Re: x3::variant vs boost::variant

Baptiste Wicht <[email protected]> Sun, 21 Feb 2016 12:19:31 +0100
Newsgroups	gmane.comp.parsers.spirit.general
Message-ID	<[email protected]>
On Sun, Feb 21, 2016 at 11:07:57AM +0100, Baptiste Wicht wrote:
> On Sun, Feb 21, 2016 at 09:55:02AM +0800, Joel de Guzman wrote:
> > On 2/20/16 5:14 PM, Baptiste Wicht wrote:
> > > On Sat, Feb 20, 2016 at 05:42:14AM +0800, Joel de Guzman wrote:
> > >> On 2/18/16 4:48 AM, Baptiste Wicht wrote:
> > >>
> > >>> The main problem that remains is with x3::variant and x3::forward_ast:
> > >>>
> > >>> I have a type like this:
> > >>>
> > >>>       using Type = boost::spirit::x3::variant<
> > >>>               SimpleType,
> > >>>               boost::spirit::x3::forward_ast<ArrayType>,
> > >>>               boost::spirit::x3::forward_ast<TemplateType>,
> > >>>               boost::spirit::x3::forward_ast<PointerType>
> > >>>           >;
> > >>>
> > >>> And a lot of code resembling this:
> > >>>
> > >>>       if(auto* ptr = boost::get<ast::SimpleType>(&type)){
> > >>>           //Do something on *ptr
> > >>>       } else if(auto* ptr = boost::get<ast::ArrayType>(&type)){
> > >>>           //Do something on *ptr
> > >>>       } else if(auto* ptr = boost::get<ast::PointerType>(&type)){
> > >>>           //...
> > >>>       }
> > >>>       //...
> > >>>
> > >>> But this does not work since ArrayType is not a member of the variant
> > >>> because of the forward_ast and I have an error from boost strict_get.
> > >>>
> > >>> I really have a lot of code like this, I won't be able (neither have the
> > >>> motivation for it) to change all of it. If there is no simple solution,
> > >>> I'll use my old AST with boost::variant and recursive_wrapper.
> > >>>
> > >>> Is there anything I can do to use boost::get with forward_ast ?
> > >>
> > >> There should be a solution. tell you what. Give me a minimal test
> > >> case and I can work on getting it compile and run as expected.
> > >
> > > Here it is: http://coliru.stacked-crooked.com/a/641b689346ff322c
> >
> > There is a solution, but it involves some meta code that can slow down CT.
> > I'm not sure if it's worth it. The only reasonable solution is to simply
> > get<forward_ast<T>>(v).
>
> Too bad, tt makes for really inelegant code :(
>
> I'll test the slowdown of my syntax with boost::variant instead of
> x3::variant to see if it's still worth it.

I just replaced all x3::variant/forward_ast by
boost::variant/recursive_wrapper and it is not good... The new parser is
70% slower than the x3::variant one :(

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