Re: [Boost-users] [proto] declaring result type for callable contexts

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"Eric" == Eric Niebler writes:

Eric> Maurizio Vitale wrote: >> Eric, It seems like result types must be defined inside a struct >> eval, but examples in the documentation show the form commented >> out below. >> >> Is this a recent change? >> >> Also, in absence of a result_type declaration inside struct eval >> {...} the type defaults to int. Would it be possible to make it >> void or some_ugly_looking_type_that_tell_me_i_is_all_wrong? int >> might happen to be what the user wants, until he decide to change >> it and hides the problem. >> >> template<typename Expr> struct my_context : >> proto::callable_context { struct eval >> {typedef double result_type;}; >> >> // doesn't work (anymore?) //typedef double result_type; }; Eric> There have been no recent changes in this area, though I have Eric> been reorganizing some code. The Context concept (ugh, not Eric> defined in the reference section, sorry) requires a nested Eric> eval struct template with a nested result_type typedef. Users Eric> can define a context that satisfies that requirement by Eric> inheriting from callable_context, which defines the nested Eric> eval<> struct for you, and is implemented in terms of member Eric> function overloads within the derived context type. That turns Eric> out to be a lot easier in most situations. Ok, while reducing the code to something small enough for posting I think I've got what was wrong. Let's see if it makes sense to you: [if my prose is too long and makes little sense, you can find my code attached to this message] - my context inherits from callable_context and defines a result_type. But it doesn't define (yet) any overload for the arithmetic operations. - I then have a data structure that I use for terminals. This also has no arithmetic operators defined. BUT it has conversion operators to some basic types (was int, it is now double to see where the type was coming from) What I think happens is that the default_context is asked to evaluate expression, knows jack about my data type but happily see a conversion operator to a builtin type, use it and then does arithmetic over it using the builtin arithmetic operator. So proto reports the right type for the evaluation of the expression, it is just that it is not what I declared with result_type. Maybe it would be good to add an assertion for this case, as forgetting even one overload under the appropriate circumstances (e.g. conversion operator to a builtin type) can cause a not immediately diagnosable problem. In the small example I cannot reproduce one aspect of what I was seeing, namely that putting result_type inside a struct eval made proto return what I was expecting (it would have been wrong, but I had enough conversion operators/constructor to make it work on the surface). My guess is that in that case proto believes in my eval and doesn't use the callable_context one which in turns would use my context and then falling back on default context. Anyhow, if the above makes sense sorry for the noise. #include template<typename> struct XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX; #define X(TYPE) \ typedef TYPE type##__LINE__; \ typedef typename XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX::XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX debug_ ## __LINE__ namespace mpl = boost::mpl; namespace proto = boost::proto; using proto::_; struct systemc_grammar : proto::or_ < proto::terminal<struct data>, proto::unary_expr<_, systemc_grammar>, proto::binary_expr<_, systemc_grammar, systemc_grammar> > {}; template <typename Expr> struct systemc_expression : proto::extends { typedef proto::extends base_type; systemc_expression (Expr const& expr_ = Expr()) : base_type (expr_) {} operator int () const { return systemc_eval (*this); } }; struct systemc_domain : proto::domain, struct systemc_grammar> {}; template<typename Expr> struct systemc_context : proto::callable_context { typedef data result_type; }; template<typename Expr> typename boost::remove_reference::type>::type systemc_eval (const Expr& expr) { typedef typename boost::remove_reference::type>::type t; typedef typename proto::result_of::eval< Expr, systemc_context<Expr> >::type tt; return proto::eval (expr, systemc_context<Expr> ()); } struct data { data () : m_data () {} operator double () const { return m_data; } double m_data; }; template<typename E> void coerce (data& target, const E& source) { BOOST_AUTO (source_data, systemc_eval (source)); X (BOOST_TYPEOF (source_data)); target = source_data & 0xf; } struct number : systemc_expression { typedef systemc_expression base_type; number () : base_type (base_type::proto_base_expr::make (data ())) {} template<typename E> number& operator=(const E& e) { coerce (proto::value (*this), e); return *this; } }; int main (int, char**) { number a,b,c; a = b*c; } Eric> In short, if you're using callable_context, don't define Eric> nested eval<> structs. If you have some code that recently Eric> stopped working, you should post it so I can see that the Eric> problem is. Eric> -- Eric Niebler BoostPro Computing http://www.boostpro.com -- Maurizio Vitale Polymath Solutions