Mapping the HTMLTidy library into the Application

namespace app
{
  // Specialise an exception type for tidylib errors
  struct tidylib_error : std::system_error
  {
    // passthrough
    using std::system_error::system_error;
    tidylib_error() = default;
    explicit tidylib_error(int c)
        : std::system_error(c, std::generic_category())
    {
    }
  };

  // Create a C++ invoking wrapper for the tidylib C API, modifying data with the returned data,
  // returing a unique_ptr to release storage on scope exit.
  struct call_free
  {
    template <class T> void operator()(T *p) { ::free(p); }
  };
  inline outcome<std::unique_ptr<char, call_free>> tidy_html(string_view &data)
  {
    char *out = nullptr;
    size_t outlen = 0;
    int errcode = ::tidy_html(&out, &outlen, data.data(), data.size());
    if(errcode != 0)
    {
      // If the error code matches a standard STL exception, throw as that.
      BOOST_OUTCOME_V2_NAMESPACE::try_throw_std_exception_from_error(std::error_code(errcode, std::generic_category()));
      // Otherwise wrap the error code into a tidylib_error exception throw
      return std::make_exception_ptr(tidylib_error(errcode));
    }
    // Reset input view to tidied html
    data = string_view(out, outlen);
    // Return a unique ptr to release storage on scope exit
    return std::unique_ptr<char, call_free>(out);
  }
}  // namespace app