c++ - boost::phoenix::insert 的评估结果

Question

我在尝试评估 boost::phoenix::insert 的结果时遇到困难将元素插入 map 。类似于常规 std::map::insert boost::phoenix::insert 返回的 Actor 对象还返回 std::pair<Iterator where, bool result> .我对该对的第二个元素感兴趣，以检查插入是否成功。说明该问题的大量精简代码示例如下:

#include <iostream>
#include <boost/phoenix/fusion.hpp>
#include <boost/phoenix/phoenix.hpp>
#include <boost/fusion/include/all.hpp>
#include <boost/fusion/include/std_pair.hpp>

int main(int, char*[])
{
  namespace phx = boost::phoenix;
  using phx::arg_names::arg1;
  using phx::arg_names::arg2;

  std::map<int, int> map;
  std::pair<int, int> value(1, 2);

  if (phx::at_c<1>(phx::insert(arg1, arg2))(map, value)) // <- Error here
    std::cout << "Success" << std::endl;
  else
    std::cout << "Fail" << std::endl;

  return 0;
}

我在使用 MSVC2012 和 boost 1.53 时遇到以下错误:

error C2440: 'return' : cannot convert from 'const bool' to 'bool &'
<some-path>\include\boost\proto\transform\call.hpp:258

clang3.2报同样的错误:

