#include <iostream>
#include <limits>

void sum( long long int &sum, int a, int b )
{
    sum = static_cast<long long int>( a ) + b;
}

int main()
{
    long long int sum;

    ::sum( sum, std::numeric_limits<int>::max(), std::numeric_limits<int>::max() );

    std::cout << "sum of "
              << std::numeric_limits<int>::max()
              <<  " + "
              << std::numeric_limits<int>::max()
              << " is equal to "
              << sum << std::endl;

    return 0;
}

sum of 2147483647 + 2147483647 is equal to 4294967294

#include <iostream>
#include <limits>

long long int sum( int a, int b )
{
    return static_cast<long long int>( a ) + b;
}

int main()
{
    long long int sum;

    sum = ::sum( std::numeric_limits<int>::max(), std::numeric_limits<int>::max() );

    std::cout << "sum of "
              << std::numeric_limits<int>::max()
              <<  " + "
              << std::numeric_limits<int>::max()
              << " is equal to "
              << sum << std::endl;

    return 0;
}

void sum();

#include <iostream>
using namespace std;
void sum(int a, int b)
{
    int sum = a + b;
    cout << sum;
}
int main()
{
    int a = 3;
    int b = 5;
    sum(a, b);
    return 0;
}

#include <iostream>
using namespace std;
int sum(int a, int b)
{
    return a + b;
}
int main()
{
    int a = 3;
    int b = 5;
    int s = sum(a, b);
    cout << s << endl;
    return 0;
}