Language: C++

using namespace std;

void sum80(int* a, int* b);
int get80(string str, int* num);

int main(void){
	string a80, b80;
	int a[81], b[81];
	int i, N;
	cin>>N;
	i=0;
	while(i<N){
		cin>>a80;
		cin>>b80;
		if(get80(a80, a)<0 || get80(b80, b)<0){
			cout<<"overflow"<<endl;
		}else{
			sum80(a, b);
		}
		i++;
	}
	return 0;
}

int get80(string str, int* num){
	int d=str.length();
	if(d<=80){
		for(int i=0; i<d; i++){
			num[i]=str[d-1-i]-'0';
		}
		for(int i=d;i<81; i++){
			num[i]=0;
		}
		return d;
	}else{
		return -1;
	}

}

void sum80(int* a, int* b){
	int s[81], amari;
	amari=0;
	for(int i=0; i<81; i++){
		s[i]=(a[i]+b[i]+amari)%10;
		if(a[i]+b[i]+amari>=10){
			amari=1;
		}else{
			amari=0;
		}
	}
	if(s[80]!=0){
		cout<<"overflow"<<endl;
	}else{
		bool flag=false;
		for(int i=79; i>=0; i--){
			if(flag==true || s[i]!=0){
				flag=true;
				cout<<s[i];
			}
		}
		cout<<endl;
	}
}

Compiler output:
a.cc:4:11: error: 'string' was not declared in this scope
    4 | int get80(string str, int* num);
      |           ^~~~~~
a.cc:1:1: note: 'std::string' is defined in header '<string>'; this is probably fixable by adding '#include <string>'
  +++ |+#include <string>
    1 | using namespace std;
a.cc:4:23: error: expected primary-expression before 'int'
    4 | int get80(string str, int* num);
      |                       ^~~
a.cc:4:31: error: expression list treated as compound expression in initializer [-fpermissive]
    4 | int get80(string str, int* num);
      |                               ^
a.cc: In function 'int main()':
a.cc:7:9: error: 'string' was not declared in this scope
    7 |         string a80, b80;
      |         ^~~~~~
a.cc:7:9: note: 'std::string' is defined in header '<string>'; this is probably fixable by adding '#include <string>'
a.cc:10:9: error: 'cin' was not declared in this scope
   10 |         cin>>N;
      |         ^~~
a.cc:1:1: note: 'std::cin' is defined in header '<iostream>'; this is probably fixable by adding '#include <iostream>'
  +++ |+#include <iostream>
    1 | using namespace std;
a.cc:13:22: error: 'a80' was not declared in this scope
   13 |                 cin>>a80;
      |                      ^~~
a.cc:14:22: error: 'b80' was not declared in this scope
   14 |                 cin>>b80;
      |                      ^~~
a.cc:15:25: error: 'get80' cannot be used as a function
   15 |                 if(get80(a80, a)<0 || get80(b80, b)<0){
      |                    ~~~~~^~~~~~~~
a.cc:15:44: error: 'get80' cannot be used as a function
   15 |                 if(get80(a80, a)<0 || get80(b80, b)<0){
      |                                       ~~~~~^~~~~~~~
a.cc:16:25: error: 'cout' was not declared in this scope
   16 |                         cout<<"overflow"<<endl;
      |                         ^~~~
a.cc:16:25: note: 'std::cout' is defined in header '<iostream>'; this is probably fixable by adding '#include <iostream>'
a.cc:16:43: error: 'endl' was not declared in this scope
   16 |                         cout<<"overflow"<<endl;
      |                                           ^~~~
a.cc:1:1: note: 'std::endl' is defined in header '<ostream>'; this is probably fixable by adding '#include <ostream>'
  +++ |+#include <ostream>
    1 | using namespace std;
a.cc: At global scope:
a.cc:25:5: error: redefinition of 'int get80'
   25 | int get80(string str, int* num){
      |     ^~~~~
a.cc:4:5: note: 'int get80' previously defined here
    4 | int get80(string str, int* num);
      |     ^~~~~
a.cc:25:11: error: 'string' was not declared in this scope
   25 | int get80(string str, int* num){
      |           ^~~~~~
a.cc:25:11: note: 'std::string' is defined in header '<string>'; this is probably fixable by adding '#include <string>'
a.cc:25:23: error: expected primary-expression before 'int'
   25 | int get80(string str, int* num){
      |                       ^~~
a.cc: In function 'void sum80(int*, int*)':
a.cc:53:17: error: 'cout' was not declared in this scope
   53 |                 cout<<"overflow"<<endl;
      |                 ^~~~
a.cc:53:17: note: 'std::cout' is defined in header '<iostream>'; this is probably fixable by adding '#include <iostream>'
a.cc:53:35: error: 'endl' was not declared in this scope
   53 |                 cout<<"overflow"<<endl;
      |                                   ^~~~
a.cc:53:35: note: 'std::endl' is defined in header '<ostream>'; this is probably fixable by adding '#include <ostream>'
a.cc:59:33: error: 'cout' was not declared in this scope
   59 |                                 cout<<s[i];
      |                                 ^~~~
a.cc:59:33: note: 'std::cout' is defined in header '<iostream>'; this is probably fixable by adding '#include <iostream>'
a.cc:62:17: error: 'cout' was not declared in this scope
   62 |                 cout<<endl;
      |                 ^~~~
a.cc:62:17: note: 'std::cout' is defined in header '<iostream>'; this is probably fixable by adding '#include <iostream>'
a.cc:62:23: error: 'endl' was not declared in this scope
   62 |                 cout<<endl;
      |                       ^~~~
a.cc:62:23: note: 'std::endl' is defined in header '<ostream>'; this is probably fixable by adding '#include <ostream>'