使用 boost::spirit 的简单表达式

Question

我需要解析simple_expression ::= limit int_number (days | hours | minutes)。我写了语法代码

struct Parser: grammar<std::string::const_iterator, boost::spirit::ascii::space_type>
{
public:
  Parser(ConditionTree& a_lTree):
    Parser::base_type(limit_expression),
    m_lTree(a_lTree)
  {
    using boost::spirit::qi::uint_;

    using boost::spirit::qi::_1;
    using boost::spirit::qi::_2;

    limit_expression = limit_days_operator | limit_hours_operator | limit_minutes_operator ;

    limit_days_operator = ( string("limit") > uint_ > string("days") )[ phoenix::bind( &ConditionTree::AddDaysLimitOperator, m_lTree, _2) ]  ;
    limit_hours_operator = ( string("limit") > uint_ > string("hours") )[ phoenix::bind( &ConditionTree::AddHoursLimitOperator, m_lTree, _2) ]  ;
    limit_minutes_operator = ( string("limit") > uint_ > string("minutes") )[ phoenix::bind( &ConditionTree::AddMinutesLimitOperator, m_lTree, _2) ] ;

    BOOST_SPIRIT_DEBUG_NODE(limit_expression);

    BOOST_SPIRIT_DEBUG_NODE(limit_days_operator);
    BOOST_SPIRIT_DEBUG_NODE(limit_hours_operator);
    BOOST_SPIRIT_DEBUG_NODE(limit_minutes_operator);
  }

  rule<std::string::const_iterator, boost::spirit::ascii::space_type> limit_expression;
  rule<std::string::const_iterator, boost::spirit::ascii::space_type> limit_days_operator;
  rule<std::string::const_iterator, boost::spirit::ascii::space_type> limit_hours_operator;
  rule<std::string::const_iterator, boost::spirit::ascii::space_type> limit_minutes_operator;

  ConditionTree& m_lTree;
}


void main()
{

  ConditionTree oTree;

  Parser parser(oTree);

  std::string strTest("limit5minutes");

  std::string::const_iterator it_begin(strTest.begin());
  std::string::const_iterator it_end(strTest.end());

  bool result = phrase_parse(it_begin, it_end, parser, space);
}

但它无法编译并出现以下 2 个错误： /usr/include/boost/spirit/home/support/argument.hpp:103: ошибка: no matching function for call to 'assertion_failed(mpl_::failed************ (boost::spirit::result_of::get_arg<boost::fusion::vector1<unsigned int&>, 1>::index_is_out_of_bounds::************)())'

/usr/include/boost/spirit/home/phoenix/bind/detail/member_function_ptr.hpp:103: ошибка: invalid initialization of reference of type 'const unsigned int&' from expression of type 'mpl_::void_'

在线 limit_days_operator = ( string("limit") > uint_ > string("days") )[ phoenix::bind( &ConditionTree::AddDaysLimitOperator, m_lTree, _2) ] ;

我试图将语义动作移动到 uint_ ：

limit_days_operator = string("limit") > uint_ [ phoenix::bind( &ConditionTree::AddDaysLimitOperator, m_lTree, _1) ] > string("days")  ;
limit_hours_operator = string("limit") > uint_ [ phoenix::bind( &ConditionTree::AddHoursLimitOperator, m_lTree, _1) ] > string("hours")  ;

然后解析器正确读取limit5days，但不正确读取limit5minutes，因为正如我所见，limit5days 与limit5hours 没有区别。

Answer 1

这里发生了很多事情。然而，当我修复一些小东西试图阅读它并使其完整（SSCCE）时，它也编译了：Live On Coliru

我在这里简要说明一些要点：

• 语义操作中的绑定表达式按值复制m_lTree，这样你就不能改变成员。此处需要phx::ref
• 期望点导致无法成功解析 "days" 限制以外的任何内容
• 使用lit，除非您真的想将字符串的值作为属性使用。更好的是，只需写 "limit" >> uint_ >> "days"，因为重载会完成剩下的工作

您已经注意到，该语法中有许多反模式，使其变得复杂：

• Boost Spirit: "Semantic actions are evil"?
• reference-type member m_lTree 导致 Action At A Distance 和紧密耦合。

但是：

话虽如此，我将通过 avoiding semantic actions 简化事情，并将解析和评估分开。 只需解析成Limit 结构：

qi::rule<Iterator, ConditionTree::Limit(), Skipper> limit_expression;

limit_expression = "limit" >> qi::uint_ >> unit_;

处理没有单独表达式分支的单元：

unit_.add("days",    ConditionTree::Limit::days)
         ("hours",   ConditionTree::Limit::hours)
         ("minutes", ConditionTree::Limit::minutes);

因为，现在，语法只解析 Limit 对象，您可以在解析后原子地进行评估：

ConditionTree::Limit limit;
if (phrase_parse(iter, end, parser, ascii::space, limit))
{
    AddLimit(oTree, limit);
}

将解析与评估分开使您可以添加诸如

之类的内容

• 重复计算同一表达式（不重复解析）
• 在求值前简化表达式树
• 评估前验证
• 调试中

• 更有趣的是，允许您以函数式风格编写ApplyLimit，而不是改变对象：

  ConditionTree ApplyLimit(ConditionTree const& ct, Limit limit) { 
      return ct + limit; // do something here
  }
  

• 但最重要的是：它极大地简化了语法并为您节省了数小时的时间，而这些时间本来应该花在其他方面更好

看到这个Live On Coliru，输出：

void AddLimit(ConditionTree&, ConditionTree::Limit): 5 minutes

上市

#include <boost/fusion/adapted/struct.hpp>
#include <boost/spirit/include/qi.hpp>

namespace qi    = boost::spirit::qi;
namespace ascii = boost::spirit::ascii;

struct ConditionTree {
    struct Limit {
        unsigned value;
        enum unit_t { days, hours, minutes } unit;
    };

    friend void AddLimit(ConditionTree& ct, Limit limit) {
        std::cout << "AddLimit: " << limit.value;
        switch (limit.unit) {
            case Limit::days:    std::cout << " days\n"; break;
            case Limit::hours:   std::cout << " hours\n"; break;
            case Limit::minutes: std::cout << " minutes\n"; break;
        }
    }
};

BOOST_FUSION_ADAPT_STRUCT(ConditionTree::Limit, (unsigned,value)(ConditionTree::Limit::unit_t,unit))

template <typename Iterator = std::string::const_iterator, typename Skipper = ascii::space_type>
struct Parser: qi::grammar<Iterator, ConditionTree::Limit(), ascii::space_type>
{
    public:
        Parser() : Parser::base_type(limit_expression)
    {
        unit_.add("days", ConditionTree::Limit::days)
                 ("hours", ConditionTree::Limit::hours)
                 ("minutes", ConditionTree::Limit::minutes);

        limit_expression = "limit" >> qi::uint_ >> unit_;
    }

    qi::symbols<char, ConditionTree::Limit::unit_t> unit_;
    qi::rule<Iterator, ConditionTree::Limit(), Skipper> limit_expression;
};

int main()
{
    ConditionTree oTree;

    Parser<> parser;

    std::string strTest("limit5minutes");

    std::string::const_iterator iter(strTest.begin()), end(strTest.end());

    ConditionTree::Limit limit;
    if (phrase_parse(iter, end, parser, ascii::space, limit))
    {
        AddLimit(oTree, limit);
    }
}