Re: x3::variant vs boost::variant

Baptiste Wicht <[email protected]> Sun, 21 Feb 2016 11:07:57 +0100
Newsgroups	gmane.comp.parsers.spirit.general
Message-ID	<[email protected]>
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.

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