无法在Boost.Spirit中从占位符构造std :: string

时间:2016-07-06 10:22:20

标签: c++ boost-spirit boost-spirit-qi

我已经开始研究基于Boost.Spirit的简单解析器,它将解析类似C ++的文件(唯一的C ++ - ish部分是内置模板类型;例如map<string, smart_ptr<int>> name_object_map; - 但这是构建的 - 在编译器中,用户无法声明模板类)。然而,语法旨在包含数据结构声明,而不是表达式,除了用于初始化枚举器声明的常量表达式; enum E { a = 4 * 5 + 3 };有效。这对我来说目前不是问题,因为我无法以我想要的方式解析E:)

我在阅读了文档和示例后,昨天做了以下内容,但它没有编译:

#include <boost/spirit/include/phoenix.hpp>
#include <boost/spirit/include/qi.hpp>
#include <boost/spirit/include/qi_char_class.hpp>
#include <cassert>
#include <memory>
#include <string>
#include <utility>

struct context {};

class foo {
  std::string name;
  const context *ctx;

public:
  foo(const std::string &name, const context *ctx) : name(name), ctx(ctx) {}
};

using foo_ref = std::shared_ptr<foo>;

template <typename Iterator>
struct skipper : boost::spirit::qi::grammar<Iterator> {
  skipper() : skipper::base_type(start) {
    using namespace boost::spirit;
    qi::char_type char_;
    ascii::space_type space;

    start = space                             // tab/space/cr/lf
            | "/*" >> *(char_ - "*/") >> "*/" // C-style comments
        ;
  }

  boost::spirit::qi::rule<Iterator> start;
};

template <typename Iterator>
struct the_parser : boost::spirit::qi::grammar<Iterator, std::vector<foo_ref>(),
                                               skipper<Iterator>> {
  the_parser() : the_parser::base_type(start), current_context(&root) {
    using namespace boost::spirit;
    namespace phx = boost::phoenix;

    identifier = qi::lexeme[qi::alpha >> *qi::alnum];
    start = *(foo_decl); // currently, no semantic action attached.
                         // This will create the root decl in ast.

    foo_decl = (lit("foo") >> identifier)[qi::_val = std::make_shared<foo>(
                                               qi::_1, current_context)] >>
               qi::char_('{') >> qi::char_('}') >> qi::char_(';');
    BOOST_SPIRIT_DEBUG_NODES((identifier)(start)(foo_decl));
  }
  boost::spirit::qi::rule<Iterator, std::string(), skipper<Iterator>>
      identifier;
  boost::spirit::qi::rule<Iterator, std::vector<foo_ref>(), skipper<Iterator>>
      start;
  boost::spirit::qi::rule<Iterator, foo_ref(), skipper<Iterator>> foo_decl;
  context root;
  const context *current_context;
};

int main() {
  the_parser<std::string::const_iterator> parser;
  std::vector<foo_ref> root;

  const std::string content = "foo john_doe { };";
  auto first = content.cbegin(), last = content.cend();
  bool r = boost::spirit::qi::phrase_parse(
      first, last, parser, skipper<std::string::const_iterator>(), root);
  assert(r && first == last);
}

使用Mac上的clang进行编译(第一行是std::make_shared):

error: no matching constructor for initialization of 'foo'
          __second_(_VSTD::forward<_Args2>(_VSTD::get<_I2>(__second_args))...)
          ^         ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
...
note: candidate constructor not viable: no known conversion from 'const boost::phoenix::actor<boost::spirit::argument<0> >' to 'const std::string' (aka
  'const basic_string<char, char_traits<char>, allocator<char> >') for 1st argument
  foo(const std::string &name, const context *ctx) : name(name), ctx(ctx) {}
  ^

foo_decl的语义操作中,它无法构造foo(通过std::make_shared),因为第一个属性的结果无法转换为std::string。但是,如果我添加一个类成员std::string s,并执行此操作,则可以:

foo_decl = (lit("foo") >> identifier)[boost::phoenix::ref(s) = qi::_1] >>
                 qi::char_('{') >> qi::char_('}') >> qi::char_(';');

同样,如果我尝试std::cout,我可以看到john_doe已打印出来。

如果我使用phoenix绑定成员函数调用,它也可以工作:

foo_decl = (lit("foo") >> identifier)[qi::_val =
                   boost::phoenix::bind(&the_parser, this, qi::_1)] >>
           qi::char_('{') >> qi::char_('}') >> qi::char_(';');

foo_ref make_foo(const std::string &n) {
  return std::make_shared(n, current_context);
}

最后三个解决方法意味着存在从decltype(qi::_1)std::string的隐式转换序列;这不正确吗?

