template<bool writeout> double sumNumbers(const std::vector<double> &numbers, double *out_partialsums) {
    double total = 0.0;
    for (int i = 0; i < numbers.size(); i++) {
        total += numbers[i];

        /* ... */

        if (writeout) {
            out_partialsums[i] = total;
        }
    }
    return total;
}

auto result = sumNumbers<false>(numbers, NULL)