使用 Boost Spirit X3 和 Fusion 解析具有单个成员的结构变体-解网

问：

我正在尝试使用包含单个成员的融合适应结构类型来解析。经过几个小时的尝试找出问题，我能够用以下代码重现该问题：std::Variant

struct TestStruct {
    float value;
};

BOOST_FUSION_ADAPT_STRUCT(TestStruct, value)

typedef std::variant<TestStruct, std:string> TestVariant;

auto TestStructRule = x3::rule<struct test_struct, TestStruct>{} = x3::float_ >> ",";

auto TestVariantRule = x3::rule<struct test_variant, TestVariant>{} = TestStruct | "default" >> x3::attr(std::String{"default"});

这会导致以下生成错误：

boost/boost/spirit/home/x3/support/traits/move_to.hpp:67:18: error: no viable overloaded '='

            dest = std::move(fusion::front(src));

boost/boost/spirit/home/x3/support/traits/move_to.hpp:79:13: note: in instantiation of function template specialization 'boost::spirit::x3::traits::detail::move_to_plain<TestStruct, std::__1::variant<TestStruct, std::string> >' requested here

            move_to_plain(std::forward<Source>(src), dest, is_single_element_sequence);

...

/Library/Developer/CommandLineTools/usr/include/c++/v1/variant:1214:12: note: candidate function not viable: no known conversion from 'typename remove_reference<float &>::type' (aka 'float') to 'const std::__1::variant<TestStruct, std::String>' for 1st argument

  variant& operator=(const variant&) = default;
       ^
/Library/Developer/CommandLineTools/usr/include/c++/v1/variant:1215:12: note: candidate function not viable: no known conversion from 'typename remove_reference<float &>::type' (aka 'float') to 'std::__1::variant<TestStruct, std::String>' for 1st argument

  variant& operator=(variant&&) = default;

我发现令人困惑的是，如果我添加第二个成员，以下代码实际上会正确编译和运行：TestStruct

struct TestStruct {
    float value1;
    float value2; // <-- Added second member
};

BOOST_FUSION_ADAPT_STRUCT(TestStruct, value1, value2)

typedef std::variant<TestStruct, std:string> TestVariant;

auto TestStructRule = x3::rule<struct test_struct, TestStruct>{} = x3::float_ >> "," >> x3::float_ >> ",";

auto TestVariantRule = x3::rule<struct test_variant, TestVariant>{} = TestStruct | "default" >> x3::attr(std::String{"default"});

我还在 SO 上读到，当尝试使用旧版本的 Spirit 解析这些结构时，融合适应的单场结构存在已知问题。

使用 Spirit X3 解决（或绕过）此问题的最佳方法是什么？

C++ 加速精神 X3 助推融合

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auto assign = [](auto& ctx) { _val(ctx) = _attr(ctx); };

手动执行属性传播：

auto test_struct
    = x3::rule<struct test_struct, TestStruct>{}
    = x3::float_ >> ",";

auto test_variant
    = x3::rule<struct test_variant, TestVariant>{}
    = test_struct [assign] | "default" >> x3::attr("default"s)[assign];

作品： Live On Coliru

事实上，“官方”传播看起来更像是¹

auto assign = [](auto& ctx) {
    x3::traits::move_to(_attr(ctx), _val(ctx));
};

而且它似乎不起作用：Live On Coliru

¹ 但不完全是，在此之前有很多元编程条件

使用 Boost Spirit X3 和 Fusion 解析具有单个成员的结构变体

Parsing variant of struct with a single member using Boost Spirit X3 and Fusion

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使用 Boost Spirit X3 和 Fusion 解析具有单个成员的结构变体

Parsing variant of struct with a single member using Boost Spirit X3 and Fusion

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