如果你能告诉我我的错误或我对语义行为和占位符如何运作的理解上的差距,我会非常高兴。对于std::make_shared无效的原因,我觉得很奇怪。

谢谢!

1 个答案:

答案 0 :(得分:2)

首先:

使用第一个链接中的phoenix :: function:

<强> Live On Coliru

#define BOOST_SPIRIT_DEBUG
#include <boost/spirit/include/phoenix.hpp>
#include <boost/spirit/include/qi.hpp>
#include <cassert>
#include <memory>
#include <string>
#include <utility>

namespace {
    template <typename T> struct make_shared_f {
        template <typename... A> struct result { typedef std::shared_ptr<T> type; };

        template <typename... A> typename result<A...>::type operator()(A &&... a) const {
            return std::make_shared<T>(std::forward<A>(a)...);
        }
    };

    template <typename T> using make_shared_ = boost::phoenix::function<make_shared_f<T> >;
}

struct context {};

class foo {
    std::string name;
    const context *ctx;

  public:
    foo(const std::string &name, const context *ctx) : name(name), ctx(ctx) {}
};

using foo_ref = std::shared_ptr<foo>;

template <typename Iterator> struct skipper : boost::spirit::qi::grammar<Iterator> {
    skipper() : skipper::base_type(start) {
        using namespace boost::spirit;
        qi::char_type char_;
        ascii::space_type space;

        start = space                             // tab/space/cr/lf
                | "/*" >> *(char_ - "*/") >> "*/" // C-style comments
            ;
    }

    boost::spirit::qi::rule<Iterator> start;
};

template <typename Iterator>
struct the_parser : boost::spirit::qi::grammar<Iterator, std::vector<foo_ref>(), skipper<Iterator> > {
    the_parser() : the_parser::base_type(start), current_context(&root) {
        using namespace boost::spirit;
        namespace phx = boost::phoenix;

        identifier = qi::alpha >> *qi::alnum;
        // This will create the root decl in ast.

        foo_decl = ("foo" >> identifier) [qi::_val = make_shared_<foo>{}(qi::_1, current_context)] >>
                   '{' >> '}' >> ';';

        start      = *foo_decl; // currently, no semantic action attached.
        BOOST_SPIRIT_DEBUG_NODES((identifier)(start)(foo_decl));
    }
    boost::spirit::qi::rule<Iterator, std::string()> identifier;
    boost::spirit::qi::rule<Iterator, foo_ref(), skipper<Iterator> > foo_decl;
    boost::spirit::qi::rule<Iterator, std::vector<foo_ref>(), skipper<Iterator> > start;
    context root;
    const context *current_context;
};

int main() {
    the_parser<std::string::const_iterator> parser;
    std::vector<foo_ref> root;

    const std::string content = "foo johndoe { };";
    auto first = content.cbegin(), last = content.cend();
    bool r = boost::spirit::qi::phrase_parse(first, last, parser, skipper<std::string::const_iterator>(), root);
    if (r)
        std::cout << "success\n";
    else
        std::cout << "failed\n";

    if (first != last)
        std::cout << "remaining unparsed: '" << std::string(first,last) << "'\n";

}

打印

success

随着

的调试输出
<start>
  <try>foo johndoe { };</try>
  <foo_decl>
    <try>foo johndoe { };</try>
    <identifier>
      <try>johndoe { };</try>
      <success> { };</success>
      <attributes>[[j, o, h, n, d, o, e]]</attributes>
    </identifier>
    <success></success>
    <attributes>[0x60600000ebb0]</attributes>
  </foo_decl>
  <foo_decl>
    <try></try>
    <fail/>
  </foo_decl>
  <success></success>
  <attributes>[[0x60600000ebb0]]</attributes>
</start>