以Boost Spirit和Fusion的关联方式解析结构

Question

我试图将键值字符串解析为结构。某些键值可能不存在或可能按不同的顺序，所以我想用boost::fusion来调整结构，然后用at_key<>指令解析它。

#include <iostream> 
#include <string> 
#include <boost/spirit/include/qi.hpp> 
#include <boost/spirit/include/phoenix.hpp> 
#include <boost/fusion/adapted.hpp> 
#include <boost/fusion/sequence.hpp> 

using namespace std; 
namespace qi = boost::spirit::qi; 
namespace ascii = boost::spirit::ascii; 
namespace phx = boost::phoenix; 

using boost::fusion::at_key; 

typedef string::const_iterator iter_type; 

struct Couple { 
    int a; 
    int b; 
    Couple() : a(0), b(0) {} 
}; 

namespace keys { 
    struct first; 
    struct second; 
} 

BOOST_FUSION_ADAPT_ASSOC_STRUCT(
    Couple, 
    (int, a, keys::first) 
    (int, b, keys::second) 
    ) 


struct G: qi::grammar< iter_type, Couple(), ascii::space_type > 
{ 
    G() : G::base_type(start_rule) { 
     using qi::_val; 
     using qi::_1; 
     using qi::_2; 

     start_rule = 
         ("first" >> qi::int_ 
           [ at_key<keys::first>(_val) = _1 ] 
         ) 
        ^
         ("second" >> qi::int_ 
           [ at_key<keys::second>(_val) = _1 ] 
         ); 
    } 

    qi::rule< iter_type, Couple(), ascii::space_type > start_rule; 
}; 

int main() { 
    Couple couple; 
    string example = "second 2 first 1"; 
    iter_type begin(example.begin()); 
    iter_type end(example.end()); 

    // test at_key -- compiles with no error 
    at_key<keys::second>(couple) = 5; 

    bool ok = qi::phrase_parse(begin, end, G(), ascii::space, couple); 
    if (ok) 
     cout << couple.a << " " << couple.b << endl; 
    else 
     cout << "Parse failed" << endl; 

