我正在尝试使用包含单个成员的 fusion 适应结构类型来解析 std::Variant
。经过几个小时的尝试找出问题后,我能够使用以下代码重现该问题:
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"});
这会导致以下构建错误:
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;
令我困惑的是,如果我向 TestStruct
添加第二个成员,以下代码实际上会正确编译并运行:
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"});
我还读到,当尝试使用旧版本的 Spirit 解析这些结构时, fusion 适应的单字段结构存在已知问题。
解决(或解决)Spirit X3 这个问题的最佳方法是什么?
最佳答案
我不想这么说,但这是一个众所周知的限制,而且它不断出现。我有点放弃尝试修复它。这是一种边缘情况,通常很容易解决或避免。
参见例如
- boost::spirit::x3 attribute compatibility rules, intuition or code?
- How to move results parsed results into a struct using spirit x3
- https://github.com/boostorg/spirit/pull/178
在你的原因中,我可能会避免解析为TestStruct
,而是解析为float
。否则,解析它并使用语义操作而不是自动属性传播来传播。
我尝试过,发现在这种特定情况下很难克服这个障碍(这似乎是“逆”问题,其中暴露 TestResult 的已经正确匹配的规则仍然会带来问题。显然,该属性合成规则又错了)。
暴力破解
auto assign = [](auto& ctx) { _val(ctx) = _attr(ctx); };
手动进行属性传播:
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];
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));
};
它似乎不起作用: Live On Coliru
¹ 但不完全是,在此之前还有很多元编程条件
关于c++ - 使用 Boost Spirit X3 和 Fusion 解析具有单个成员的结构变体,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/62628317/