grokk-exs/4.1/sum_tail.cpp

#include <iostream>

int sumStep(int n, int s)
{
    return (n != 0)
         ?  sumStep(n - 1, n + s)
         :  s;
}

int sum(int n)
{
    return sumStep(n, 0);
}

int main()
{
    int n = 5;
    std::cout << "For n = " << n << " sum is " << sum(n) << '\n';
}