How to copy a std::string to unsigned char array?

Here's one way:

#include <algorithm>
#include <iostream>

auto main() -> int
{
    unsigned char trap[256];
    std::string sample = ".1.3.6.1.4";
    std::copy( sample.begin(), sample.end(), trap );
    trap[sample.length()] = 0;
    std::cout << trap << std::endl;
}

It can be a good idea to additionally check whether the buffer is sufficiently large.