Найти 1+2+3+...+n рекурсивно и итеративно

unsigned int operator () () const { return nResult; }

const int sum = Sum<5>::value;

const int sum = Sum<0>::value;

template<>
struct Sum<0>{
        enum { value = 0 };
};

template<int N>
struct Sum
{
        enum { value = N + Sum<N-1>::value };
};
 
template<>
struct Sum<0>
{
        enum { value = 0 };
};

const int sum = Sum<2>::value;

template<2>
struct Sum
{
        enum { value = 2 + Sum<2-1>::value };
};

template<1>
struct Sum
{
        enum { value = 1 + Sum<1-1>::value };
};

template<>
struct Sum<0>
{
        enum { value = 0 };
};

#include <iostream>
#include <cstdlib>
 
unsigned int sum_iter(unsigned int result, unsigned int counter)
{
    if(!counter)
        return result;
    else
        return sum_iter(result + counter, --counter);
}
 
unsigned int sum(unsigned int n)
{
    return sum_iter(0, n);
}
 
int main()
{
    unsigned int n;
    std::cout << "Input n: ";
    std::cin >> n;
    std::cout << "Sum from 0 to " << n << " = " << sum(n) << std::endl;
    return EXIT_SUCCESS;
}

#include <iostream>
#include <algorithm>
#include <iterator>
#include <vector>
 
class Sum {
        unsigned int nResult;
public:
        Sum() : nResult(0) {}
        Sum(unsigned int start){
                for ( nResult = 0; start; nResult += start-- )
                        ;
        }
        bool operator () (int test){
            return test > nResult;
        }
 
        friend std::ostream & operator << (std::ostream & ost, const Sum & sum){
                ost << sum.nResult;
                return ost;
        }
};
 
int main(){
        const size_t size = 5;
        int arr[size] = { 8, 12, 5, 14, 21 };
        std::vector<int> vec(arr, arr + size);
        int num = 5;
        Sum sum(num);
        std::vector<int>::iterator fnd;
 
        std::cout << "Array:\n";
        std::copy(vec.begin(), vec.end(), std::ostream_iterator<int>(std::cout, " "));
        std::cout << "\nGiven number:\n" << num << std::endl;
        std::cout << "Sum of numbers from 1 to " << num << " = " << sum << std::endl;
        if ( ( fnd = std::find_if(vec.begin(), vec.end(), sum) ) == vec.end() )
            std::cout << "All elements in array less than it." << std::endl;
        else
            std::cout << "First element in array greater it is " << *fnd << std::endl;
 
        return 0;
}