我想我会在这里尝试一下。
需要的是围绕迭代器和属性类型进行一些类型擦除。这已经非常接近一个界面了qi::rule
古时候。
为了完整起见,我们实际上还可以删除或转换上下文(例如,在惰性规则内传播船长),但为了简单起见,我在这里选择了。
在许多情况下,要延迟调用的解析器无论如何都可能是词素(如我将使用的示例中所示)
在我们的用例中,让我们解析这些输入:
integer_value: 42
quoted_string: "hello world"
bool_value: true
double_value: 3.1415926
我们将使用变体属性类型,并从创建lazy_rule
解析器将允许我们删除类型:
using Value = boost::variant<int, bool, double, std::string>;
using It = std::string::const_iterator;
using Rule = x3::any_parser<It, Value>;
传递懒惰的话题
现在,我们从哪里“得到”这个懒惰的主题呢?
在灵气中,我们有纳比亚莱克把戏 https://stackoverflow.com/search?q=user%3A85371+nabialek。这将使用qi::locals<>
or 继承属性 https://www.boost.org/doc/libs/1_72_0/libs/spirit/doc/html/spirit/abstracts/attributes/nonterminal_attributes.html,这基本上都归结为使用 Phoenix 懒惰的演员(qi::_r1
or qi::_a
等)评估一个值从解析器上下文在运行时。
在 X3 中,没有 Phoenix,我们必须自己使用语义动作来操纵上下文。
其基本构建块是x3::with<T>[]指示 https://stackoverflow.com/search?q=user%3A85371+x3+with+context1.这是我们最终将用作解析器的内容:
x3::symbols<Rule> options;
现在我们可以将任何解析表达式添加到选项中,例如:options.add("anything", x3::eps);
.
auto const parser = x3::with<Rule>(Rule{}) [
set_context<Rule>[options] >> ':' >> lazy<Rule>
];
这增加了一个Rule
上下文的值,可以设置(set_context
)和“执行”(lazy
).
就像我说的,我们必须手动操作上下文,所以让我们定义一些执行此操作的助手:
template <typename Tag>
struct set_context_type {
template <typename P>
auto operator[](P p) const {
auto action = [](auto& ctx) {
x3::get<Tag>(ctx) = x3::_attr(ctx);
};
return x3::omit [ p [ action ] ];
}
};
template <typename Tag>
struct lazy_type : x3::parser<lazy_type<Tag>> {
using attribute_type = typename Tag::attribute_type; // TODO FIXME?
template<typename It, typename Ctx, typename RCtx, typename Attr>
bool parse(It& first, It last, Ctx& ctx, RCtx& rctx, Attr& attr) const {
auto& subject = x3::get<Tag>(ctx);
It saved = first;
x3::skip_over(first, last, ctx);
if (x3::as_parser(subject).parse(first, last,
std::forward<Ctx>(ctx),
std::forward<RCtx>(rctx), attr)) {
return true;
} else {
first = saved;
return false;
}
}
};
template <typename T> static const set_context_type<T> set_context{};
template <typename T> static const lazy_type<T> lazy{};
这就是全部内容了。
演示时间
在此演示中,我们运行上述输入(在函数中run_tests()
)并且它将使用如下所示的解析器:
auto run_tests = [=] {
for (std::string const& input : {
"integer_value: 42",
"quoted_string: \"hello world\"",
"bool_value: true",
"double_value: 3.1415926",
})
{
Value attr;
std::cout << std::setw(36) << std::quoted(input);
if (phrase_parse(begin(input), end(input), parser, x3::space, attr)) {
std::cout << " -> success (" << attr << ")\n";
} else {
std::cout << " -> failed\n";
}
}
};
首先我们将运行:
options.add("integer_value", x3::int_);
options.add("quoted_string", as<std::string> [
// lexeme is actually redundant because we don't use surrounding skipper yet
x3::lexeme [ '"' >> *('\\' >> x3::char_ | ~x3::char_('"')) >> '"' ]
]);
run_tests();
将打印:
"integer_value: 42" -> success (42)
"quoted_string: \"hello world\"" -> success (hello world)
"bool_value: true" -> failed
"double_value: 3.1415926" -> failed
现在,我们可以通过扩展来演示该解析器的动态本质options
:
options.add("double_value", x3::double_);
options.add("bool_value", x3::bool_);
run_tests();
输出变为:
"integer_value: 42" -> success (42)
"quoted_string: \"hello world\"" -> success (hello world)
"bool_value: true" -> success (true)