Compilation finished with errors:
In file included from source.cpp:2:
In file included from /usr/local/include/boost/phoenix/fusion.hpp:14:
In file included from /usr/local/include/boost/phoenix/fusion/at.hpp:14:
In file included from /usr/local/include/boost/phoenix/core/expression.hpp:10:
In file included from /usr/local/include/boost/phoenix/core/as_actor.hpp:10:
In file included from /usr/local/include/boost/phoenix/core/actor.hpp:17:
In file included from /usr/local/include/boost/phoenix/core/domain.hpp:12:
In file included from /usr/local/include/boost/proto/matches.hpp:43:
In file included from /usr/local/include/boost/proto/transform/when.hpp:22:
/usr/local/include/boost/proto/transform/call.hpp:255:24: error: binding of reference to type 'bool' to a value of type 'const bool' drops qualifiers
                return typename detail::poly_function_traits<Fun, Fun(a0, a1)>::function_type()(
                       ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/local/include/boost/phoenix/core/meta_grammar.hpp:74:24: note: in instantiation of member function 'boost::proto::call<boost::proto::functional::at (boost::phoenix::evaluator (*)(boost::proto::_child_c<1>), boost::proto::_value (*)(boost::proto::_child_c<0>))>::impl2<const boost::phoenix::actor<boost::proto::exprns_::basic_expr<boost::phoenix::tag::at_c, boost::proto::argsns_::list2<boost::proto::exprns_::basic_expr<boost::proto::tagns_::tag::terminal, boost::proto::argsns_::term<mpl_::int_<1> >, 0>, boost::phoenix::actor<boost::proto::exprns_::basic_expr<boost::phoenix::detail::tag::function_eval, boost::proto::argsns_::list3<boost::proto::exprns_::basic_expr<boost::proto::tagns_::tag::terminal, boost::proto::argsns_::term<boost::phoenix::stl::insert>, 0>, boost::phoenix::actor<boost::proto::exprns_::basic_expr<boost::proto::tagns_::tag::terminal, boost::proto::argsns_::term<boost::phoenix::argument<1> >, 0> >, boost::phoenix::actor<boost::proto::exprns_::basic_expr<boost::proto::tagns_::tag::terminal, boost::proto::argsns_::term<boost::phoenix::argument<2> >, 0> > >, 3> > >, 2> > &, boost::phoenix::vector3<const boost::phoenix::actor<boost::proto::exprns_::basic_expr<boost::phoenix::tag::at_c, boost::proto::argsns_::list2<boost::proto::exprns_::basic_expr<boost::proto::tagns_::tag::terminal, boost::proto::argsns_::term<mpl_::int_<1> >, 0>, boost::phoenix::actor<boost::proto::exprns_::basic_expr<boost::phoenix::detail::tag::function_eval, boost::proto::argsns_::list3<boost::proto::exprns_::basic_expr<boost::proto::tagns_::tag::terminal, boost::proto::argsns_::term<boost::phoenix::stl::insert>, 0>, boost::phoenix::actor<boost::proto::exprns_::basic_expr<boost::proto::tagns_::tag::terminal, boost::proto::argsns_::term<boost::phoenix::argument<1> >, 0> >, boost::phoenix::actor<boost::proto::exprns_::basic_expr<boost::proto::tagns_::tag::terminal, boost::proto::argsns_::term<boost::phoenix::argument<2> >, 0> > >, 3> > >, 2> > *, std::map<int, int, std::less<int>, std::allocator<std::pair<const int, int> > > &, std::pair<int, int> &> &, const boost::phoenix::default_actions &, false>::operator()' requested here
                return what()(e, phoenix::env(s), actions(s));
                       ^
/usr/local/include/boost/phoenix/core/meta_grammar.hpp:34:9: note: in instantiation of member function 'boost::phoenix::evaluator::impl<const boost::phoenix::actor<boost::proto::exprns_::basic_expr<boost::phoenix::tag::at_c, boost::proto::argsns_::list2<boost::proto::exprns_::basic_expr<boost::proto::tagns_::tag::terminal, boost::proto::argsns_::term<mpl_::int_<1> >, 0>, boost::phoenix::actor<boost::proto::exprns_::basic_expr<boost::phoenix::detail::tag::function_eval, boost::proto::argsns_::list3<boost::proto::exprns_::basic_expr<boost::proto::tagns_::tag::terminal, boost::proto::argsns_::term<boost::phoenix::stl::insert>, 0>, boost::phoenix::actor<boost::proto::exprns_::basic_expr<boost::proto::tagns_::tag::terminal, boost::proto::argsns_::term<boost::phoenix::argument<1> >, 0> >, boost::phoenix::actor<boost::proto::exprns_::basic_expr<boost::proto::tagns_::tag::terminal, boost::proto::argsns_::term<boost::phoenix::argument<2> >, 0> > >, 3> > >, 2> > &, const boost::phoenix::vector2<boost::phoenix::vector3<const boost::phoenix::actor<boost::proto::exprns_::basic_expr<boost::phoenix::tag::at_c, boost::proto::argsns_::list2<boost::proto::exprns_::basic_expr<boost::proto::tagns_::tag::terminal, boost::proto::argsns_::term<mpl_::int_<1> >, 0>, boost::phoenix::actor<boost::proto::exprns_::basic_expr<boost::phoenix::detail::tag::function_eval, boost::proto::argsns_::list3<boost::proto::exprns_::basic_expr<boost::proto::tagns_::tag::terminal, boost::proto::argsns_::term<boost::phoenix::stl::insert>, 0>, boost::phoenix::actor<boost::proto::exprns_::basic_expr<boost::proto::tagns_::tag::terminal, boost::proto::argsns_::term<boost::phoenix::argument<1> >, 0> >, boost::phoenix::actor<boost::proto::exprns_::basic_expr<boost::proto::tagns_::tag::terminal, boost::proto::argsns_::term<boost::phoenix::argument<2> >, 0> > >, 3> > >, 2> > *, std::map<int, int, std::less<int>, std::allocator<std::pair<const int, int> > > &, std::pair<int, int> &> &, const boost::phoenix::default_actions &> &, boost::proto::envns_::empty_env>::operator()' requested here
