At Stackoverflow there are several Questions & Answers which are related to use {boost, std}::string_view, e.g.:
parsing from std::string into a boost::string_view using boost::spirit::x3 with overloading x3's move_to
namespace boost { namespace spirit { namespace x3 { namespace traits {
template <typename It>
void move_to(It b, It e, boost::string_view& v)
{
v = boost::string_view(&*b, e-b);
}
} } } }
parse into vector using boost::spirit::x3 where further the information about the compatibility of the attributes is given
namespace boost { namespace spirit { namespace x3 { namespace traits {
template <>
struct is_substitute<raw_attribute_type,boost::string_view> : boost::mpl::true_
{};
} } } }
llonesmiz wrote an example at wandbox which compiles with boost 1.64, but failed with boost 1.67 now with
opt/wandbox/boost-1.67.0/gcc-7.3.0/include/boost/spirit/home/x3/support/traits/container_traits.hpp:177:15: error: 'class boost::basic_string_view<char, std::char_traits<char> >' has no member named 'insert'
c.insert(c.end(), first, last);
~~^~~~~~
the same error I faced in my project.
The problem raises also by use of std::string
even with the explicite use of Sehe's as<> "directive", see also at wandbox:
#include <iostream>
#include <string>
#include <string_view>
namespace boost { namespace spirit { namespace x3 { namespace traits {
template <typename It>
void move_to(It b, It e, std::string_view& v)
{
v = std::string_view(&*b, e-b);
}
} } } } // namespace boost
#include <boost/spirit/home/x3.hpp>
namespace boost { namespace spirit { namespace x3 { namespace traits {
template <>
struct is_substitute<raw_attribute_type, std::string_view> : boost::mpl::true_
{};
} } } } // namespace boost
namespace parser
{
namespace x3 = boost::spirit::x3;
using x3::char_;
using x3::raw;
template<typename T>
auto as = [](auto p) { return x3::rule<struct _, T>{ "as" } = x3::as_parser(p); };
const auto str = as<std::string_view>(raw[ +~char_('_')] >> '_');
const auto str_vec = *str;
}
int main()
{
std::string input = "hello_world_";
std::vector<std::string_view> strVec;
boost::spirit::x3::parse(input.data(), input.data()+input.size(), parser::str_vec, strVec);
for(auto& x : strVec) { std::cout << x << std::endl; }
}
As far I've seen, the problem starts with boost 1.65. What has been changed and how to fix it?
Finaly, I've a question about the requirement of contiguous storage mentioned by sehe: I understand the requirements of this, but does the parser can violate this? - In my opinion the parser has to fail even on backtracking, so this can't happen by spirit. By use of the error_handler, the memory storage address which refers the string_view is at last on parse level valid. I conclude it's save to use string_view as far the references are in the scope under this condition, isn't it?
The problem here seems to be with the is_container trait:
template <typename T>
using is_container = mpl::bool_<
detail::has_type_value_type<T>::value &&
detail::has_type_iterator<T>::value &&
detail::has_type_size_type<T>::value &&
detail::has_type_reference<T>::value>;
In Qi, that would have been specializable:
template <> struct is_container<std::string_view> : std::false_type {};
However in X3 it started being a template alias, which cannot be specialized.
This is a tough issue, as it seems that there is simply no customization point to get X3 to do what we need here.
I've tried to dig deeper. I have not seen a "clean" way around this. In fact, the attribute coercion trick can help, though, if you use it to "short out" the heuristic that causes the match:
In this situation we can coerce the parser's attribute to specifically be non-compatible, and things will start working.
move_toThis, too, is an area of contention. Simply adding the overload like:
template <typename It>
inline void move_to(It b, It e, std::string_view& v) {
v = std::string_view(&*b, std::distance(b,e));
}
is not enough to make it the best overload.
The base template is
template <typename Iterator, typename Dest>
inline void move_to(Iterator first, Iterator last, Dest& dest);
To actually make it stick, we need to specialize. However, specializing and function templates is not a good match. In particular, we can't partially specialize, so we'll end up hard-coding the template arguments:
template <>
inline void move_to<Iterator, std::string_view>(Iterator b, Iterator e, std::string_view& v) {
v = std::string_view(&*b, std::distance(b,e));
}
This is making me question whether
move_tois "user-serviceable" at all, much likeis_container<>above, it just seems not designed for extension.I do realize I've applied it in the past myself, but I also learn as I go.
Instead of declaring the rule's attribute std::string_view (leaving X3's type magic room to "do the right thing"), let's etch in stone the intended outcome of raw[] (and leave X3 to do the rest of the magic using move_to):
namespace parser {
namespace x3 = boost::spirit::x3;
const auto str
= x3::rule<struct _, boost::iterator_range<Iterator> >{"str"}
= x3::raw[ +~x3::char_('_')] >> '_';
const auto str_vec = *str;
}
This works. See it Live On Wandbox
Prints
hello
world
That seems brittle. E.g. it'll break if you change Iterator to char const* (or, use std::string const input = "hello_world_", but not both).
Here's a better take (I think):
namespace boost { namespace spirit { namespace x3 {
template <typename Char, typename CharT, typename Iterator>
struct default_transform_attribute<std::basic_string_view<Char, CharT>, boost::iterator_range<Iterator>> {
using type = boost::iterator_range<Iterator>;
template <typename T> static type pre(T&&) { return {}; }
static void post(std::basic_string_view<Char, CharT>& sv, boost::iterator_range<Iterator> const& r) {
sv = std::basic_string_view<Char, CharT>(std::addressof(*r.begin()), r.size());
}
};
} } }
Now, the only hoop left to jump is that the rule declaration mentions the iterator type. You can hide this too:
namespace parser {
namespace x3 = boost::spirit::x3;
template <typename It> const auto str_vec = [] {
const auto str
= x3::rule<struct _, boost::iterator_range<It> >{"str"}
= x3::raw[ +~x3::char_('_')] >> '_';
return *str;
}();
}
auto parse(std::string_view input) {
auto b = input.begin(), e = input.end();
std::vector<std::string_view> data;
parse(b, e, parser::str_vec<decltype(b)>, data);
return data;
}
int main() {
for(auto& x : parse("hello_world_"))
std::cout << x << "\n";
}
This at once demonstrates that it works with non-pointer iterators.
Note: for completeness you'd want to statically assert the iterator models the ContiguousIterator concept (c++17)
#include <iostream>
#include <string>
#include <string_view>
#include <boost/spirit/home/x3.hpp>
namespace boost { namespace spirit { namespace x3 {
template <typename Char, typename CharT, typename Iterator>
struct default_transform_attribute<std::basic_string_view<Char, CharT>, boost::iterator_range<Iterator>> {
using type = boost::iterator_range<Iterator>;
template <typename T> static type pre(T&&) { return {}; }
static void post(std::basic_string_view<Char, CharT>& sv, boost::iterator_range<Iterator> const& r) {
sv = std::basic_string_view<Char, CharT>(std::addressof(*r.begin()), r.size());
}
};
} } }
namespace parser {
namespace x3 = boost::spirit::x3;
template <typename It> const auto str_vec = [] {
const auto str
= x3::rule<struct _, boost::iterator_range<It> >{"str"}
= x3::raw[ +~x3::char_('_')] >> '_';
return *str;
}();
}
auto parse(std::string_view input) {
auto b = input.begin(), e = input.end();
std::vector<std::string_view> data;
parse(b, e, parser::str_vec<decltype(b)>, data);
return data;
}
int main() {
for(auto& x : parse("hello_world_"))
std::cout << x << "\n";
}