I would like to split a string into parts:
input = "part1/part2/part3/also3"
and fill the structure that consist of three std::string with these parts.
struct strings
{
std::string a; // <- part1
std::string b; // <- part2
std::string c; // <- part3/also3
};
However my parser seems to merge the parts together and store it into the first std::string.
Here is the code on coliru
#include <iostream>
#include <boost/spirit/include/qi.hpp>
#include <boost/fusion/include/adapted.hpp>
namespace qi = ::boost::spirit::qi;
struct strings
{
std::string a;
std::string b;
std::string c;
};
BOOST_FUSION_ADAPT_STRUCT(strings,
(std::string, a) (std::string, b) (std::string, c))
template <typename It>
struct split_string_grammar: qi::grammar<It, strings ()>
{
split_string_grammar (int parts)
: split_string_grammar::base_type (split_string)
{
assert (parts > 0);
using namespace qi;
split_string = repeat (parts-1) [part > '/'] > last_part;
part = +(~char_ ("/"));
last_part = +char_;
BOOST_SPIRIT_DEBUG_NODES ((split_string) (part) (last_part))
}
private:
qi::rule<It, strings ()> split_string;
qi::rule<It, std::string ()> part, last_part;
};
int main ()
{
std::string const input { "one/two/three/four" };
auto const last = input.end ();
auto first = input.begin ();
// split into 3 parts.
split_string_grammar<decltype (first)> split_string (3);
strings ss;
bool ok = qi::parse (first, last, split_string, ss);
std::cout << "Parsed: " << ok << "\n";
if (ok) {
std::cout << "a:" << ss.a << "\n";
std::cout << "b:" << ss.b << "\n";
std::cout << "c:" << ss.c << "\n";
}
}
The output is:
Parsed: 1
a:onetwo
b:three/four
c:
while I expected:
Parsed: 1
a:one
b:two
c:three/four
I'd like not to modify the grammar heavily and leave "repeat" statement in it, because the "real" grammar is much more complex of course and I will need to have it there. Just need to find the way to disable the concatenations. I tried
repeat (parts-1) [as_string[part] > '/']
but that does not compile.
The trouble here is specifically that qi::repeat is documented to expose a container of element-types.
Now, because the exposed attribute type of the rule (strings) is not a container-type, Spirit "knows" how to flatten the values.
Of course it's not what you wanted in this case, but usually this heuristic makes for really convenient accumulation of string values.
You could witness the reverse fix by getting rid of the non-container (sequence) target attribute:
//#define BOOST_SPIRIT_DEBUG
#include <iostream>
#include <boost/spirit/include/qi.hpp>
#include <boost/fusion/include/adapted.hpp>
namespace qi = ::boost::spirit::qi;
using strings = std::vector<std::string>;
template <typename It>
struct split_string_grammar: qi::grammar<It, strings ()>
{
split_string_grammar (int parts)
: split_string_grammar::base_type (split_string)
{
assert (parts > 0);
using namespace qi;
split_string = repeat (parts-1) [part > '/']
> last_part
;
part = +(~char_ ("/"))
;
last_part = +char_
;
BOOST_SPIRIT_DEBUG_NODES ((split_string) (part) (last_part))
}
private:
qi::rule<It, strings ()> split_string;
qi::rule<It, std::string ()> part, last_part;
};
int main ()
{
std::string const input { "one/two/three/four" };
auto const last = input.end ();
auto first = input.begin ();
// split into 3 parts.
split_string_grammar<decltype (first)> split_string (3);
strings ss;
bool ok = qi::parse (first, last, split_string, ss);
std::cout << "Parsed: " << ok << "\n";
if (ok) {
for(auto i = 0ul; i<ss.size(); ++i)
std::cout << static_cast<char>('a'+i) << ":" << ss[i] << "\n";
}
}
Of course you want to keep the struct/sequence adaptation (?); In this case that's really tricky because as soon as you use any kind of Kleene operator (*,%) or qi::repeat you'll have the attribute transformation rules as outlined above, ruining your mood.
Luckily, I just remembered I have a hacky solution based on the auto_ parser. Note the caveat in this older answer though:
Read empty values with boost::spirit
CAVEAT Specializing for
std::stringdirectly like this might not be the best idea (it might not always be appropriate and might interact badly with other parsers).
By default create_parser<std::string> is not defined, so you might decide this usage is good enough for your case:
#include <boost/fusion/adapted/struct.hpp>
#include <boost/spirit/include/qi.hpp>
namespace qi = boost::spirit::qi;
struct strings {
std::string a;
std::string b;
std::string c;
};
namespace boost { namespace spirit { namespace traits {
template <> struct create_parser<std::string> {
typedef proto::result_of::deep_copy<
BOOST_TYPEOF(
qi::lexeme [+(qi::char_ - '/')] | qi::attr("(unspecified)")
)
>::type type;
static type call() {
return proto::deep_copy(
qi::lexeme [+(qi::char_ - '/')] | qi::attr("(unspecified)")
);
}
};
}}}
BOOST_FUSION_ADAPT_STRUCT(strings, (std::string, a)(std::string, b)(std::string, c))
template <typename Iterator>
struct google_parser : qi::grammar<Iterator, strings()> {
google_parser() : google_parser::base_type(entry, "contacts") {
using namespace qi;
entry =
skip('/') [auto_]
;
}
private:
qi::rule<Iterator, strings()> entry;
};
int main() {
using It = std::string::const_iterator;
google_parser<It> p;
std::string const input = "part1/part2/part3/also3";
It f = input.begin(), l = input.end();
strings ss;
bool ok = qi::parse(f, l, p >> *qi::char_, ss, ss.c);
if (ok)
{
std::cout << "a:" << ss.a << "\n";
std::cout << "b:" << ss.b << "\n";
std::cout << "c:" << ss.c << "\n";
}
else
std::cout << "Parse failed\n";
if (f!=l)
std::cout << "Remaining unparsed: '" << std::string(f,l) << "'\n";
}
Prints
a:part1
b:part2
c:part3/also3
In reponse to the OP's own answer I wanted to challenge myself to write it more generically indeed.
