问题
I have a case where I'd like to filter the value that comes up as a synthesized attribute inside of a rule through a std::map.
- The map is pre-generated and will not change during the parsing.
- The nature of the map and the real parser means that the lookup should never fail (although the actual number of elements may be quite large)
- The usual approach perfectly suited to this problem (use a symbol table) isn't appropriate for the real case. In the real problem, the lookup is conditional based on an attribute that won't be apparent until much later in the parse (in a somewhat removed rule).
My attempt:
#define BOOST_SPIRIT_USE_PHOENIX_V3
#include <boost/spirit/include/qi.hpp>
#include <boost/spirit/include/phoenix.hpp>
#include <boost/foreach.hpp>
#include <string>
#include <iostream>
#include <vector>
#include <map>
namespace qi = boost::spirit::qi;
namespace phx = boost::phoenix;
int main() {
std::map<unsigned int, unsigned int> myMap;
myMap[1] = 100; myMap[2] = 200; myMap[3] = 300;
std::string test = "1 2 3";
std::vector<unsigned int> results;
qi::rule<std::string::iterator, unsigned int()> r
= qi::uint_ [qi::_val = phx::at(myMap, qi::_1)];
qi::parse(test.begin(), test.end(), ( r % " " ), results);
BOOST_FOREACH(unsigned int &x, results) {
std::cout << x << "\n";
}
}
I guess I was under the impression, due to the phoenix support for stl containers, that this should work. But I get a compile error on the rule line. This error goes away if I replace the semantic action with the classic pointless [qi::_val = qi::_1] (not surprisingly).
The compiler error under MSVS10 is shockingly long, as usual, but here's the first mention of my code file: (at C:\code\Compiler2\spirit_test.cpp(25)... line 25 is the rule r)
C:\boost_1_50_0\boost/spirit/home/qi/nonterminal/rule.hpp(191) : see reference to function template instantiatio
n 'void boost::spirit::qi::rule<Iterator,T1>::define<boost::mpl::false_,Expr>(boost::spirit::qi::rule<Iterator,T1> &,con
st Expr &,boost::mpl::true_)' being compiled
with
[
Iterator=std::_String_iterator<char,std::char_traits<char>,std::allocator<char>>,
T1=unsigned int (void),
Expr=boost::proto::exprns_::expr<boost::proto::tagns_::tag::subscript,boost::proto::argsns_::list2<const boo
st::spirit::terminal<boost::spirit::tag::uint_> &,const boost::phoenix::actor<boost::proto::exprns_::basic_expr<boost::p
roto::tagns_::tag::assign,boost::proto::argsns_::list2<boost::proto::exprns_::expr<boost::proto::tagns_::tag::terminal,b
oost::proto::argsns_::term<boost::spirit::attribute<0>>,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::at_impl>,0>,boost::phoenix::actor<boost::proto::exprns
_::basic_expr<boost::proto::tagns_::tag::terminal,boost::proto::argsns_::term<boost::reference_wrapper<std::map<unsigned
int,unsigned int>>>,0>>,boost::phoenix::actor<boost::spirit::argument<0>>>,3>>>,2>> &>,2>
]
C:\code\Compiler2\spirit_test.cpp(25) : see reference to function template instantiation 'boost::spirit::qi
::rule<Iterator,T1>::rule<boost::proto::exprns_::expr<Tag,Args,Arity>>(const Expr &,const std::string &)' being compiled
with
[
Iterator=std::_String_iterator<char,std::char_traits<char>,std::allocator<char>>,
T1=unsigned int (void),
Tag=boost::proto::tagns_::tag::subscript,
Args=boost::proto::argsns_::list2<const boost::spirit::terminal<boost::spirit::tag::uint_> &,const boost::ph