    return 0; 
} 

的问题是，代码不编译（升压1.50.0，G ++ 4.5.0，MinGW的），显然未能在at_key<>规则：

In file included from D:\projects\workspace\boost/boost/fusion/support/category_of.hpp:10:0, 
       from D:\projects\workspace\boost/boost/fusion/include/category_of.hpp:10, 
       from D:\projects\workspace\boost/boost/proto/fusion.hpp:20, 
       from D:\projects\workspace\boost/boost/proto/core.hpp:21, 
       from D:\projects\workspace\boost/boost/proto/proto.hpp:12, 
       from D:\projects\workspace\boost/boost/spirit/home/support/meta_compiler.hpp:19, 
       from D:\projects\workspace\boost/boost/spirit/home/qi/meta_compiler.hpp:14, 
       from D:\projects\workspace\boost/boost/spirit/home/qi/action/action.hpp:14, 
       from D:\projects\workspace\boost/boost/spirit/home/qi/action.hpp:14, 
       from D:\projects\workspace\boost/boost/spirit/home/qi.hpp:14, 
       from D:\projects\workspace\boost/boost/spirit/include/qi.hpp:16, 
       from ..\src\spirit02_test.cpp:11: 
D:\projects\workspace\boost/boost/fusion/support/detail/category_of.hpp: In instantiation of 'boost::fusion::detail::fusion_category_of<const boost::phoenix::actor<boost::spirit::attribute<0> > >': 
D:\projects\workspace\boost/boost/fusion/support/category_of.hpp:44:58: instantiated from 'boost::fusion::extension::category_of_impl<boost::fusion::non_fusion_tag>::apply<const boost::phoenix::actor<boost::spirit::attribute<0> > >' 
D:\projects\workspace\boost/boost/fusion/support/category_of.hpp:66:9: instantiated from 'boost::fusion::traits::category_of<const boost::phoenix::actor<boost::spirit::attribute<0> > >' 
D:\projects\workspace\boost/boost/fusion/support/category_of.hpp:73:9: instantiated from 'boost::fusion::traits::is_associative<const boost::phoenix::actor<boost::spirit::attribute<0> > >' 
D:\projects\workspace\boost/boost/mpl/if.hpp:67:11: instantiated from 'boost::mpl::if_<boost::fusion::traits::is_associative<const boost::phoenix::actor<boost::spirit::attribute<0> > >, boost::fusion::result_of::key_of<mpl_::arg<1> >, boost::fusion::result_of::value_of<mpl_::arg<1> > >' 
D:\projects\workspace\boost/boost/fusion/algorithm/query/find.hpp:45:9: instantiated from 'boost::fusion::result_of::find<const boost::phoenix::actor<boost::spirit::attribute<0> >, keys::first>' 
D:\projects\workspace\boost/boost/fusion/sequence/intrinsic/at_key.hpp:38:17: instantiated from 'boost::fusion::extension::at_key_impl<boost::fusion::non_fusion_tag>::apply<const boost::phoenix::actor<boost::spirit::attribute<0> >, keys::first>' 
D:\projects\workspace\boost/boost/fusion/sequence/intrinsic/at_key.hpp:71:9: instantiated from 'boost::fusion::result_of::at_key<const boost::phoenix::actor<boost::spirit::attribute<0> >, keys::first>' 
..\src\spirit02_test.cpp:54:35: instantiated from here 
D:\projects\workspace\boost/boost/fusion/support/detail/category_of.hpp:15:38: error: no type named 'category' in 'const struct boost::phoenix::actor<boost::spirit::attribute<0> >' 
In file included from D:\projects\workspace\boost/boost/proto/args.hpp:21:0, 
       from D:\projects\workspace\boost/boost/proto/core.hpp:14, 
       from D:\projects\workspace\boost/boost/proto/proto.hpp:12, 
       from D:\projects\workspace\boost/boost/spirit/home/support/meta_compiler.hpp:19, 
       from D:\projects\workspace\boost/boost/spirit/home/qi/meta_compiler.hpp:14, 
       from D:\projects\workspace\boost/boost/spirit/home/qi/action/action.hpp:14, 
       from D:\projects\workspace\boost/boost/spirit/home/qi/action.hpp:14, 
       from D:\projects\workspace\boost/boost/spirit/home/qi.hpp:14, 
       from D:\projects\workspace\boost/boost/spirit/include/qi.hpp:16, 
       from ..\src\spirit02_test.cpp:11: 
D:\projects\workspace\boost/boost/mpl/if.hpp: In instantiation of 'boost::mpl::if_<boost::fusion::traits::is_associative<const boost::phoenix::actor<boost::spirit::attribute<0> > >, boost::fusion::result_of::key_of<mpl_::arg<1> >, boost::fusion::result_of::value_of<mpl_::arg<1> > >': 
D:\projects\workspace\boost/boost/fusion/algorithm/query/find.hpp:45:9: instantiated from 'boost::fusion::result_of::find<const boost::phoenix::actor<boost::spirit::attribute<0> >, keys::first>' 
D:\projects\workspace\boost/boost/fusion/sequence/intrinsic/at_key.hpp:38:17: instantiated from 'boost::fusion::extension::at_key_impl<boost::fusion::non_fusion_tag>::apply<const boost::phoenix::actor<boost::spirit::attribute<0> >, keys::first>' 
D:\projects\workspace\boost/boost/fusion/sequence/intrinsic/at_key.hpp:71:9: instantiated from 'boost::fusion::result_of::at_key<const boost::phoenix::actor<boost::spirit::attribute<0> >, keys::first>' 
..\src\spirit02_test.cpp:54:35: instantiated from here 
D:\projects\workspace\boost/boost/mpl/if.hpp:67:11: error: 'value' is not a member of 'boost::fusion::traits::is_associative<const boost::phoenix::actor<boost::spirit::attribute<0> > >' 
D:\projects\workspace\boost/boost/mpl/if.hpp:70:41: error: 'value' is not a member of 'boost::fusion::traits::is_associative<const boost::phoenix::actor<boost::spirit::attribute<0> > >' 
In file included from D:\projects\workspace\boost/boost/fusion/sequence/intrinsic.hpp:20:0, 
       from D:\projects\workspace\boost/boost/fusion/include/intrinsic.hpp:10, 
       from D:\projects\workspace\boost/boost/proto/fusion.hpp:22, 
       from D:\projects\workspace\boost/boost/proto/core.hpp:21, 
       from D:\projects\workspace\boost/boost/proto/proto.hpp:12, 
       from D:\projects\workspace\boost/boost/spirit/home/support/meta_compiler.hpp:19, 
       from D:\projects\workspace\boost/boost/spirit/home/qi/meta_compiler.hpp:14, 
       from D:\projects\workspace\boost/boost/spirit/home/qi/action/action.hpp:14, 
       from D:\projects\workspace\boost/boost/spirit/home/qi/action.hpp:14, 
       from D:\projects\workspace\boost/boost/spirit/home/qi.hpp:14, 
       from D:\projects\workspace\boost/boost/spirit/include/qi.hpp:16, 
       from ..\src\spirit02_test.cpp:11: 
D:\projects\workspace\boost/boost/fusion/sequence/intrinsic/at_key.hpp: In instantiation of 'boost::fusion::extension::at_key_impl<boost::fusion::non_fusion_tag>::apply<const boost::phoenix::actor<boost::spirit::attribute<0> >, keys::first>': 
D:\projects\workspace\boost/boost/fusion/sequence/intrinsic/at_key.hpp:71:9: instantiated from 'boost::fusion::result_of::at_key<const boost::phoenix::actor<boost::spirit::attribute<0> >, keys::first>' 
..\src\spirit02_test.cpp:54:35: instantiated from here 
D:\projects\workspace\boost/boost/fusion/sequence/intrinsic/at_key.hpp:38:17: error: no type named 'type' in 'struct boost::fusion::result_of::find<const boost::phoenix::actor<boost::spirit::attribute<0> >, keys::first>' 
..\src\spirit02_test.cpp: In constructor 'G::G()': 
..\src\spirit02_test.cpp:54:35: error: no matching function for call to 'at_key(const boost::spirit::_val_type&)' 
In file included from D:\projects\workspace\boost/boost/fusion/sequence/intrinsic.hpp:20:0, 
       from D:\projects\workspace\boost/boost/fusion/include/intrinsic.hpp:10, 
       from D:\projects\workspace\boost/boost/proto/fusion.hpp:22, 
       from D:\projects\workspace\boost/boost/proto/core.hpp:21, 
       from D:\projects\workspace\boost/boost/proto/proto.hpp:12, 
       from D:\projects\workspace\boost/boost/spirit/home/support/meta_compiler.hpp:19, 
       from D:\projects\workspace\boost/boost/spirit/home/qi/meta_compiler.hpp:14, 
       from D:\projects\workspace\boost/boost/spirit/home/qi/action/action.hpp:14, 
       from D:\projects\workspace\boost/boost/spirit/home/qi/action.hpp:14, 
       from D:\projects\workspace\boost/boost/spirit/home/qi.hpp:14, 
       from D:\projects\workspace\boost/boost/spirit/include/qi.hpp:16, 
       from ..\src\spirit02_test.cpp:11: 
D:\projects\workspace\boost/boost/fusion/sequence/intrinsic/at_key.hpp: At global scope: 
D:\projects\workspace\boost/boost/fusion/sequence/intrinsic/at_key.hpp: In instantiation of 'boost::fusion::extension::at_key_impl<boost::fusion::non_fusion_tag>::apply<const boost::phoenix::actor<boost::spirit::attribute<0> >, keys::second>': 
D:\projects\workspace\boost/boost/fusion/sequence/intrinsic/at_key.hpp:71:9: instantiated from 'boost::fusion::result_of::at_key<const boost::phoenix::actor<boost::spirit::attribute<0> >, keys::second>' 
..\src\spirit02_test.cpp:58:36: instantiated from here 
D:\projects\workspace\boost/boost/fusion/sequence/intrinsic/at_key.hpp:38:17: error: no type named 'type' in 'struct boost::fusion::result_of::find<const boost::phoenix::actor<boost::spirit::attribute<0> >, keys::second>' 
..\src\spirit02_test.cpp: In constructor 'G::G()': 
..\src\spirit02_test.cpp:58:36: error: no matching function for call to 'at_key(const boost::spirit::_val_type&)' 

