Re: [Boost] [Describe] Review starts Monday 1st March 2021 - 10th March 2021

From: Peter Dimov (pdimov_at_[hidden])
Date: 2021-03-02 21:48:07

Maximilian Riemensberger wrote:
> Hi,
> 
> Thank you, Peter, for sharing this library.

You are welcome. :-) Thanks for the feedback.

> While reading through the documentation of describe and considering how to
> use it in a C++14 code base a few questions came to my mind.
> 
> 1) I saw that describe does not support describing reference member
> variables.  Is there a fundamental limitation that prohibits this?  We have quite
> a few `boost::asio::io_context&` members or similar members.  (Yes, we
> haven't found time to update them to executor members.) We wouldn't be
> able to describe those.

Reference (and bitfield) members are a problem for reflection libraries (and
even some core proposals) because it's not possible to form a pointer to member
for them (and without injection, there's no good way to describe them in
a useful way.)

> 2) Overloaded functions are not supported. Again, what are the reasons for
> this limitation?  Is it fundamentally impossible?

The problem is basically that &X::f doesn't work when f is overloaded. There
are probably ways around that, such as f.ex. using something like

(int (X::*)(float) const, f)

in place of just `f` (ugh), but I haven't yet settled on a solution I'd be
comfortable with.

(Actually I know what solution I'd be comfortable with - adding
__describe_members to clang, gcc and msvc - but that's a difficult endeavor.)

...

> Is it possible to support attaching extra data to the enum or their descriptor.
> As an example consider an error code enum. Without describe we would have
> something like
> 
> enum class my_errors {
>      success,
>      some_thing_went_wrong,
>      invalid_path,
>      wrong_argument,
>      unknown_error
> };
> 
> char const* my_errors_s[5] = {
>      "success",
>      "some thing went wrong",
>      "invalid path",
>      "wrong argument",
>      "unknown error"
> };
> 
> std::errc my_errors_map[] = {
>      0,
>      std::errc::io_error,
>      std::errc::file_or_directory_not_found,
>      std::errc::invalid_argument,
>      std::errc::io_error
> };

That's an interesting example. I'll have to think about it.