问题描述
假设我有一个要与Spirit Qi一起解析的结构,其定义如下:
Suppose I have a structure that I want to parse into with Spirit Qi, that is defined as such:
struct data_
{
bool export;
std::wstring name;
data_() : export(false) {}
};
此外,假设该结构已适应如下融合:
Also, suppose the struct has been adapted to fusion like this:
BOOST_FUSION_ADAPT_STRUCT(
data_,
(bool, export)
(std::wstring, name)
)
关联的规则是:
qi::rule<Iterator, data_(), skipper<Iterator> > rule_data;
rule_data = -lexeme["SpecialText" >> !(alnum | '_')] [ boost::phoenix::at_c<0> = true ] // If this string is found, , set "export" to true
> lexeme["Name" >> !(alnum | '_')] // this is supposed to go into the "name" member
到目前为止,它可以编译的很好.但是,名称"现在保持为空!
This compiles, so far, so good. However, "name" stays empty now!
从本质上讲,我在问:鉴于"SpecialText"在"Name"之前,我该如何正确地为"export"而不是字符串合成一个布尔属性?
So essentially, I am asking: Given that "SpecialText" precedes "Name", how would I synthesize a boolean attribute for "export" properly, rather than a string?
编辑拔掉头发之后,我偶然发现了"matches []"解析器,它似乎可以满足我的要求.
EDITAfter pulling my hair out on this, I randomly stumbled upon the "matches[]" parser, which seems to do what I want.
尽管如此,例如,如果我想返回某个字符串或其他数据类型而不是布尔值,该问题仍然以常规形式存在.本质上,如何通过语义动作设置struct属性的特定成员.
Nonetheless, the question still exists in the general form, for example, if I wanted to return a certain string or other data type instead of a bool.Essentially, how to set a specific member of a struct attribute via a semantic action.
推荐答案
如何设置结构成员.
给出一个结构S
struct S
{
int field1;
std::string field2;
int target_field;
bool field3;
};
您可以像这样分配一个字段(例如target_field
):
You can assign to a field (e.g. target_field
) like so:
rule<It, S()> p = int_ [ phx::bind(&S::target_field, _val) = _1 ];
现在,您可以通过执行以下操作来使bind
更具可读性:
Now, you can make the bind
more readable, by doing something like:
auto target_field_ = phx::bind(&S::target_field, _val);
p = int_ [ target_field_ = _1 ];
概念证明: 在Coliru上直播
Proof of concept: live on Coliru
#include "boost/spirit/include/qi.hpp"
#include "boost/spirit/include/phoenix.hpp"
namespace qi = boost::spirit::qi;
namespace phx = boost::phoenix;
typedef std::string::const_iterator It;
struct S
{
int field1;
std::string field2;
int target_field;
bool field3;
};
int main()
{
const std::string input("42");
It f(begin(input)), l(end(input));
S instance;
using namespace qi;
rule<It, S()> p = int_ [ phx::bind(&S::target_field, _val) = _1 ];
// or, alternatively:
auto target_field_ = phx::bind(&S::target_field, _val);
p = int_ [ target_field_ = _1 ];
if (parse(f, l, p, instance))
std::cout << "Parsed: " << instance.target_field;
}
选项2(融合序列)
您可以使用 adaptation
#include "boost/fusion/adapted/struct.hpp"
BOOST_FUSION_ADAPT_STRUCT(S, (int, field1)(std::string, field2)(int, target_field)(bool, field3))
现在,您可以在语义操作中对这些序列使用凤凰号 lazy 函数:
Now you can use phoenix lazy functions on these sequences in your semantic action:
rule<It, S()> p = int_ [ phx::at_c<2>(_val) = _1 ];
我不喜欢这种风格(因为它会把表达结构降级"为……元组),但是它可能派上用场. 在Coliru上直播
I don't prefer this style (because it 'degrades' an expressive struct to ... a tuple of sorts), but it might come in handy. Live on Coliru
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