Proto for Boost mailing list page: Re: [proto] pod_generator Question

Subject: Re: [proto] pod_generator Question
From: Nate Knight (Nate.Knight_at_[hidden])
Date: 2011-02-13 12:46:15

> The second argument to BOOST_PROTO_DEFINE_OPERATORS is a domain. When
> defining a domain you (usually) specify a generator with an expression
> wrapper. What are you using for a domain, and how do you define it?
> 
> When I want an end-user-friendly terminal type, I derive it from the
> expression wrapper class, like this:
> 
>  template<class Expr> struct my_wrapper;
> 
>  struct my_domain : domain< generator<my_wrapper> > {};
> 
>  template<class Expr> struct my_wrapper
>   : extends< Expr, my_wrapper<Expr>, my_domain > {
>  ...
>  };
> 
>  class my_int : my_wrapper< terminal<int>::type > {
>  ...
>  };
> 
> Sounds like pod-ness is important to you, so this scheme won't work. Is
> that the problem you're having?

I am trying to allow user defined types to be treated as terminals.  I provide a trait class 

template<typename T, typename Enable = void>
struct IsMap
  : mpl::false_
{};

template<typename T>
struct IsMap<T, typename T::is_map>
  : mpl::true_
{};

So that only types with a typedef is_map or those that specialize IsMap are considered valid terminals.  
I then use this with BOOST_PROTO_DEFINE_OPERATORS like

namespace MapOps
{
    BOOST_PROTO_DEFINE_OPERATORS(IsMap, map_domain)
    struct map {}; 
}

where map_domain is

struct map_domain
  : domain< pod_generator<map_expr>, Map >
{};

template<typename ProtoExpression>
struct map_expr
{
    BOOST_PROTO_BASIC_EXTENDS(ProtoExpression, map_expr, map_domain)
};

and Map is the grammar for valid expressions.

I am trying to figure out the most convenient way to have the operator overloads found properly in user code.  So far I see several options,

1) "using namespace MapOps;" where expressions are needed.
2) Have users derive their maps from MapOps::map (I lose pod-ness (in C++03) here, right?).
3) Have all maps defined in the same namespace as the operations.
4) Define the operators in the global namespace.

Any suggestions on best practice here?

Nate