将已解析序列的每个元素传递给 returns 规则属性类型的函数

Question

我想解析 CSS 颜色函数（为简单起见，所有参数都是 0 到 255 之间的数字）

rgb(r,g,b)
rgba(r,g,b,a)
hsl(h,s,l)
hsla(h,s,l,a)

进入

struct color
{
    color(std::uint8_t r, std::uint8_t g, std::uint8_t b, std::uint8_t a) : red{r}, green{g}, blue{b}, alpha{a} {}
    static color hsl(std::uint8_t h, std::uint8_t s, std::uint8_t l, std::uint8_t a) { ... }
    std::uint8_t red;
    std::uint8_t green;
    std::uint8_t blue;
    std::uint8_t alpha;
}

我有一个有效的 hsl 函数实现，可以将 h、s 和 l 转换为 rgb 值。

我还有 rules 来处理前两个函数：

constexpr auto uint8 = uint_parser<std::uint8_t>{};
const auto color_rgb = rule<struct rgb, color>{"rgb"}
                     = lit("rgb") >> '(' >> uint8 >> ',' >> uint8 >> ',' >> uint8 >> ')' >> attr(255);
const auto color_rgba = rule<struct rgba, color>{"rgba"}
                      = lit("rgba") >> '(' >> uint8 >> ',' >> uint8 >> ',' >> uint8 >> ',' >> uint8 >> ')';

这个有用，因为我用过

BOOST_FUSION_ADAPT_STRUCT(color,
                          red, green, blue, alpha)

问题出在hsl函数上。我不能做第二个 BOOST_FUSION_ADAPT_STRUCT，所以我想到在相应的值序列上使用语义操作，其中语义操作将简单地从序列构造 color。像这样：

const auto color_hsl = rule<struct hsl, color, true>{"hsl"}
                   = (lit("hsl") >> '(' >> uint8 >> ',' >> uint8 >> ',' >> uint8 >> ')' >> attr(255))[color::hsl];

这个不行，不然我也不会问这个问题。 Boost.Spirit.X3 也没有

意义上的东西

[ qi::_val = phx::construct<color>(...), qi::_1, qi::_2, qi::_3, qi::_4) ];

看来我想手动执行 BOOST_FUSION_ADAPT_STRUCT 所做的操作，但是是在语义操作中。这可能吗？我应该如何处理？我知道序列的属性应该是一个具有四个解析值的 "fusion-vector" 类元组实体。我想提取它们并将它们填充到 color::hsl 中以生成规则的属性。

Answer 1

此处适用许多提示。

I cannot do a second BOOST_FUSION_ADAPT_STRUCT

当然可以，看BOOST_FUSION_ADAPT_STRUCT_NAMED
在齐国这个一般形式似乎适用：
```
[ qi::_val = phxfunction(qi::_0) ]
```
您可以通过创建自己的 actor 类型来进一步简化它，这样您就可以只提供 "factory action": [ factory(&Foo::makeBar) ].

如果你投入 fusion::apply¹ 的实现，你就可以避免处理 Fusion 序列 manuallt
但是，您可能想了解这个 - 隐藏得很好 - 语义操作的属性兼容模式：BOOST_SPIRIT_ACTIONS_ALLOW_ATTR_COMPAT。埋在那个更改日志中：

Semantic actions now support attribute compatibility. This is a breaking change but #define BOOST_SPIRIT_ACTIONS_ALLOW_ATTR_COMPAT must be defined in order for the new behavior to kick in. By default, the old behavior is still in place.

您可以通过更少的调整获得您想要的行为。

X3真是可塑。我们可以像上面描述的那样 factory 帮手：

auto factory = [](auto f) {
    return [f](auto& ctx) {
        x3::_val(ctx) = my_apply(f, x3::_attr(ctx));
    };
};

我将快速草稿 my_apply（针对前面描述的 boost::fusion::apply）：

namespace detail {
    template <class F, class Sequence, std::size_t... I>
        constexpr decltype(auto) apply_impl(F&& f, Sequence&& t, std::index_sequence<I...>)
        {
            return std::invoke(std::forward<F>(f), boost::fusion::at_c<I>(std::forward<Sequence>(t))...);
        }
}

template <class F, class Sequence>
    constexpr decltype(auto) my_apply(F&& f, Sequence&& t)
    {
        return detail::apply_impl(
                std::forward<F>(f), std::forward<Sequence>(t),
                std::make_index_sequence<typename boost::fusion::result_of::size<std::remove_reference_t<Sequence> >::type{}>{});
    }

现在我们可以拥有解析器了：

namespace parser {
    using namespace x3;

    constexpr auto uint8  = uint_parser<std::uint8_t>{};

    auto factory = [](auto f) {
        return [f](auto& ctx) {
            x3::_val(ctx) = my_apply(f, x3::_attr(ctx));
        };
    };

    const auto color_rgb  = rule<struct rgb, ast::color>{"rgb"}
                          = lit("rgb") >> '(' >> uint8 >> ',' >> uint8 >> ',' >> uint8 >> x3::attr(255u) >> ')';
    const auto color_rgba = rule<struct rgba, ast::color>{"rgba"}
                          = lit("rgba") >> '(' >> uint8 >> ',' >> uint8 >> ',' >> uint8 >> ',' >> uint8 >> ')';
    const auto color_hsl  = rule<struct hsl, ast::color>{"hsl"}
                          = (lit("hsl") >> '(' >> uint8 >> ',' >> uint8 >> ',' >> uint8 >> attr(255u) >> ')') [factory(ast::color::hsl)];