"double_value: 3.1415926" -> success (3.14159)
注意,我添加了另一个助手as<>
这使得更容易将属性类型强制为std::string
那里。这是一个演变早期答案中的想法 https://stackoverflow.com/questions/45457868/statefulness-of-spirit-v2-and-x3/45469919#45469919
Coliru 上的完整列表
See it Live On Coliru http://coliru.stacked-crooked.com/a/2fd7c70fef082e6b
#include <boost/spirit/home/x3.hpp>
#include <iostream>
#include <iomanip>
namespace x3 = boost::spirit::x3;
namespace {
template <typename T>
struct as_type {
template <typename...> struct Tag{};
template <typename P>
auto operator[](P p) const {
return x3::rule<Tag<T, P>, T> {"as"} = x3::as_parser(p);
}
};
template <typename Tag>
struct set_lazy_type {
template <typename P>
auto operator[](P p) const {
auto action = [](auto& ctx) {
x3::get<Tag>(ctx) = x3::_attr(ctx);
};
return x3::omit [ p [ action ] ];
}
};
template <typename Tag>
struct do_lazy_type : x3::parser<do_lazy_type<Tag>> {
using attribute_type = typename Tag::attribute_type; // TODO FIXME?
template <typename It, typename Ctx, typename RCtx, typename Attr>
bool parse(It& first, It last, Ctx& ctx, RCtx& rctx, Attr& attr) const {
auto& subject = x3::get<Tag>(ctx);
It saved = first;
x3::skip_over(first, last, ctx);
if (x3::as_parser(subject).parse(first, last,
std::forward<Ctx>(ctx),
std::forward<RCtx>(rctx), attr)) {
return true;
} else {
first = saved;
return false;
}
}
};
template <typename T> static const as_type<T> as{};
template <typename T> static const set_lazy_type<T> set_lazy{};
template <typename T> static const do_lazy_type<T> do_lazy{};
}
int main() {
std::cout << std::boolalpha << std::left;
using Value = boost::variant<int, bool, double, std::string>;
using It = std::string::const_iterator;
using Rule = x3::any_parser<It, Value>;
x3::symbols<Rule> options;
auto const parser = x3::with<Rule>(Rule{}) [
set_lazy<Rule>[options] >> ':' >> do_lazy<Rule>
];
auto run_tests = [=] {
for (std::string const input : {
"integer_value: 42",
"quoted_string: \"hello world\"",
"bool_value: true",
"double_value: 3.1415926",
})
{
Value attr;
std::cout << std::setw(36) << std::quoted(input);
if (phrase_parse(begin(input), end(input), parser, x3::space, attr)) {
std::cout << " -> success (" << attr << ")\n";
} else {
std::cout << " -> failed\n";
}
}
};
std::cout << "Supporting only integer_value and quoted_string:\n";
options.add("integer_value", x3::int_);
options.add("quoted_string", as<std::string> [
// lexeme is actually redundant because we don't use surrounding skipper yet
x3::lexeme [ '"' >> *('\\' >> x3::char_ | ~x3::char_('"')) >> '"' ]
]);
run_tests();
std::cout << "\nAdded support for double_value and bool_value:\n";
options.add("double_value", x3::double_);
options.add("bool_value", x3::bool_);
run_tests();
}
打印完整输出:
Supporting only integer_value and quoted_string:
"integer_value: 42" -> success (42)
"quoted_string: \"hello world\"" -> success (hello world)
"bool_value: true" -> failed
"double_value: 3.1415926" -> failed
Added support for double_value and bool_value:
"integer_value: 42" -> success (42)
"quoted_string: \"hello world\"" -> success (hello world)
"bool_value: true" -> success (true)
"double_value: 3.1415926" -> success (3.14159)
1 遗憾的是,实际操作中缺少文档