c++ - boost spirit qi on_error 通过引用传递 error_handler struct

Question

我又遇到了灵气障碍问题。

我在一个名为 error_handler 的仿函数结构中实现了错误处理。这通过引用传递给语法构造函数(参见 Qi 的 MiniC 示例)。

然后我有on_error<fail> s 在语法的构造函数中定义:

typedef boost::phoenix::function<error_handler<> > error_handler_function;
on_error<fail>(gr_instruction,
        error_handler_function(err_handler)(L"Error: Expecting ", _4, _3));
        // more on_error<fail>s...

但是，我的error_handler有私有(private)成员。好像每次都是on_error被调用，err_handler对象被复制，因此一旦仿函数离开，更改的局部变量将被销毁。

我尝试通过引用传递处理程序:

typedef boost::phoenix::function<error_handler<>& > error_handler_function; // <--- Note the ampersand!

on_error<fail>(gr_instruction,
        error_handler_function(err_handler)(L"Error: Expecting ", _4, _3));
        // more on_error<fail>s...

但是，问题依然存在:on_error()在 err_handler 的拷贝上工作, 不是单个实例!!

我也试过 boost::phoenix::ref(err_handler) 的变体只有编译错误。

当然，必须有一个简单的解决方案来通过引用传递处理程序吗？

如有任何意见，我将不胜感激。感谢您的帮助。

Answer 1

是的，phx::bind 和 phx::function<> 将默认采用它们的包装器可调用对象。然而。

比方说^[1]，您有一个这样的错误处理程序……极简示例:

template <typename=void> struct my_error_handler {
    my_error_handler() = default;
    my_error_handler(my_error_handler const&) = delete;

    template<typename...> struct result { typedef void type; };
    template<typename... T> void operator()(T&&...) const { 
        std::cerr << "my_error_handler invoked " << proof++ << "\n";
    }
    mutable int proof = 0;
};

(如您所见，我将其明确设为不可复制，以确保编译器不会在我背后悄悄生成代码。)

现在，我不确定这是否是您不小心错过的组合，但是:

on_error<fail>(function,       phx::bind(phx::ref(err_handler), L"Error: Expecting ", _4, _3));
on_error<fail>(start,          phx::bind(phx::ref(err_handler), L"Error: Expecting ", _4, _3));
on_error<fail>(gr_instruction, phx::bind(phx::ref(err_handler), L"Error: Expecting ", _4, _3));

工作得很好，同样如此

auto ll = phx::bind(phx::ref(err_handler), L"Error: Expecting ", _4, _3);
on_error<fail>(function,       ll);
on_error<fail>(start,          ll);
on_error<fail>(gr_instruction, ll);

请注意，因为 bind 需要引用，所以我们需要确保 err_handler 的生命周期匹配(或超过)解析器的生命周期，所以我制作了 err_handler 解析器类的成员。

当我将输入传递给失败时，我的程序将能够打印 my_error_handler 调用的 proof:

std::cout << "The 'proof' in the err_handler instance is: " << p.err_handler.proof << "\n";

一如既往，一个完全集成的示例程序:

#define BOOST_SPIRIT_USE_PHOENIX_V3
//#define BOOST_SPIRIT_DEBUG
#include <boost/spirit/include/qi.hpp>
#include <boost/spirit/include/phoenix.hpp>

namespace qi  = boost::spirit::qi;
namespace phx = boost::phoenix;

namespace asmast
{
    typedef std::string label;
}

template <typename=void> struct my_error_handler {
    my_error_handler() = default;
    my_error_handler(my_error_handler const&) = delete;

    template<typename...> struct result { typedef void type; };
    template<typename... T> void operator()(T&&...) const { 
        std::cerr << "my_error_handler invoked " << proof++ << "\n";
    }
    mutable int proof = 0;
};

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

        start = lexeme["Func" >> !(alnum | '_')] > function;
        function = gr_identifier
                    >> "{"
                    >> -(
                              gr_instruction
                            | gr_label
                          //| gr_vardecl
                          //| gr_paramdecl
                        ) % eol
                    > "}";

        gr_instruction_names.add("Mov", unused);
        gr_instruction_names.add("Push", unused);
        gr_instruction_names.add("Exit", unused);

        gr_instruction = lexeme [ gr_instruction_names >> !(alnum|"_") ] > gr_operands;
        gr_operands = -(gr_operand % ',');

        gr_identifier = lexeme [ alpha >> *(alnum | '_') ];
        gr_operand    = gr_identifier | gr_string;
        gr_string     = lexeme [ '"' >> *("\"\"" | ~char_("\"")) >> '"' ];

        gr_newline = +( char_('\r')
                       |char_('\n')
                      );

        gr_label = gr_identifier >> ':' > gr_newline;

#if 1
        on_error<fail>(function,       phx::bind(phx::ref(err_handler), L"Error: Expecting ", _4, _3));
        on_error<fail>(start,          phx::bind(phx::ref(err_handler), L"Error: Expecting ", _4, _3));
        on_error<fail>(gr_instruction, phx::bind(phx::ref(err_handler), L"Error: Expecting ", _4, _3));
#else
        auto ll = phx::bind(phx::ref(err_handler), L"Error: Expecting ", _4, _3);
        on_error<fail>(function,       ll);
        on_error<fail>(start,          ll);
        on_error<fail>(gr_instruction, ll);
#endif
        // more on_error<fail>s...

        BOOST_SPIRIT_DEBUG_NODES((start)(function)(gr_instruction)(gr_operands)(gr_identifier)(gr_operand)(gr_string));
    }

    my_error_handler<> err_handler;
  private:
    qi::symbols<char, qi::unused_type> gr_instruction_names;
    qi::rule<It, Skipper> start, function, gr_instruction, gr_operands, gr_operand, gr_string;
    qi::rule<It, qi::unused_type()> gr_newline;
    qi::rule<It, asmast::label(), Skipper> gr_label, gr_identifier;
};

int main()
{
    typedef boost::spirit::istream_iterator It;
    std::cin.unsetf(std::ios::skipws);
    It f(std::cin), l;

    parser<It, qi::blank_type> p;

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

        if (f!=l) std::cerr << "trailing unparsed: '" << std::string(f,l) << "'\n";

        std::cout << "The 'proof' in the err_handler instance is: " << p.err_handler.proof << "\n";
        return ok;
    } catch(const qi::expectation_failure<It>& e)
    {
        std::string frag(e.first, e.last);
        std::cerr << e.what() << "'" << frag << "'\n";
    }

    return false;
}

输入时

Func Ident{
    Mov name, "hello" 
    Push 5
    Exit
}

它打印(作为第一行/最后一行)

my_error_handler invoked 0
my_error_handler invoked 1
...
The 'proof' in the err_handler instance is: 2

---

^[1]这不是我第一次不得不想象相关代码