我已经创建了一个数据库引擎，我可以在其中创建和修改表，并将它们添加到数据库中。为了解析 SQL 查询，我使用 EBNF 形式实现了 Boost.Spirit 库。我正确设置了解析器，它成功地解析了每条规则。

我的问题是我现在不知道如何将两者结合起来。 Boost.Spirit 解析器仅验证输入是否正确，但我需要它来实际做 某事。我查找了语义操作，但它们似乎无法处理我正在寻找的内容。

例如，如果我有这样的查询:
new_table <- SELECT (id < 5) old_table;

我希望它使用规则验证输入，然后调用函数
Table Database::Select(Table t , Condition c){ ... }
并将标记作为参数传递。

如何集成解析器？

最佳答案

Note: I opted to invent a sample grammar here for demonstration purposes, since your question doesn't show yours. Using the approach recommended here, it should not be hard to code a function to execute your queries after parsing.

我真的建议构建一个解析树。

我建议属性传播优先于语义操作。参见例如

• Boost Spirit: "Semantic actions are evil"?

Spirit 中的属性传播规则非常灵活。默认公开的属性类型在每个解析器的文档中都有很好的记录

• http://www.boost.org/doc/libs/1_48_0/libs/spirit/doc/html/spirit/qi/reference.html

例如- qi::char_会导致 boost::optional<char>和 qi::double_ | qi::int_会导致 boost::variant<double, int> .

您可能希望将解析后的元素累积到您自己发明的 AST 数据类型中，例如:

struct SelectStatement
{
    std::vector<std::string> columns, fromtables; 
    std::string whereclause; // TODO model as a vector<WhereCondition> :) 

    friend std::ostream& operator<<(std::ostream& os, SelectStatement const& ss)
    {
        return os << "SELECT [" << ss.columns.size() << " columns] from [" << ss.fromtables.size() << " tables]\nWHERE " + ss.whereclause;
    }
};

您可以通过将结构调整为 Fusion 序列来使其适应 Spirits 属性传播机制:

BOOST_FUSION_ADAPT_STRUCT(SelectStatement, 
        (std::vector<std::string>, columns)
        (std::vector<std::string>, fromtables)
        (std::string, whereclause)
       )

现在您可以将以下规则解析为该类型:

sqlident = lexeme [ alpha >> *alnum ]; // table or column name

columns  = no_case [ "select" ] >> (sqlident % ',');
tables   = no_case [ "from" ]   >> (sqlident % ',');

start    = columns >> tables 
    >> no_case [ "where" ]
    >> lexeme [ +(char_ - ';') ]
    >> ';';

您可以在此处查看此实时运行代码:http://liveworkspace.org/code/0b525234dbce22cbd8becd69f84065c1

完整的演示代码:

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

namespace qi    = boost::spirit::qi;

struct SelectStatement
{
    std::vector<std::string> columns, fromtables; 
    std::string whereclause; // TODO model as a vector<WhereCondition> :) 

    friend std::ostream& operator<<(std::ostream& os, SelectStatement const& ss)
    {
        return os << "SELECT [" << ss.columns.size() << " columns] from [" << ss.fromtables.size() << " tables]\nWHERE " + ss.whereclause;
    }
};

BOOST_FUSION_ADAPT_STRUCT(SelectStatement, 
        (std::vector<std::string>, columns)
        (std::vector<std::string>, fromtables)
        (std::string, whereclause)
       )

template <typename It, typename Skipper = qi::space_type>
    struct parser : qi::grammar<It, SelectStatement(), Skipper>
{
    parser() : parser::base_type(start)
    {
        using namespace qi;

        sqlident = lexeme [ alpha >> *alnum ]; // table or column name

        columns  = no_case [ "select" ] >> (sqlident % ',');
        tables   = no_case [ "from" ]   >> (sqlident % ',');

        start    = columns >> tables 
            >> no_case [ "where" ]
            >> lexeme [ +(char_ - ';') ]
            >> ';';

        BOOST_SPIRIT_DEBUG_NODE(start);
        BOOST_SPIRIT_DEBUG_NODE(sqlident);
        BOOST_SPIRIT_DEBUG_NODE(columns);
        BOOST_SPIRIT_DEBUG_NODE(tables);
    }

  private:
    qi::rule<It, std::string()             , Skipper> sqlident;
    qi::rule<It, std::vector<std::string>(), Skipper> columns  , tables;
    qi::rule<It, SelectStatement()         , Skipper> start;
};

template <typename C, typename Skipper>
    bool doParse(const C& input, const Skipper& skipper)
{
    auto f(std::begin(input)), l(std::end(input));

    parser<decltype(f), Skipper> p;
    SelectStatement query;

    try
    {
        bool ok = qi::phrase_parse(f,l,p,skipper,query);
        if (ok)   
        {
            std::cout << "parse success\n";
            std::cout << "query: " << query << "\n";
        }
        else      std::cerr << "parse failed: '" << std::string(f,l) << "'\n";

        if (f!=l) std::cerr << "trailing unparsed: '" << std::string(f,l) << "'\n";
        return ok;
    } catch(const qi::expectation_failure<decltype(f)>& e)
    {
        std::string frag(e.first, e.last);
        std::cerr << e.what() << "'" << frag << "'\n";
    }

    return false;
}

int main()
{
    const std::string input = "select id, name, price from books, authors where books.author_id = authors.id;";
    bool ok = doParse(input, qi::space);

    return ok? 0 : 255;
}

将打印输出:

parse success
query: SELECT [3 columns] from [2 tables]
WHERE books.author_id = authors.id

关于c++ - 使用 Boost.Spirit 在 C++ 中解析 SQL 查询，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/12627963/