oenix::actor<boost::proto::exprns_::basic_expr<boost::proto::tagns_::tag::assign,boost::proto::argsns_::list2<boost::pro
to::exprns_::expr<boost::proto::tagns_::tag::terminal,boost::proto::argsns_::term<boost::spirit::attribute<0>>,0>,boost:
:phoenix::actor<boost::proto::exprns_::basic_expr<boost::phoenix::detail::tag::function_eval,boost::proto::argsns_::list
3<boost::proto::exprns_::basic_expr<boost::proto::tagns_::tag::terminal,boost::proto::argsns_::term<boost::phoenix::stl:
:at_impl>,0>,boost::phoenix::actor<boost::proto::exprns_::basic_expr<boost::proto::tagns_::tag::terminal,boost::proto::a
rgsns_::term<boost::reference_wrapper<std::map<unsigned int,unsigned int>>>,0>>,boost::phoenix::actor<boost::spirit::arg
ument<0>>>,3>>>,2>> &>,
Arity=2,
Expr=boost::proto::exprns_::expr<boost::proto::tagns_::tag::subscript,boost::proto::argsns_::list2<const boo
st::spirit::terminal<boost::spirit::tag::uint_> &,const boost::phoenix::actor<boost::proto::exprns_::basic_expr<boost::p
roto::tagns_::tag::assign,boost::proto::argsns_::list2<boost::proto::exprns_::expr<boost::proto::tagns_::tag::terminal,b
oost::proto::argsns_::term<boost::spirit::attribute<0>>,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::at_impl>,0>,boost::phoenix::actor<boost::proto::exprns
_::basic_expr<boost::proto::tagns_::tag::terminal,boost::proto::argsns_::term<boost::reference_wrapper<std::map<unsigned
int,unsigned int>>>,0>>,boost::phoenix::actor<boost::spirit::argument<0>>>,3>>>,2>> &>,2>
]
C:\boost_1_50_0\boost/proto/transform/default.hpp(154) : error C2440: '=' : cannot convert from 'std::pair<_Ty1,_Ty2>' t
o 'unsigned int'
with
[
_Ty1=const unsigned int,
_Ty2=unsigned int
]
No user-defined-conversion operator available that can perform this conversion, or the operator cannot be called
回答1:
Reposting from comment
Try qi::_val = phx::ref(myMap)[qi::_1] rather than qi::_val = phx::at(myMap, qi::_1).
回答2:
The problem is that the return type of phoenix::at is defined in this header as container::value_type. This is a pair in the case of a map. Simply specializing the result for map makes it work (using ref as noted by ildjarn).
#define BOOST_SPIRIT_USE_PHOENIX_V3
#include <boost/spirit/include/qi.hpp>
#include <boost/spirit/include/phoenix.hpp>
#include <boost/foreach.hpp>
#include <string>
#include <iostream>
#include <vector>
#include <map>
namespace qi = boost::spirit::qi;
namespace phx = boost::phoenix;
namespace boost { namespace phoenix { namespace stl {
template <typename This, typename Key, typename Value, typename Compare, typename Allocator, typename Index>
struct at_impl::result<This(std::map<Key,Value,Compare,Allocator>&, Index)>
{
typedef Value & type;
};
template <typename This, typename Key, typename Value, typename Compare, typename Allocator, typename Index>
struct at_impl::result<This(std::map<Key,Value,Compare,Allocator> const&, Index)>
{
typedef Value const& type;
};
}}}
int main() {
std::map<unsigned int, unsigned int> myMap;
myMap[1] = 100; myMap[2] = 200; myMap[3] = 300;
std::string test = "1 2 3";
std::vector<unsigned int> results;
qi::rule<std::string::iterator, unsigned int()> r
= qi::uint_ [qi::_val = phx::at(phx::cref(myMap), qi::_1)];
qi::parse(test.begin(), test.end(), ( r % " " ), results);
BOOST_FOREACH(unsigned int &x, results) {
std::cout << x << "\n";
}
}
来源：https://stackoverflow.com/questions/14205154/filter-the-synthesized-attribute-through-a-stdmap-in-a-boost-spirit-semantic-a