spirit默认的语义动作和字符串组成

boost::spirit default semantic action and string composition

本文关键字：字符串 默认 语义 spirit      更新时间：2023-10-16

我有以下代码:

#include <gtest/gtest.h>
#include <boost/config/warning_disable.hpp>
#include <boost/spirit/include/qi.hpp>
#include <boost/spirit/include/qi_eps.hpp>
#include <boost/spirit/include/phoenix_core.hpp>
#include <boost/spirit/include/phoenix_operator.hpp>
#include <boost/spirit/include/phoenix_object.hpp>
#include <boost/spirit/include/phoenix_bind.hpp>
#include <boost/spirit/include/classic_core.hpp>
#include <boost/spirit/include/classic_confix.hpp>
#include <boost/spirit/include/classic_chset.hpp>
#include <boost/spirit/include/classic_utility.hpp>
#include <boost/fusion/include/cons.hpp>
#include <boost/fusion/include/at_c.hpp>
#include <boost/fusion/include/std_pair.hpp>
#include <boost/bind.hpp>
#include <boost/optional/optional_io.hpp>
#include <boost/variant.hpp>
namespace spi = boost::spirit;
namespace qi = boost::spirit::qi;
TEST(TestBoost, cpp_comment)
{
    using qi::char_;
    using qi::omit;
    using qi::eoi;
    typedef std::string::const_iterator iter;
    const std::string example = "/* this should be ignored */";
    auto b = std::begin(example);
    auto e = std::end(example);
    qi::rule<iter, std::string()> cpp_comment = char_('/') >> char_('/') >> *(char_ - 'n') >> (char_('n') | omit[eoi]);
    qi::rule<iter, std::string()> c_comment = char_('/') >> char_('*') >> *(char_ - "*/") >> char_('*') >> char_('/');
    qi::rule<iter, std::string()> shell_comment = char_('#') >> *(char_ - 'n') >> (char_('n') | omit[eoi]);
    qi::rule<iter, std::string()> comment = cpp_comment
            | c_comment
            | shell_comment
            ;
    std::string result;
    EXPECT_TRUE(qi::parse(b, e, comment, result));
    EXPECT_EQ(b, e);
    EXPECT_EQ(result, example);
}

失败，出现以下错误:

[----------] 1 test from TestBoost
[ RUN      ] TestBoost.cpp_comment
tests/spirit.cpp:56: Failure
      Expected: result
      Which is: "//* this should be ignored */"
To be equal to: example
      Which is: "/* this should be ignored */"
[  FAILED  ] TestBoost.cpp_comment (0 ms)
[----------] 1 test from TestBoost (0 ms total)

我不明白为什么。可能在boost文档的某个地方提到了这种行为，但我找不到它。有人知道为什么会这样吗?

如果我把语义动作像这样:

qi::rule<iter, std::string()> comment = cpp_comment[spi::_val = spi::_1]
            | c_comment[spi::_val = spi::_1]
            | shell_comment[spi::_val = spi::_1]
            ;

或

qi::rule<iter, std::string()> comment = cpp_comment[spi::_val += spi::_1]
            | c_comment[spi::_val += spi::_1]
            | shell_comment[spi::_val += spi::_1]
            ;

它工作，但我真的很想知道为什么原来的代码不能工作。

这是回溯容器属性的经典问题:

  • boost::spirit::qi重复解析输出
    • 使用Boost::Spirit (V2.4)解析到容器

这里的想法是使用qi::hold，或者在这种情况下使用qi::raw更好，因为它看起来像是要将整个匹配的输入序列作为属性公开:

qi::rule<iter, std::string()>
      cpp_comment   = "//" >> *~char_('n')   >> (eol|eoi),
      c_comment     = "/*" >> *(char_ - "*/") >> "*/",
      shell_comment = '#'  >> *~char_('n')   >> (eol|eoi),
      comment       = qi::raw [ cpp_comment | c_comment | shell_comment ];

Live On Coliru

#include <boost/spirit/include/qi.hpp>
#include <cassert>
namespace qi = boost::spirit::qi;
void test() {
    using qi::char_;
    using qi::eol;
    using qi::eoi;
    std::string const example = "/* this should be ignored */";
    qi::rule<std::string::const_iterator, std::string()>
          cpp_comment   = "//" >> *~char_('n')   >> (eol|eoi),
          c_comment     = "/*" >> *(char_ - "*/") >> "*/",
          shell_comment = '#'  >> *~char_('n')   >> (eol|eoi),
          comment       = qi::raw [ cpp_comment | c_comment | shell_comment ];
    std::string result;
    bool ok = qi::parse(std::begin(example), std::end(example), comment >> eoi, result);
    assert(ok);
    std::cout << "expected: " << example << "n";
    std::cout << "actual:   " << result << "n";
    assert(result == example);
}
int main() {
    test();
}

打印

expected: /* this should be ignored */
actual:   /* this should be ignored */

相关文章：