Re: [boost] [Proto] Access to member in proto expression tree

From: Eric Niebler (eric_at_[hidden])
Date: 2007-07-01 10:57:18

dan marsden wrote:
> Hi
> 
> Does proto support member access in its tree generation in any way. I'm looking at a few of the phoenix statement forms, particularly:
> 
> if_(pred)[statements].else_[statements]
> 
> and
> 
> try_[statements].catch_<exception_type>()[statements].catch_call[statements]
> 
> I need to specify member access to else_ and catch_<exception_type> etc. Is this something I'll have to hand roll, or does proto have some extension mechanism I've missed that will let me add these to the expression tree generation process?

Yes, proto::extends is the extension mechanism.

> As phoenix expressions have operator(), I'm already using an expression wrapper around all my proto expressions, so I presume that is where I would add any special member support?

Yep. You could be to put all such members into the Phoenix expression 
wrapper and use a grammar to flag or disallow invalid uses. The members 
would just be Proto terminals, like "terminal<phoenix::tag::else_>::type 
const else_".

A better approach might be to partially specialize your wrapper such 
that members like else_ only show up where they make sense. So, 
something like:

template<typename Expr, typename EnableIf = void>
struct phoenix_expr
  : proto::extends<Expr, phoenix_expr, phoenix_domain>
{
   // default phoenix stuff here, including
   // operator()
};

// Here is the grammar for if_ expressions
struct IfGrammar
  : proto::subscript<
      proto::terminal<phoenix::tag::if_>
      proto::_
    >
{};

template<typename Expr>
struct phoenix_expr<
    Expr
  , typename enable_if<proto::matches<Expr, IfGrammar> >::type
 >
  : proto::extends<Expr, phoenix_expr, phoenix_domain>
{
   // default phoenix stuff here, including
   // operator()

   terminal<phoenix::tag::else_>::type const else_;
};

Obviously, you'd want to structure this such that both the primary 
template and the specialization inherit from a common base which has the 
operator() overloads and any other stuff that would be common to them.

HTH,

-- 
Eric Niebler
Boost Consulting
www.boost-consulting.com