c++ - 使用 Boost Spirit 来解析一个文本文件，同时跳过它的大部分内容

Question

我有以下 std::string :

<lots of text not including "label A" or "label B">    
label A: 34
<lots of text not including "label A" or "label B">
label B: 45
<lots of text not including "label A" or "label B">
...

我想提取所有出现的 label A 之后的单个整数或 label B并将它们放在相应的 vector<int> a, b 中.一种简单但不优雅的方法是使用 find("label A")和 find("label B")并解析以先到者为准。有没有一种用Spirit来表达的简洁的方式？你如何跳过除label A以外的所有内容？或 label B ？

Answer 1

你可以

omit [ eol >> *char_ - ("\nlabel A:") ] >> eol

示例: Live On Coliru

还有 seek[]存储库中的指令。下面等价于上面的:

 repo::seek [ eol >> &lit("int main") ] 

这是一个解析原始样本的样本:

*repo::seek [ eol >> "label" >> char_("A-Z") >> ':' >> int_ ],

这将解析为 std::vector<std::pair<char, int> >没有别的。

On Coliru Too :

#if 0
<lots of text not including "label A" or "label B">    
label A: 34
<lots of text not including "label A" or "label B">
label B: 45
<lots of text not including "label A" or "label B">
...
#endif
#include <boost/fusion/adapted/std_pair.hpp>
#include <boost/spirit/include/qi.hpp>
#include <boost/spirit/include/phoenix.hpp>
#include <boost/spirit/repository/include/qi_seek.hpp>
#include <fstream>

namespace qi   = boost::spirit::qi;
namespace repo = boost::spirit::repository::qi;

int main()
{
    std::ifstream ifs("main.cpp");
    ifs >> std::noskipws;

    boost::spirit::istream_iterator f(ifs), l;

    std::vector<std::pair<char, int> > parsed;
    using namespace qi;
    bool ok = phrase_parse(
            f, l, 
            *repo::seek [ eol >> "label" >> char_("A-Z") >> ':' >> int_ ],
            blank,
            parsed
        );

    if (ok)
    {
        std::cout << "Found:\n";
        for (auto& p : parsed)
            std::cout << "'" << p.first << "' has value " << p.second << "\n";
    }
    else
        std::cout << "Fail at: '" << std::string(f,l) << "'\n";
}

注意事项:

• seek确实公开了匹配的属性，这非常强大:

  repo::seek [ eol >> "label" >> char_("ABCD") >> ':' ] 
  

  将“吃掉”标签，但将标签字母('A'、'B'、'C' 或 'D')公开为属性。

• 跳过时的性能可能非常令人惊讶，请阅读文档中的警告 http://www.boost.org/doc/libs/1_55_0/libs/spirit/repository/doc/html/spirit_repository/qi_components/directives/seek.html

输出是

Found:
'A' has value 34
'B' has value 45