        BOOST_PROTO_TRANSFORM(evaluator)
        ^
/usr/local/include/boost/proto/transform/impl.hpp:228:9: note: expanded from macro 'BOOST_PROTO_TRANSFORM'
        BOOST_PROTO_TRANSFORM_(PrimitiveTransform, void)                                                        \
        ^
/usr/local/include/boost/proto/transform/impl.hpp:213:16: note: expanded from macro 'BOOST_PROTO_TRANSFORM_'
        return boost::proto::detail::apply_transform<transform_type(Expr const &, State const &)>()(e, s, d);   \
               ^
/usr/local/include/boost/phoenix/core/meta_grammar.hpp:139:16: note: in instantiation of function template specialization 'boost::phoenix::evaluator::operator()<const boost::phoenix::actor<boost::proto::exprns_::basic_expr<boost::phoenix::tag::at_c, boost::proto::argsns_::list2<boost::proto::exprns_::basic_expr<boost::proto::tagns_::tag::terminal, boost::proto::argsns_::term<mpl_::int_<1> >, 0>, boost::phoenix::actor<boost::proto::exprns_::basic_expr<boost::phoenix::detail::tag::function_eval, boost::proto::argsns_::list3<boost::proto::exprns_::basic_expr<boost::proto::tagns_::tag::terminal, boost::proto::argsns_::term<boost::phoenix::stl::insert>, 0>, boost::phoenix::actor<boost::proto::exprns_::basic_expr<boost::proto::tagns_::tag::terminal, boost::proto::argsns_::term<boost::phoenix::argument<1> >, 0> >, boost::phoenix::actor<boost::proto::exprns_::basic_expr<boost::proto::tagns_::tag::terminal, boost::proto::argsns_::term<boost::phoenix::argument<2> >, 0> > >, 3> > >, 2> > &, const boost::phoenix::vector2<boost::phoenix::vector3<const boost::phoenix::actor<boost::proto::exprns_::basic_expr<boost::phoenix::tag::at_c, boost::proto::argsns_::list2<boost::proto::exprns_::basic_expr<boost::proto::tagns_::tag::terminal, boost::proto::argsns_::term<mpl_::int_<1> >, 0>, boost::phoenix::actor<boost::proto::exprns_::basic_expr<boost::phoenix::detail::tag::function_eval, boost::proto::argsns_::list3<boost::proto::exprns_::basic_expr<boost::proto::tagns_::tag::terminal, boost::proto::argsns_::term<boost::phoenix::stl::insert>, 0>, boost::phoenix::actor<boost::proto::exprns_::basic_expr<boost::proto::tagns_::tag::terminal, boost::proto::argsns_::term<boost::phoenix::argument<1> >, 0> >, boost::phoenix::actor<boost::proto::exprns_::basic_expr<boost::proto::tagns_::tag::terminal, boost::proto::argsns_::term<boost::phoenix::argument<2> >, 0> > >, 3> > >, 2> > *, std::map<int, int, std::less<int>, std::allocator<std::pair<const int, int> > > &, std::pair<int, int> &> &, const boost::phoenix::default_actions &> &>' requested here
        return e(expr, ctx);
               ^
/usr/local/include/boost/phoenix/core/detail/preprocessed/actor_operator_10.hpp:31:385: note: in instantiation of function template specialization 'boost::phoenix::eval<boost::phoenix::actor<boost::proto::exprns_::basic_expr<boost::phoenix::tag::at_c, boost::proto::argsns_::list2<boost::proto::exprns_::basic_expr<boost::proto::tagns_::tag::terminal, boost::proto::argsns_::term<mpl_::int_<1> >, 0>, boost::phoenix::actor<boost::proto::exprns_::basic_expr<boost::phoenix::detail::tag::function_eval, boost::proto::argsns_::list3<boost::proto::exprns_::basic_expr<boost::proto::tagns_::tag::terminal, boost::proto::argsns_::term<boost::phoenix::stl::insert>, 0>, boost::phoenix::actor<boost::proto::exprns_::basic_expr<boost::proto::tagns_::tag::terminal, boost::proto::argsns_::term<boost::phoenix::argument<1> >, 0> >, boost::phoenix::actor<boost::proto::exprns_::basic_expr<boost::proto::tagns_::tag::terminal, boost::proto::argsns_::term<boost::phoenix::argument<2> >, 0> > >, 3> > >, 2> >, boost::phoenix::vector2<boost::phoenix::vector3<const boost::phoenix::actor<boost::proto::exprns_::basic_expr<boost::phoenix::tag::at_c, boost::proto::argsns_::list2<boost::proto::exprns_::basic_expr<boost::proto::tagns_::tag::terminal, boost::proto::argsns_::term<mpl_::int_<1> >, 0>, boost::phoenix::actor<boost::proto::exprns_::basic_expr<boost::phoenix::detail::tag::function_eval, boost::proto::argsns_::list3<boost::proto::exprns_::basic_expr<boost::proto::tagns_::tag::terminal, boost::proto::argsns_::term<boost::phoenix::stl::insert>, 0>, boost::phoenix::actor<boost::proto::exprns_::basic_expr<boost::proto::tagns_::tag::terminal, boost::proto::argsns_::term<boost::phoenix::argument<1> >, 0> >, boost::phoenix::actor<boost::proto::exprns_::basic_expr<boost::proto::tagns_::tag::terminal, boost::proto::argsns_::term<boost::phoenix::argument<2> >, 0> > >, 3> > >, 2> > *, std::map<int, int, std::less<int>, std::allocator<std::pair<const int, int> > > &, std::pair<int, int> &> &, const boost::phoenix::default_actions &> >' requested here
