Re: x3::variant vs boost::variant

On 2/21/16 7:19 PM, Baptiste Wicht wrote:
> 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 :(

X3 variant is quite fast, primarily because of the move issue with boost variant
recursive_wrapper. I complained about that, but lost the debate. That's why you
have  x3::variant/forward_ast now.

In the future, please consider using visitors instead of switch-by-type.

Regards,
-- 
Joel de Guzman
http://www.ciere.com
http://boost-spirit.com
http://www.cycfi.com/


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