Re: [boost] boost::proto overloads of operator()

From: Eric Niebler (eric_at_[hidden])
Date: 2007-04-16 00:44:44

Sorry for the delay. I'm back from my trip and getting caught up.

Maurizio Vitale wrote:
> 
> This is a short example that shows the problem (I'm using GCC 4.1.2)
> (ARITY 1 fails for different reasons)
> 
> #define BOOST_PROTO_MAX_ARITY 2
> #include <boost/xpressive/proto/proto.hpp>
> #include <boost/xpressive/proto/debug.hpp>
> 
> struct udt_tag {};
> boost::proto::terminal<udt_tag>::type const udt = {{}};
> 
> struct my_type : boost::proto::extends<boost::proto::terminal<udt_tag>::type, my_type> {
>   my_type (int i) {};
>   my_type () {};
> };
> 
> int main() {
>   my_type x;
> 
>   x(1,2);
>   return 0;
> }

Ah. There's a bug in proto/debug.hpp. It's fixed now. Thanks for the report.

> 
> eric.cpp(16): error: no instance of overloaded function "my_type::operator()" matches the argument list
>             argument types are: (int, int)
>             object type is: my_type
>     x(1,2);
>     ^

This is not a bug. This expression *looks* like it has arity 2, but it 
actually has arity 3, the three children being x, 1 and 2. By defining 
BOOST_PROTO_MAX_ARITY to 2, there is no operator() that accepts enough 
arguments.

<snip>

> At the moment it seems that only terminals will require data members, but if the
> plan I outlined above is viable, expression will need additional methods (for 
> converting to builtin and user-defined data types).
> 
> So the reason I'm wrapping expression is to be able to add the appropriate
> conversion operators.

 From later mails, it seems you now have this well in hand.

> Thanks for all the hand-holding during my first steps with proto.

No problem.

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