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源代码在线查看： null.hpp
#ifndef BOOST_PP_IS_ITERATING /////////////////////////////////////////////////////////////////////////////// /// \file null.hpp /// Definintion of null_context\, an evaluation context for /// proto::eval() that simply evaluates each child expression, doesn't /// combine the results at all, and returns void. // // Copyright 2008 Eric Niebler. Distributed under the Boost // Software License, Version 1.0. (See accompanying file // LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) #ifndef BOOST_PROTO_CONTEXT_NULL_HPP_EAN_06_24_2007 #define BOOST_PROTO_CONTEXT_NULL_HPP_EAN_06_24_2007 #include // must be first include #include #include #include #include #include #include // must be last include namespace boost { namespace proto { namespace context { template typename Expr , typename Context , long Arity BOOST_PROTO_WHEN_BUILDING_DOCS(= Expr::proto_arity::value) > struct null_eval {}; template struct null_eval { typedef void result_type; void operator()(Expr &, Context &) const {} }; #define BOOST_PROTO_EVAL_N(Z, N, DATA) \ proto::eval(proto::child_c(expr), ctx); \ /**/ #define BOOST_PP_ITERATION_PARAMS_1 \ (3, (1, BOOST_PROTO_MAX_ARITY, )) \ /**/ #include BOOST_PP_ITERATE() #undef BOOST_PROTO_EVAL_N /// null_context /// struct null_context { /// null_context::eval /// template struct eval : null_eval {}; }; }}} #endif #else #define N BOOST_PP_ITERATION() template struct null_eval { typedef void result_type; void operator ()(Expr &expr, Context &ctx) const { BOOST_PP_REPEAT(N, BOOST_PROTO_EVAL_N, ~) } }; #undef N #endif
