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Subject: Re: [std-proposals] enum class value name string via something similar to typeid().name() ?
From: Sebastian Büttner (sebastian.buettner_at_[hidden])
Date: 2019-11-09 18:50:26

The second one I belief would work but it will give you the name of the
variable, so "value" instead the one of the corresponding enumerator.

Unfortunately it is not easily possible to provide a universal solution for
enumerator_name_by_value(value) because any value that fits into the underlying
type of an enum type can be converted to that enum. Thus this is valid:

enum Color {
  red /* = 0 */,
  green /* = 1 */,
  blue /* = 2 */

Color col = 3;
std::string enum_str = enumerator_name_by_value(col);

This is valid code (correct me if I am wrong please). As of http://eel.is/c+
+draft/dcl.init#list-3.8 direct-list-initialization is performed not checking
whether the value we actually assign has an corresponding enumerator or not.

What should enumerator_name_by_value() return? An empty string? Some
placeholder like name like "Undefined"? I think this is application dependend
and thus cannot be done in the language it self. A viable library solution to
this problem is the one you saw in the Reflection paper.

At least for now I cannot (sadly) think of a universal solution to this
problem that is nicer to spell than the existing example... However as you
only would require once such function for a given default behavior and
formatting taste it would be at least feasible to handle all of your
enumerations by one library function. Which would be a great win imo :-).


On Samstag, 9. November 2019 23:13:40 UTC Jonny Grant wrote:
> Many thanks Sebastian
> Yes, that example looks useful!
> Would be nice to be able to simply call directly:
> std::string enum_str = std::meta::name_of(value);
> or at least:
> std::string enum_str = std::meta::name_of(reflexpr(value));
> or even nicer:
> std::string enum_str = value.str();
> Let's see how things end up.
> Thank you
> Jonny
> On 09/11/2019 20:57, Sebastian Büttner wrote:
> > I aggree on both your statements.
> >
> > However I still think that the problem is just one of many in that domain.
> > With reflection at hand we will be able to solve this and even way more
> > like enumerating class members and so on.
> >
> > WG21 still has no common direction on this, however significant work has
> > already been done. It is still a long way to go tho. There is at least one
> > major proposal on this in flight (with some serious opposition to it).
> >
> > You might want to have a look at P1240 (Scalable Reflection in C++,
> > http://
> > www.open-std.org/jtc1/sc22/wg21/docs/papers/2019/p1240r1.pdf).
> >
> > The first example on page 2 is already what you are looking for (kinda at
> > least).
> >
> > Sebastian
> >
> > On Samstag, 9. November 2019 19:40:41 UTC Jonny Grant wrote:
> >> Seems a shame if we need to write a python script to generate the
> >> to_string_str for each enum.
> >>
> >>
> >>
> >> Don't you just hate it when you get a debug log that shows :-

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