Re: [Boost-users] [proto] Handling String Terminals

Subject: Re: [Boost-users] [proto] Handling String Terminals
From: Eric Niebler (eric_at_[hidden])
Date: 2010-02-07 17:45:05

On 2/8/2010 9:05 AM, David A. Greene wrote:
> If I have a grammar like this:
>
> typedef boost::proto::literal<std::string>::type StringTerminal;
>
> typedef or_<
>    StringTerminal,
>    ...
>> Rule;
>
> and I do this:
>
>    checkMatch<Rule>(boost::proto::lit(std::string("str")));
>    checkMatch<Rule>(std::string("str"));
>    checkMatch<Rule>("str");
>
> where checkMatch is a wrapper around matches<>, the first
> line is the only one that passes.  This is presumably because
> the string needs to be "protoized" and to match StringTerminal
> it has to be cast to std::string explicitly.
<snip>

proto::convertible_to is your friend here, as well as proto::as_expr. 
See below:

   #include <boost/proto/proto.hpp>
   namespace proto = boost::proto;

   struct Rule
     : proto::terminal< proto::convertible_to< std::string > >
   {};

   template< typename Rule, typename T >
   void checkMatch( T const & t )
   {
     typedef typename proto::result_of::as_expr< T >::type expr_type;
     BOOST_MPL_ASSERT(( proto::matches< expr_type, Rule > ));
   }

   int main()
   {
     checkMatch<Rule>(proto::lit(std::string("str")));
     checkMatch<Rule>(std::string("str"));
     checkMatch<Rule>("str");
   }

As its name implies, proto::convertible_to matches terminal types that 
have an implicit conversion. proto::result_of::as_expr converts 
non-proto types to proto::terminals and leaves proto expression types alone.

HTH,

-- 
Eric Niebler
BoostPro Computing
http://www.boostpro.com