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 <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;
}