Re: [boost] [system] Was: Re: [asio] Breaking changes committed to cvs HEAD

From: Christopher Kohlhoff (chris_at_[hidden])
Date: 2006-11-14 06:39:17

Johan Nilsson <r.johan.nilsson_at_[hidden]> wrote:
> Imagine that you're implementing a "middle-layer" library,
> calling down to "lower-layer" methods that do the actual O/S
> specific calls. The lower-layer methods might be C methods, or
> a third-party library.
>
> Examples for comparison:
[...]

In my experience the lower layer tends to look more like:

void foo()
{
  error_code ec;
  foo(ec); // Called foo_impl() in your examples
  if (ec) throw system_error(ec);
}

error_code foo(error_code& ec)
{
#ifdef BOOST_WINDOWS
  ... windows implementation ...
  ec = error_code(::GetLastError(), native_ecat);
#else
  ... posix implementation ...
  ec = error_code(errno, native_ecat);
#endif
  return ec;
}

At least, this is my understanding of how error_code and
system_error should be used. IMHO if the lowest level wrappers
use the error_code class in this way, there's no need for a
publicly available wrapper for ::GetLastError()/errno.

Having said that, I do have some "portable" helper functions
inside of asio to do effectively what you're proposing, However
they're only truly useful on a subset of the functions and are
mostly a hangover from before I switched asio to use
error_code/system_error. See clear_error() and error_wrapper()
in this file:

http://boost.cvs.sourceforge.net/*checkout*/boost/boost/boost/asio/detail/socket_ops.hpp?revision=1.5

Cheers,
Chris