Language: C++

#include <iostream>
#include <iomanip>
#include <cassert>
#include <string>

using namespace std;

typedef long long ll;

const ll BASE = 10000;
const int LogB = 4;
const int MAX_DIGIT = 100;

struct Bigint {
	ll digit[MAX_DIGIT];
	int size;
	Bigint (int sz = 1, ll a = 0) : size(sz) {
		memset(digit, 0, sizeof(digit));
		digit[0] = a;
	}
};

const Bigint ZERO(1,0), ONE(1, 1);

bool operator < (Bigint x, Bigint y) {
	if (x.size != y.size) return x.size < y.size;
	for (int i=x.size-1; i>=0; i--) if (x.digit[i] != y.digit[i]) return x.digit[i] < y.digit[i];
	return false;
}

bool operator > (Bigint x, Bigint y) { return y < x; }
bool operator <= (Bigint x, Bigint y) { return !(y < x); }
bool operator >= (Bigint x, Bigint y) { return !(x < y); }
bool operator != (Bigint x, Bigint y) { return x < y || y < x; }
bool operator == (Bigint x, Bigint y) { return !(x < y) && !(y < x); }

Bigint normal(Bigint x) {
	ll c = 0;
	for (int i=0; i<x.size; i++) {
		while (x.digit[i] < 0)
			x.digit[i+1] -= 1, x.digit[i] += BASE;
		ll a = x.digit[i] + c;
		x.digit[i] = a % BASE;
		c = a / BASE;
	}
	for (; c>0; c /= BASE) x.digit[x.size++] = c % BASE;
	while (x.size > 1 && x.digit[x.size-1] == 0) x.size--;
	return x;
}

Bigint convert(ll a) { return normal(Bigint(1, a)); }

Bigint convert(const string& s) {
	Bigint x;
	int i = s.size() % LogB;
	if (i > 0) i -= LogB;
	for (; i<(int)s.size(); i += LogB) {
		ll a = 0;
		for (int j=0; j<LogB; j++) a = 10 * a + (i+j >= 0 ? s[i+j] - '0' : 0);
		x.digit[x.size++] = a;
	}
	reverse(x.digit, x.digit+x.size);
	return normal(x);
}

ostream &operator << (ostream& os, Bigint x) {
	os << x.digit[x.size-1];
	for (int i=x.size-2; i>=0; i--) os << setw(LogB) << setfill('0') << x.digit[i];
	return os;
}

istream &operator >> (istream& is, Bigint &x) {
	string s; is >> s;
	x = convert(s);
	return is;
}

Bigint operator + (Bigint x, Bigint y) {
	if (x.size < y.size) x.size = y.size;
	for (int i=0; i<y.size; i++) x.digit[i] += y.digit[i];
	return normal(x);
}

Bigint operator - (Bigint x, Bigint y) {
	assert(x >= y);
	for (int i=0; i<y.size; i++) x.digit[i] -= y.digit[i];
	return normal(x);
}

Bigint operator * (Bigint x, Bigint y) {
	Bigint z(x.size + y.size);
	for (int i=0; i< x.size; i++)
		for (int j=0; j<y.size; j++)
			z.digit[i+j] += x.digit[i] * y.digit[j];
	return normal(z);
}

Bigint operator * (Bigint x, ll a) {
	for (int i=0; i<x.size; i++)
		x.digit[i] *= a;
	return normal(x);
}

pair<Bigint, ll> divmod(Bigint x, ll a) {
	ll c = 0, t;
	for (int i=x.size-1; i>=0; i--) {
		t = BASE*c + x.digit[i];
		x.digit[i] = t / a;
		c = t % a;
	}
	return pair<Bigint, ll>(normal(x), c);
}

Bigint operator / (Bigint x, ll a) { return divmod(x, a).first; }

ll operator % (Bigint x, ll a) { return divmod(x, a).second; }

pair<Bigint, Bigint> divmod(Bigint x, Bigint y) {
	if (x.size < y.size) return pair<Bigint, Bigint>(ZERO, x);
	int F =BASE / (y.digit[y.size-1] + 1);
	x = x * F; y = y * F;
	Bigint z(x.size - y.size + 1);
	for (int k = z.size - 1, i = x.size-1; k >= 0; k--, i--) {
		z.digit[k] = (i+1 < x.size ? x.digit[i+1] : 0) * BASE + x.digit[i];
		z.digit[k] /= y.digit[y.size-1];
		Bigint t(k + y.size);
		for (int j=0; j < y.size; j++) t.digit[k+j] = z.digit[k] * y.digit[j];
		t = normal(t);
		while (x < t) {
			z.digit[k] -= 1;
			for (int j = 0;j<y.size; j++) t.digit[k+j] -= y.digit[j];
			t = normal(t);
		}
		x = x-t;
	}
	return pair<Bigint, Bigint>(normal(z), x/F);
}

Bigint operator / (Bigint x, Bigint y) { return divmod(x, y).first; }

Bigint operator % (Bigint x, Bigint y) { return divmod(x, y).second; }

int main() {
	int n;
	cin >> n;
	string s1, s2;
	
	for (int i=0; i<n; i++) {
		cin >> s1 >> s2;
		Bigint x = convert(s1), y = convert(s2);
		Bigint z = x + y;
		if (z.size > 80) cout << "overflow" << endl;
		else cout << z << endl;
	}
	
	return 0;
}

Compiler output:
a.cc: In constructor 'Bigint::Bigint(int, ll)':
a.cc:18:17: error: 'memset' was not declared in this scope
   18 |                 memset(digit, 0, sizeof(digit));
      |                 ^~~~~~
a.cc:4:1: note: 'memset' is defined in header '<cstring>'; this is probably fixable by adding '#include <cstring>'
    3 | #include <cassert>
  +++ |+#include <cstring>
    4 | #include <string>
a.cc: In function 'Bigint convert(const std::string&)':
a.cc:62:9: error: 'reverse' was not declared in this scope
   62 |         reverse(x.digit, x.digit+x.size);
      |         ^~~~~~~