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

#include <iostream>
 
class Sum {
    unsigned int nResult;
public:
    Sum() : nResult(0) {}
    Sum(unsigned int start){
        for ( nResult = 0; start; nResult += start-- )
            ;
    }
    friend std::ostream & operator << (std::ostream & ost, const Sum & sum){
        ost << sum.nResult;
        return ost;
    }
};
 
int main(){
    int val;
    
    std::cout << "Enter some positive numbers (negative - exit)" << std::endl;
    
    while ( true ){
        std::cout << "> ";
        std::cin >> val;
        if ( val < 0 )
            break;
        std::cout << "  " << Sum(val) << std::endl;
    }
    
    return 0;
}

#include <iostream>
#include <string>
 
class Hello
{
   std::string f;
public:
    Hello():f("Hello, world!"){}
    ~Hello(){}
    friend std::ostream& operator <<(std::ostream& os, const Hello &Ob);
};
 
std::ostream& operator <<(std::ostream& os, const Hello& Ob)
{
   os<<Ob.f<<'\n';
   return os;
}
 
int main()
{
    Hello Str;
    std::cout<<Str;
    return 0;
}

for ( nResult = 0; start; nResult += start-- );

void operator()(unsigned int start)
{
   for ( nResult = 0; start; nResult += start-- )
      ;
}