[LLVMdev] Users of llvm::error_code all inherit from _do_message

[Michael Spencer]

On Fri, Mar 14, 2014 at 2:52 PM, Justin Bogner <mail at justinbogner.com> wrote:
> There are several libraries in LLVM that create their own `error_code`
> categories, so that they can return errors in a consistent way and with
> convenient messages.
>
> To do this, these libraries create an error category type that inherits
> from `_do_message` in system_error.h. There are a number of ways that
> this is wrong:
>
> 1. From the name, it's quite clear that _do_message is an implementation
>    detail.
>
> 2. The only function defined in _do_message is specifically intended for
>    system errors that report via errno, which is not what the custom
>    categories are doing.
>
> As far as I can tell, the custom categories are only inheriting from
> _do_message is apparently to get around the fact that error_category's
> constructor is private, and that _do_message is a friend class of
> error_category.
>
> If we want to continue using error_code the way we currently are, I
> think the right thing to do here is to make error_category's constructor
> protected and stop inheriting from _do_message.  Would anyone object to
> that?

Hmm, I'm not sure why error_category's constructor is private. It's
not what the spec says.

This change is fine, but we should eventually move over to
std::{error_code,error_category}. We'll need to investigate first how
to properly handle windows_error though, I believe that on MSVC the
system_category properly maps to the Windows error codes. Not sure how
MinGW handles it though.

- Michael Spencer