Simple std dev calculation using STL algorithms.

cpp_std_dev_calculation.cpp

#include <iostream>
#include <numeric>
#include <cmath>
#include <algorithm>
#include <vector>
#include <string>

int main()
{
    std::vector<int> values{2, 4, 4, 4, 5, 5, 7, 9};
    
    const auto sum = std::accumulate(values.begin(), values.end(), 0);
    const double mean = static_cast<double>(sum) / static_cast<double>(values.size());
    
    const auto sq_sum = std::accumulate(values.begin(), values.end(), 0.0, [mean](double sum, int value) {
       return sum + std::pow(value - mean, 2.0); 
    });

    const auto variance = sq_sum / static_cast<double>(values.size());
    auto std_dev = std::sqrt(variance);
    std::cout << std::to_string(std_dev) << std::endl;
    return 0;
}

Example calculation and explanation is here.