os is a Python lib and thus not available in C++.
You could use e.g. std::filesystem to use a similar as pathlib:
#include <iostream>
#include <filesystem>
namespace fs = std::filesystem;

int main() 
{
    fs::path dir ("/tmp");
    fs::path file ("foo.txt");
    fs::path full_path = dir / file;
    std::cout << full_path << std::endl;
    return 0;
}

Code taken from here 20.