What is the simplest way to make a semantic action that extracts a string from a typical identifier parser based on boost::spirit::x3::lexeme
?
I thought it might be possible to bypass needing to unpack the attribute and just use iterators into the input stream but apparently x3::_where
does not do what I think it does.
The following yields output
being empty. I expected it to contain "foobar_hello".
namespace x3 = boost::spirit::x3;
using x3::_where;
using x3::lexeme;
using x3::alpha;
auto ctx_to_string = [&](auto& ctx) {
_val(ctx) = std::string(_where(ctx).begin(), _where(ctx).end());
};
x3::rule<class identifier_rule_, std::string> const identifier_rule = "identifier_rule";
auto const identifier_rule_def = lexeme[(x3::alpha | '_') >> *(x3::alnum | '_')][ctx_to_string];
BOOST_SPIRIT_DEFINE(identifier_rule);
int main()
{
std::string input = "foobar_hello";
std::string output;
auto result = x3::parse(input.begin(), input.end(), identifier_rule, output);
}
Do I need to somehow extract the string from the boost::fusion objects in x3::_attr(ctx)
or am I doing something wrong?
You can simply use automatic attribute propagation, meaning you don't need the semantic action(1)
#include <iostream>
#include <iomanip>
#define BOOST_SPIRIT_X3_DEBUG
#include <boost/spirit/home/x3.hpp>
namespace x3 = boost::spirit::x3;
namespace P {
x3::rule<class identifier_rule_, std::string> const identifier_rule = "identifier_rule";
auto const identifier_rule_def = x3::lexeme[(x3::alpha | x3::char_('_')) >> *(x3::alnum | x3::char_('_'))];
BOOST_SPIRIT_DEFINE(identifier_rule)
}
int main() {
std::string const input = "foobar_hello";
std::string output;
auto result = x3::parse(input.begin(), input.end(), P::identifier_rule, output);
}
Prints
<identifier_rule>
<try>foobar_hello</try>
<success></success>
<attributes>[f, o, o, b, a, r, _, h, e, l, l, o]</attributes>
</identifier_rule>
Note I changed
'_'
tox3::char_('_')
to capture the underscores (x3::lit
does not capture what it matches)
If you insist on semantic actions,
rule<..., std::string, true>
to also force automatic attrobute propagation_where
points to what you hope: http://coliru.stacked-crooked.com/a/336c057dabc86a84use x3::raw[]
to expose a controlled source iterator range (http://coliru.stacked-crooked.com/a/80a69ae9b99a4c61)
auto ctx_to_string = [](auto& ctx) {
std::cout << "\nSA: '" << _attr(ctx) << "'" << std::endl;
_val(ctx) = std::string(_attr(ctx).begin(), _attr(ctx).end());
};
x3::rule<class identifier_rule_, std::string> const identifier_rule = "identifier_rule";
auto const identifier_rule_def = x3::raw[ lexeme[(x3::alpha | '_') >> *(x3::alnum | '_')] ] [ctx_to_string];
BOOST_SPIRIT_DEFINE(identifier_rule)
Note now the
char_('_')
doesn't make a difference anymore
consider using the built-in attribute helpers: http://coliru.stacked-crooked.com/a/3e3861330494e7c9
auto ctx_to_string = [](auto& ctx) {
using x3::traits::move_to;
move_to(_attr(ctx), _val(ctx));
};
Note how this approximates the builtin attribute propagation, though it's much less flexible than letting Spirit manage it
(1) mandatory link: Boost Spirit: "Semantic actions are evil"?