserve(N);

	for(int i=1; i<=N; i++){

		string slhs, srhs;
		cin >> slhs >> srhs;

		reverse(slhs.begin(), slhs.end());
		reverse(srhs.begin(), srhs.end());

		input.push_back(make_pair(slhs,srhs));
	}

	for(auto p=input.begin(); p!=input.end(); p++){
		auto slhs = p->first, srhs = p->second;

		if(slhs.size()>MAX_DIGIT || srhs.size()>MAX_DIGIT){
			cout << ERROR << endl;
			continue;
		}

		int lhs[MAX_DIGIT], rhs[MAX_DIGIT], res[MAX_DIGIT];
		fill(lhs, lhs+MAX_DIGIT, 0);
		fill(rhs, rhs+MAX_DIGIT, 0);
		fill(res, res+MAX_DIGIT, 0);
		auto atoi_digit = [] (char x) -> int {
			return atoi(&x);
		};
		transform(slhs.begin(), slhs.end(), lhs, atoi_digit);
		transform(srhs.begin(), srhs.end(), rhs, atoi_digit);

		int headDigit;
		for (headDigit=MAX_DIGIT-1; headDigit>=0; headDigit--){
			if(lhs[headDigit]!=0 || rhs[headDigit]!=0){
				break;
			}
		}
		if(headDigit != MAX_DIGIT-1){
			headDigit++;
		}

		int carry = 0;
		for (int j=0; j<=headDigit; j++){
			auto d = div(lhs[j]+rhs[j]+carry,10);
			carry = d.quot;
			res[j] = d.rem;
		}
		if(headDigit==MAX_DIGIT-1 && carry !=0){
			cout << ERROR << endl;
			continue;
		}


		for (; headDigit>=1; headDigit--){
			if(res[headDigit]!=0){
				break;
			}
		}
		for (int j=headDigit; j>=0; j--){
			cout << res[j];
		}
		cout << endl;
	}
}

C++: #include<bits/stdc++.h>
using namespace std;
char strn[1000],str1[101],str2[101];
void add(int one,int two)
{
    int n,carry=0;
    for(int i=0;one-i>=0||two-i>=0||carry==1;i++){
        if(one-i>=0&&two-i>=0)n=(str1[one-i]-'0')+(str2[two-i]-'0');
        else if(one-i>=0)n=(str1[one-i]-'0');
        else if(two-i>=0)n=(str2[two-i]-'0');
        else n=0;
        if(n+carry>9){
            strn[i]=n+'0'+carry-10;
            carry=1;
        }
        else {
            strn[i]=n+'0'+carry;
            carry=0;
        }
    }
}
int main()
{
    int n,firs,seco,co;
    cin>>n;
    for(int i=0;i<n;i++){
        cin>>str1>>str2;
        firs=strlen(str1);
        seco=strlen(str2);
        if(firs>80||seco>80)cout<<"overflow"<<endl;
        else {
            add(firs,seco);
            co=strlen(strn);
            if(co>80)cout<<"overflow"<<endl;
            else {
                for(int i=co-1;i>0;i--)printf("%c",strn[i]);
                cout<<endl;
            }
        }

    }
}

C++: #include<stdio.h>
#include<string>
#include<algorithm>
using namespace std;
int max(int a,int b){return a>b?a:b;}
int main(){
	int n;
	scanf("%d",&n);
	for(int d=0;d<n;d++){
		char a[100];
		char b[100];
		scanf(" %s",a);
		scanf(" %s",b);
		string s,t;
		int i=0;
		s.append(a);
		t.append(b);
		reverse(s.begin(),s.end());
		reverse(t.begin(),t.end());
		string ans;
		int k=0;
		for(i=0;i<max(s.size(),t.size())+1;i++){
			int x,y;
			if(i>=s.size())x=0;
			else x=s[i]-'0';
			if(i>=t.size())y=0;
			else y=t[i]-'0';
			int z=x+y+k;
			k=z/10;
			char c=z%10+'0';
			if(i!=max(s.size(),t.size())||c!='0')ans.append(1,c);
		}
		reverse(ans.begin(),ans.end());
		if(ans.size()>=81)printf("overflow\n");
		else printf("%s\n",ans.c_str());
	}
	return 0;
}