Sum of two large numbers in C++ code example

Example 1: sum of 2 numbers in cpp

#include<iostream>
int add(int,int);
int main()
{
	using namespace std;
	int a,b;
	cout<<"Enter first number: ";
	cin>>a;
	cout<<"Enter second number: ";
	cin>>b;
	cout<<"Sum = "<<add(a,b);
}
int add(int x,int y)
{
	return(x+y);
}

Example 2: sum of two numbers c++

#include<iostream>
using namespace std;
int main()
{
	double number1,number2;
  	double sum0;
  	sum=number1+number2;
  	cout<<sum;
}

Example 3: Sum of two large numbers in C++

string sum(string a, string b){
    string res="";
    while(a.length() < b.length()) a="0"+a;
    while(b.length() < a.length()) b="0"+b;
    int carry=0;
    for(int i=a.length()-1;i>=0;i--)
    {
        int tmp=a[i]-48 + b[i]-48 + carry;
        carry=tmp/10;
        tmp=tmp%10;
        res=(char)(tmp+48)+res;
    }
    if(carry>0) res="1"+res;
    return res;
}