Comment by pjmlp

Some stuff that could be improved with modern C++,

Instead of

    static bool file_exists(const std::string &p) {
        struct stat st{}; return ::stat(p.c_str(), &st) == 0;
    }
      

Use the filesystem API,

    std::filesystem::exists(path);

-- https://en.cppreference.com/w/cpp/filesystem.html