    const auto color = skip(space) [ color_rgba | color_rgb | color_hsl ];
}

并测试它：

int main() {
    for (std::string const input : {
            "rgb(1,2,3)",
            "rgba(4,5,6,7)",
            "hsl(8,9,10)" }) 
    {
        std::cout << " ----- Parsng " << std::quoted(input) << " --------\n";
        auto begin = input.begin(), end = input.end();

        ast::color result;
        bool success = parse(begin, end, parser::color, result);

        if (success) {
            std::cout << "parsed: ";
            std::cout << result << "\n";
        } else {
            std::cout << "failed\n";
        }

        if (begin != end) {
            std::cout << "Remaining unparsed: " << std::quoted(std::string(begin, end)) << std::endl;
        }
    }
}

打印：

Live On Coliru

 ----- Parsng "rgb(1,2,3)" --------
parsed: rgba(1,2,3,255)
 ----- Parsng "rgba(4,5,6,7)" --------
parsed: rgba(4,5,6,7)
 ----- Parsng "hsl(8,9,10)" --------
TODO: implement static ast::color ast::color::hsl(uint8_t, uint8_t, uint8_t, uint8_t)(8,9,10,255)
parsed: rgba(8,9,10,255)

完整列表

Live On Coliru

#include <iostream>
#include <iomanip>
#include <boost/spirit/home/x3.hpp>
#include <boost/fusion/adapted/struct.hpp>
#include <boost/fusion/include/size.hpp>

namespace x3 = boost::spirit::x3;


namespace ast {
    using std::uint8_t;

    struct color {
        uint8_t red, green, blue, alpha;

        color(uint8_t r=0, uint8_t g=0, uint8_t b=0, uint8_t a=255) : red{r}, green{g}, blue{b}, alpha{a} {}

        static color hsl(uint8_t h, uint8_t s, uint8_t l, uint8_t a) { 
            std::cerr << "TODO: implement " << __PRETTY_FUNCTION__ << "(" << 1*h << "," << 1*s << "," << 1*l << "," << 1*a << ")\n";
            return {h,s,l,a}; }
    };

    static inline std::ostream& operator<<(std::ostream& os, color const& c) {
        return os << "rgba(" << 1*c.red << "," << 1*c.green << "," << 1*c.blue << "," << 1*c.alpha << ")";
    }
}

BOOST_FUSION_ADAPT_STRUCT(ast::color, red, green, blue, alpha);

namespace {
    namespace detail {
        template <class F, class Sequence, std::size_t... I>
            constexpr decltype(auto) apply_impl(F&& f, Sequence&& t, std::index_sequence<I...>)
            {
                return std::invoke(std::forward<F>(f), boost::fusion::at_c<I>(std::forward<Sequence>(t))...);
            }
    }

    template <class F, class Sequence>
        constexpr decltype(auto) my_apply(F&& f, Sequence&& t)
        {
            return detail::apply_impl(
                    std::forward<F>(f), std::forward<Sequence>(t),
                    std::make_index_sequence<typename boost::fusion::result_of::size<std::remove_reference_t<Sequence> >::type{}>{});
        }
}

namespace parser {
    using namespace x3;

    constexpr auto uint8  = uint_parser<std::uint8_t>{};

    auto factory = [](auto f) {
        return [f](auto& ctx) {
            x3::_val(ctx) = my_apply(f, x3::_attr(ctx));
        };
    };

    const auto color_rgb  = rule<struct rgb, ast::color>{"rgb"}
                          = lit("rgb") >> '(' >> uint8 >> ',' >> uint8 >> ',' >> uint8 >> x3::attr(255u) >> ')';
    const auto color_rgba = rule<struct rgba, ast::color>{"rgba"}
                          = lit("rgba") >> '(' >> uint8 >> ',' >> uint8 >> ',' >> uint8 >> ',' >> uint8 >> ')';
    const auto color_hsl  = rule<struct hsl, ast::color>{"hsl"}
                          = (lit("hsl") >> '(' >> uint8 >> ',' >> uint8 >> ',' >> uint8 >> attr(255u) >> ')') [factory(ast::color::hsl)];

    const auto color = skip(space) [ color_rgba | color_rgb | color_hsl ];
} 

int main() {
    for (std::string const input : {
            "rgb(1,2,3)",
            "rgba(4,5,6,7)",
            "hsl(8,9,10)" }) 
    {
        std::cout << " ----- Parsng " << std::quoted(input) << " --------\n";
        auto begin = input.begin(), end = input.end();

        ast::color result;
        bool success = parse(begin, end, parser::color, result);

        if (success) {
            std::cout << "parsed: ";
            std::cout << result << "\n";
        } else {
            std::cout << "failed\n";
        }

        if (begin != end) {
            std::cout << "Remaining unparsed: " << std::quoted(std::string(begin, end)) << std::endl;
        }
    }
}

¹ 它似乎不存在。当然，您可以复制到 std::tuple 并使用 std::apply (also experimental)

将已解析序列的每个元素传递给 returns 规则属性类型的函数

Passing each element of a parsed sequence to a function that returns a rule's attribute type

css

c++

boost-spirit

boost-spirit-x3

c++17

完整列表