Re: [Boost-users] [boost][proto] Type inference using transform

Subject: Re: [Boost-users] [boost][proto] Type inference using transform
From: Manjunath Kudlur (keveman_at_[hidden])
Date: 2010-05-06 14:09:01

> Thanks, Joel. (Note to self : RTFM more often :)). I also discovered
> that I didn't have to make the readonly and readwrite classes
> templated. The following also works :
>
This is even simpler :

template<typename T>
struct readonly
  : proto::callable {
  typedef const T& result_type;
};

template<typename T>
struct readwrite
  : proto::callable {
  typedef T& result_type;
};

template<int N, typename T>
struct argtype
  : proto::or_<
    proto::when<proto::assign<arg<N>, proto::_>, readwrite<T>()>
  , proto::otherwise<readonly<T>()>
>
{};

namespace boost { namespace proto {
template<class T> struct is_callable< readonly<T> > : mpl::true_ {};
template<class T> struct is_callable< readwrite<T> > : mpl::true_ {};
} }

Manjunath
http://nonchalantlytyped.net/blog/musings/

> struct readonly
>  : proto::callable {
>
>  template<typename Sig>
>  struct result;
>
>  template<typename This, typename T>
>  struct result<This(T)>
>  {
>    typedef const T& type;
>  };
> };
>
> struct readwrite
>  : proto::callable {
>
>  template<typename Sig>
>  struct result;
>
>  template<typename This, typename T>
>  struct result<This(T)>
>  {
>    typedef T& type;
>  };
> };
>
> template<int N, typename T>
> struct argtype
>  : proto::or_<
>    proto::when<proto::assign<arg<N>, proto::_>, readwrite(T())>
>  , proto::otherwise<readonly(T())>
>>
> {};
>
> Manjunath
> http://nonchalantlytyped.net/blog/musings/
>
>> struct arg_tag
>> {};
>>
>> namespace proto = boost::proto;
>> namespace mpl = boost::mpl;
>>
>> using namespace std;
>>
>> template<int N>
>> struct arg
>>  : proto::or_<proto::nullary_expr<arg_tag, mpl::int_<N> > >
>> {};
>>
>> template<typename T>
>> struct readonly
>>  : proto::callable {
>>
>>  template<typename Sig>
>>  struct result;
>>
>>  template<typename This,class X>
>>  struct result<This(X)>
>>  {
>>   typedef const T& type;
>>  };
>> };
>>
>> template<typename T>
>> struct readwrite
>>  : proto::callable {
>>
>>  template<typename Sig>
>>  struct result;
>>
>>  template<typename This,class X>
>>  struct result<This(X)>
>>  {
>>   typedef T& type;
>>  };
>> };
>>
>> template<int N, typename T>
>> struct argtype
>>  : proto::or_<
>>   proto::when<proto::assign<arg<N>, proto::_>, readwrite<T>(proto::_)>
>>  , proto::otherwise<readonly<T>(proto::_)>
>>>
>> {};
>>
>> proto::nullary_expr<arg_tag, mpl::int_<0> >::type const _1 = {{}};
>> proto::nullary_expr<arg_tag, mpl::int_<1> >::type const _2 = {{}};
>>
>>
>> namespace boost {  namespace proto
>> {
>>  template<class T> struct is_callable<readwrite<T> > : true_ {};
>>  template<class T> struct is_callable<readonly<T> > : true_ {};
>> }}
>>
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>