我需要解析 simple_expression ::= limit int_number (days | hours | minutes)
.我为语法写代码
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) ] ;
我尝试将语义操作移至 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了limit5days
, 但不正确 limit5minutes
,因为,正如我所见,它不能不同 limit5days
来自 limit5hours
.
最佳答案
这里发生了很多事情。然而,当我完成一些小事情的修复并尝试阅读它并使其完整时 (SSCCE),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(bind)表达式按值复制
m_lTree
,这样您就不能改变成员。phx::ref
是必需的 - 期望点使得无法成功解析“天”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,只需编写"limit">> uint_ >> "days"
因为重载会完成其余工作
正如您所注意到的,此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和紧耦合。
但是:
综上所述,我会通过 avoiding semantic actions 来简化事情,并将解析和评估分开。
简单地解析成一个 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一个 Limit
对象,您可以在解析后原子地进行求值:
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类似
的内容- 同一个表达式的重复求值(不重复解析)
- 在求值前简化表达式树
- 评估前验证
- 调试
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节省了您生命中的时间,而这些时间本可以花在其他地方
看到这个 Live On Coliru ,输出:
void AddLimit(ConditionTree&, ConditionTree::Limit): 5 minutes
list
#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);
}
}
关于c++ - 使用 boost::spirit 的简单表达式,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/24933455/