        template <typename This, typename A0 , typename A1> struct result<This(A0 & , A1 &)> : result_of::actor<proto_base_expr, A0 & , A1 &> {}; template <typename A0 , typename A1> typename result_of::actor<proto_base_expr, A0 & , A1 &>::type operator()(A0 & a0 , A1 & a1) const { typedef vector3< const actor<Expr> *, A0 & , A1 & > env_type; env_type env = {this, a0 , a1}; return phoenix::eval(*this, phoenix::context(env, default_actions())); } template <typename A0 , typename A1> typename result_of::actor<proto_base_expr, A0 & , A1 &>::type operator()(A0 & a0 , A1 & a1) { typedef vector3< const actor<Expr> *, A0 & , A1 & > env_type; env_type env = {this, a0 , a1}; return phoenix::eval(*this, phoenix::context(env, default_actions())); } template <typename This, typename A0 , typename A1> struct result<This(A0 & , A1 const&)> : result_of::actor<proto_base_expr, A0 & , A1 const&> {}; template <typename A0 , typename A1> typename result_of::actor<proto_base_expr, A0 & , A1 const&>::type operator()(A0 & a0 , A1 const& a1) const { typedef vector3< const actor<Expr> *, A0 & , A1 const& > env_type; env_type env = {this, a0 , a1}; return phoenix::eval(*this, phoenix::context(env, default_actions())); } template <typename A0 , typename A1> typename result_of::actor<proto_base_expr, A0 & , A1 const&>::type operator()(A0 & a0 , A1 const& a1) { typedef vector3< const actor<Expr> *, A0 & , A1 const& > env_type; env_type env = {this, a0 , a1}; return phoenix::eval(*this, phoenix::context(env, default_actions())); } template <typename This, typename A0 , typename A1> struct result<This(A0 const& , A1 &)> : result_of::actor<proto_base_expr, A0 const& , A1 &> {}; template <typename A0 , typename A1> typename result_of::actor<proto_base_expr, A0 const& , A1 &>::type operator()(A0 const& a0 , A1 & a1) const { typedef vector3< const actor<Expr> *, A0 const& , A1 & > env_type; env_type env = {this, a0 , a1}; return phoenix::eval(*this, phoenix::context(env, default_actions())); } template <typename A0 , typename A1> typename result_of::actor<proto_base_expr, A0 const& , A1 &>::type operator()(A0 const& a0 , A1 & a1) { typedef vector3< const actor<Expr> *, A0 const& , A1 & > env_type; env_type env = {this, a0 , a1}; return phoenix::eval(*this, phoenix::context(env, default_actions())); } template <typename This, typename A0 , typename A1> struct result<This(A0 const& , A1 const&)> : result_of::actor<proto_base_expr, A0 const& , A1 const&> {}; template <typename A0 , typename A1> typename result_of::actor<proto_base_expr, A0 const& , A1 const&>::type operator()(A0 const& a0 , A1 const& a1) const { typedef vector3< const actor<Expr> *, A0 const& , A1 const& > env_type; env_type env = {this, a0 , a1}; return phoenix::eval(*this, phoenix::context(env, default_actions())); } template <typename A0 , typename A1> typename result_of::actor<proto_base_expr, A0 const& , A1 const&>::type operator()(A0 const& a0 , A1 const& a1) { typedef vector3< const actor<Expr> *, A0 const& , A1 const& > env_type; env_type env = {this, a0 , a1}; return phoenix::eval(*this, phoenix::context(env, default_actions())); }
                                                                                                                                                                                                                                                                                                                                                                                                ^
source.cpp:16:44: note: in instantiation of function template specialization 'boost::phoenix::actor<boost::proto::exprns_::basic_expr<boost::phoenix::tag::at_c, boost::proto::argsns_::list2<boost::proto::exprns_::basic_expr<boost::proto::tagns_::tag::terminal, boost::proto::argsns_::term<mpl_::int_<1> >, 0>, boost::phoenix::actor<boost::proto::exprns_::basic_expr<boost::phoenix::detail::tag::function_eval, boost::proto::argsns_::list3<boost::proto::exprns_::basic_expr<boost::proto::tagns_::tag::terminal, boost::proto::argsns_::term<boost::phoenix::stl::insert>, 0>, boost::phoenix::actor<boost::proto::exprns_::basic_expr<boost::proto::tagns_::tag::terminal, boost::proto::argsns_::term<boost::phoenix::argument<1> >, 0> >, boost::phoenix::actor<boost::proto::exprns_::basic_expr<boost::proto::tagns_::tag::terminal, boost::proto::argsns_::term<boost::phoenix::argument<2> >, 0> > >, 3> > >, 2> >::operator()<std::map<int, int, std::less<int>, std::allocator<std::pair<const int, int> > >, std::pair<int, int> >' requested here
  if (phx::at_c<1>(phx::insert(arg1, arg2))(map, value))
                                           ^
1 error generated.

