1 ///////////////////////////////////////////////////////////////////////////////
2 /// \file null.hpp
3 /// Definintion of null_context\<\>, an evaluation context for
4 /// proto::eval() that simply evaluates each child expression, doesn't
5 /// combine the results at all, and returns void.
6 //
7 //  Copyright 2008 Eric Niebler. Distributed under the Boost
8 //  Software License, Version 1.0. (See accompanying file
9 //  LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
10 
11 #ifndef BOOST_PROTO_CONTEXT_NULL_HPP_EAN_06_24_2007
12 #define BOOST_PROTO_CONTEXT_NULL_HPP_EAN_06_24_2007
13 
14 #include <boost/preprocessor/iteration/iterate.hpp>
15 #include <boost/preprocessor/repetition/repeat.hpp>
16 #include <boost/proto/proto_fwd.hpp>
17 #include <boost/proto/eval.hpp>
18 #include <boost/proto/traits.hpp>
19 
20 namespace boost { namespace proto { namespace context
21 {
22 
23     template<
24         typename Expr
25       , typename Context
26       , long Arity          // = Expr::proto_arity_c
27     >
28     struct null_eval
29     {};
30 
31     template<typename Expr, typename Context>
32     struct null_eval<Expr, Context, 0>
33     {
34         typedef void result_type;
operator ()boost::proto::context::null_eval35         void operator()(Expr &, Context &) const
36         {}
37     };
38 
39     // Additional specializations generated by the preprocessor
40     #include <boost/proto/context/detail/null_eval.hpp>
41 
42     /// null_context
43     ///
44     struct null_context
45     {
46         /// null_context::eval
47         ///
48         template<typename Expr, typename ThisContext = null_context const>
49         struct eval
50           : null_eval<Expr, ThisContext>
51         {};
52     };
53 
54 }}}
55 
56 #endif
57