Re: [Boost-users] [phoenix] v3 extension mechanism for template functions

19 Apr 2011


      On Apr 18, 8:34 pm, alfC  wrote:
...
Hi,
 In Phoenix2 I used to have the following code to add expressions of
type pow<N>(... expr...) which where useful for expressions involving
Boost.Units. Is it possible to do something similar with Phoenix3. The
usage if macros in the tutorial examples obscures how to add template
parameters to the phoenix expression. In Phoenix2 it was straight
forward, but I can figure out how to do this in Phoenix3.
ok, this is the farthest I could go with Phoenix3. It seems that the
tag:: struct argument in phoenix::expr is a template template.

The following is the latest attempt, which doesn't compile, giving the
error:
boost/phoenix/core/is_nullary.hpp:124:9: error: base type ‘const
boost::proto::exprns_::basic_expr >, 0l> > >,
1l>&’ fails to be a struct or class type


#include
namespace boost{
	namespace phoenix{
		namespace tag{
			template<long N>
			struct pow{};
		}

		namespace expression
		{
		    template 
		    struct pow
			: boost::phoenix::expr<
			    tag::pow<N>
			  , A0
			>
			{};
		}
	}
	namespace phoenix_impl{
		template<long N>
		struct pow_impl{
			template<class Sig> struct result;
			template
		        struct result{
				typedef double type; // just for testing
			};
		        template
			typename result::type
			operator()(Arg const& arg, Cont const& ctx) const {
				return boost::units::pow<N>(eval(arg, ctx)).value(); //value, just
for testing
		        }
		};
	}
	namespace phoenix{
		template 
		typename expression::pow::type const
		pow(Arg const& arg){
			return expression::pow::make(arg);
		}
	}
}