The main thing is to to write set_field_ in such a way that it doesn't know/assume more than required about the destination sequence type.
With a bit of Boost Fusion magic that became:
struct set_field_
{
template <typename Seq, typename Value>
void operator() (Seq& seq, Value const& src, unsigned idx) const {
fus::fold(seq, 0u, Visit<Value> { idx, src });
}
private:
template <typename Value>
struct Visit {
unsigned target_idx;
Value const& value;
template <typename B>
unsigned operator()(unsigned i, B& dest) const {
if (target_idx == i) {
boost::spirit::traits::assign_to(value, dest);
}
return i + 1;
}
};
};
It has the added flexibility of applying Spirit's attribute compatibility rules¹. So, you can use the same grammar with both the following types:
struct strings {
std::string a, b, c;
};
struct alternative {
std::vector<char> first;
std::string second;
std::string third;
};
To drive the point home, I made the adaptation of the second struct reverse the field order:
BOOST_FUSION_ADAPT_STRUCT(strings, a, b, c)
BOOST_FUSION_ADAPT_STRUCT(alternative, third, second, first) // REVERSE ORDER :)
Without further ado, the demo program:
#define BOOST_SPIRIT_USE_PHOENIX_V3
#define BOOST_RESULT_OF_USE_DECLTYPE
#include <boost/fusion/adapted.hpp>
#include <boost/fusion/algorithm/iteration.hpp>
#include <boost/spirit/include/qi.hpp>
#include <boost/spirit/include/phoenix.hpp>
namespace qi = boost::spirit::qi;
namespace fus = boost::fusion;
struct strings {
std::string a, b, c;
};
struct alternative {
std::vector<char> first;
std::string second;
std::string third;
};
BOOST_FUSION_ADAPT_STRUCT(strings, a, b, c)
BOOST_FUSION_ADAPT_STRUCT(alternative, third, second, first) // REVERSE ORDER :)
// output helpers for demo:
namespace {
inline std::ostream& operator<<(std::ostream& os, strings const& data) {
return os
<< "a:\"" << data.a << "\" "
<< "b:\"" << data.b << "\" "
<< "c:\"" << data.c << "\" ";
}
inline std::ostream& operator<<(std::ostream& os, alternative const& data) {
os << "first: vector<char> { \""; os.write(&data.first[0], data.first.size()); os << "\" } ";
os << "second: \"" << data.second << "\" ";
os << "third: \"" << data.third << "\" ";
return os;
}
}
struct set_field_
{
template <typename Seq, typename Value>
void operator() (Seq& seq, Value const& src, unsigned idx) const {
fus::fold(seq, 0u, Visit<Value> { idx, src });
}
private:
template <typename Value>
struct Visit {
unsigned target_idx;
Value const& value;
template <typename B>
unsigned operator()(unsigned i, B& dest) const {
if (target_idx == i) {
boost::spirit::traits::assign_to(value, dest);
}
return i + 1;
}
};
};
boost::phoenix::function<set_field_> const set_field = {};
template <typename It, typename Target>
struct split_string_grammar: qi::grammar<It, Target(), qi::locals<unsigned> >
{
split_string_grammar (int parts)
: split_string_grammar::base_type (split_string)
{
assert (parts > 0);
using namespace qi;
using boost::phoenix::val;
_a_type _current; // custom placeholder
split_string =
eps [ _current = 0u ]
> repeat (parts-1)
[part [ set_field(_val, _1, _current++) ] > '/']
> last_part [ set_field(_val, _1, _current++) ];
part = +(~char_ ("/"));
last_part = +char_;
BOOST_SPIRIT_DEBUG_NODES ((split_string) (part) (last_part))
}
private:
qi::rule<It, Target(), qi::locals<unsigned> > split_string;
qi::rule<It, std::string()> part, last_part;
};
template <size_t N = 3, typename Target>
void run_test(Target target) {
using It = std::string::const_iterator;
std::string const input { "one/two/three/four" };
It first = input.begin(), last = input.end();
split_string_grammar<It, Target> split_string(N);
bool ok = qi::parse (first, last, split_string, target);
if (ok) {
std::cout << target << '\n';
} else {
std::cout << "Parse failed\n";
}
if (first != last)
std::cout << "Remaining input left unparsed: '" << std::string(first, last) << "'\n";
}
int main ()
{
run_test(strings {});
run_test(alternative {});
}
Output:
a:"one" b:"two" c:"three/four"
first: vector<char> { "three/four" } second: "two" third: "one"
¹ as with BOOST_SPIRIT_ACTIONS_ALLOW_ATTR_COMPAT