Proto for Boost mailing list page: Re: [proto] Using a derived class as terminals in Boost.proto

Subject: Re: [proto] Using a derived class as terminals in Boost.proto
From: Eric Niebler (eric_at_[hidden])
Date: 2016-04-14 15:57:28

Proto grammars and transforms handle this better than evaluators, which are
deprecated. It would pay to look into some examples that use transforms.
Sorry, that's all the advice I have time for at the moment.

\e
On Apr 14, 2016 10:33 AM, "Mathias Gaunard" <mathias.gaunard_at_[hidden]>
wrote:

> I'd try to use IsVector.
> I'm not sure how to do this with a grammar (maybe someone can pitch in)
> but you could do something like this
>
> enable_if< IsVector<typename proto::result_of::value<Expr>::type> >
>
> On 14 April 2016 at 18:04, Frank Winter <fwinter_at_[hidden]> wrote:
>
>> I made some progress. If I specialize struct VectorSubscriptCtx::eval
>> with Vector10, like
>>
>>
>> struct VectorSubscriptCtx
>> {
>> VectorSubscriptCtx(std::size_t i) : i_(i) {}
>>
>> template<typename Expr, typename EnableIf = void>
>> struct eval
>> : proto::default_eval<Expr, VectorSubscriptCtx const>
>> {};
>>
>> template<typename Expr>
>> struct eval<
>> Expr
>> , typename boost::enable_if<
>> proto::matches<Expr, proto::terminal< Vector10 > >
>> >::type
>> >
>> {
>> //..
>> }
>> };
>>
>> then it works (is was specialized with Vector). It also works when using
>> the Boost _ literal (match anything), like
>>
>> template<typename Expr>
>> struct eval<
>> Expr
>> , typename boost::enable_if<
>> proto::matches<Expr, proto::terminal< _ > >
>> >::type
>>
>>
>> However, I feel this is not good style. Can this be expressed with the
>> is_base_of trait instead?
>>
>>
>>
>>
>>
>> On 04/14/2016 10:10 AM, Mathias Gaunard wrote:
>>
>>> On 14 April 2016 at 14:43, Frank Winter <fwinter_at_[hidden]
>>> <mailto:fwinter_at_[hidden]>> wrote:
>>>
>>>     Hi all!
>>>
>>>     Suppose you'd want to implement a simple EDSL (Embedded Domain
>>>     Specific Language) with Boost.proto with the following requirements:
>>>
>>>          Custom class 'Vector' as terminal
>>>          Classes derived from 'Vector' are working terminals too, e.g.
>>>     Vector10
>>>
>>>     [...]
>>>
>>>     template<typename T>
>>>     struct IsVector
>>>        : mpl::false_
>>>     {};
>>>
>>>
>>>     template<>
>>>     struct IsVector< Vector >
>>>        : mpl::true_
>>>     {};
>>>
>>>
>>> Surely this should be true for all types derived from Vector.
>>>
>>> template<typename T, typename Enable = void>
>>> struct IsVector
>>>    : mpl::false_
>>> {};
>>>
>>> template<typename T>
>>> struct IsVector<T, typanem enable_if< is_base_of<Vector, T> >::type>
>>>    : mpl::true_
>>> {};
>>>
>>>
>>> _______________________________________________
>>> proto mailing list
>>> proto_at_[hidden]
>>> http://lists.boost.org/mailman/listinfo.cgi/proto
>>>
>>>
>>
>> _______________________________________________
>> proto mailing list
>> proto_at_[hidden]
>> http://lists.boost.org/mailman/listinfo.cgi/proto
>>
>
>
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