我即将用完如何正确评估插入结果的想法。任何帮助将不胜感激。

编辑:我的问题的更广泛的背景是我正在尝试使用 boost::spirit::qi 解析类似 C++ 的枚举。我发现的任何试图实现相同目的的示例都不会检查重复的枚举成员。这是包含两个相关规则的代码片段:

  enumerationMember = identifier[at_c<0>(_val) = _1] >
    // If there is an explicit value defined use it.
    ((lit('=') > int_[at_c<1>(_val) = _1]) |
    // Otherwise use the value of argument _r1 instead.
    eps[at_c<1>(_val) = _r1]);
  enumeration = lit("enum") > typeName[at_c<0>(_val) = _1] > lit(':') >
    enumerationType[at_c<1>(_val) = _1] > braceOpen >
    // Initialize _a with 0.
    eps[_a = 0] >
    // Zero or one comma separated list of members.
    -(enumerationMember(_a)[//_pass = boost::phoenix::at_c<1>( <- I'm looking for something like this..
      insert(at_c<2>(_val), _1)]
      // Set _a to the value of the last member + 1.
      [_a = at_c<1>(_1) + 1]
      % lit(',')) >
    braceClose;

Answer 1

根据@llonesmiz 的评论，我找到了解决方法。核心问题似乎是参数没有在整个嵌套操作中正确传递(或者我在使用 boost::phoenix 时做一些根本性的错误):

phx::at_c<1>(phx::insert(arg1, arg2))( map , 值)

at_c 返回的操作应将两个参数映射和值转发给 insert 返回的操作。

相比之下，以下内容确实有效:

phx::at_c<1>(phx::insert(arg1, arg2)(map, value))()

但是，在 boost::spirit::qi 解析器中，我无法直接访问 map，因为它是我的返回值之一，我需要惰性求值(无论如何，这就是 boost::phoenix 的全部意义)。所以它似乎并不直接适用于这个(简化的)解析器规则:

  enumeration = lit("enum") > typeName[at_c<0>(_val) = _1] > braceOpen >
    // Initialize _a with 0.
    eps[_a = 0] >
    // Zero or one comma separated lists of members.
    -(enumerationMember(_a)
      // Only pass if the new member is unique.
      [_pass = at_c<1>(insert(at_c<2>(_val), _1))] // <- Error
      // Set _a to the value of the last member + 1.
      [_a = at_c<1>(_1) + 1]
      % lit(',')) >
    braceClose;

我找到的解决方法是使用两个语义操作并将插入结果临时存储到局部变量中:

  enumeration = lit("enum") > typeName[at_c<0>(_val) = _1] > braceOpen >
    // Initialize _a with 0.
    eps[_a = 0] >
    // Zero or one comma separated lists of members.
    -(enumerationMember(_a)
      // Temporarily store the result of insert into the local variable _b.
      [_b = insert(at_c<2>(_val), _1)]
      // Only pass if the previous insert was successful.
      [_pass = at_c<1>(_b)]
      // Set _a to the value of the last member + 1.
      [_a = at_c<1>(_1) + 1]
      % lit(',')) >
    braceClose;

名为 enumeration 的规则的定义现在如下所示:

boost::spirit::qi::rule<Iterator, Enumeration(),
  boost::spirit::qi::locals<
    int, // _a
    std::pair<std::map<std::string, int>::iterator, bool> // _b
  >, space_type> enumeration;

我想有一些比使用本地存储插入结果更优雅的方法。因此，我暂时保留这个问题，以便给出更优雅的答案。