如果我用一个简单的规则（无关联），一切都编译并运行正常，但这种方法是有些脆弱：

// A non-associative solution 
    //start_rule %= (("first" >> qi::int_)^("second" >> qi::int_)); 

为什么我不能在语义动作中使用at_key？有没有更好的方法来做一个“关联”解析成非关联结构？

Answer 1

你需要一个懒惰版本的at_key。该patch for this可悲的是从未进入灵（还）：

/*============================================================================= 
    Copyright (c) 2005-2008 Hartmut Kaiser 
    Copyright (c) 2005-2007 Joel de Guzman 
    Copyright (c) 2011  Michael Caisse 

    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 PHOENIX_SEQUENCE_AT_KEY_HPP 
#define PHOENIX_SEQUENCE_AT_KEY_HPP 

#include <boost/fusion/include/at_key.hpp> 
#include <boost/spirit/home/phoenix/core/actor.hpp> 
#include <boost/spirit/home/phoenix/core/compose.hpp> 
#include <boost/type_traits/remove_reference.hpp> 

namespace boost { namespace phoenix 
{ 
    template <typename Key> 
    struct at_key_eval 
    { 
     template <typename Env, typename Tuple> 
     struct result 
     { 
      typedef typename Tuple::template result<Env>::type tuple; 
      typedef typename 
       fusion::result_of::at_key< 
        typename remove_reference<tuple>::type, Key 
       >::type 
      type; 
     }; 

     template <typename RT, typename Env, typename Tuple> 
     static RT 
     eval(Env const& env, Tuple const& t) 
     { 
      return fusion::at_key<Key>(t.eval(env)); 
     } 
    }; 

    template <typename Key, typename Tuple> 
    inline actor<typename as_composite<at_key_eval<Key>, Tuple>::type> 
    at_key(Tuple const& tup) 
    { 
     return compose<at_key_eval<Key> >(tup); 
    } 

}} 

#endif 

如果添加它，你可以使用boost::phoenix::at_key：

using boost::phoenix::at_key; 

事情会编译和正常工作。当然，链接的修补程序也会修改boost/spirit/home/phoenix/fusion.hpp以包含此新标题。