use boost spirit parse int pair to vector - c++

Question

The string content is like:
20 10 5 3...
it is a list of pair of int. How to use spirit parse it to std::vector<std::pair<int, int>>?
std::string line;
std::vector<std::pair<int, int>> v;
boost::spirit::qi::phrase_parse(
line.cbegin(),
line.cend(),
(
???
),
boost::spirit::qi::space
);

Answer 1

You could do a simple parser expression like *(int_ >> int_) (see the tutorial and these documentation pages).
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#include <boost/spirit/include/qi.hpp>
#include <boost/fusion/include/std_pair.hpp>
namespace qi = boost::spirit::qi;
int main() {
std::string line = "20 10 5 3";
std::vector<std::pair<int, int>> v;
qi::phrase_parse(line.cbegin(), line.cend(), *(qi::int_ >> qi::int_), qi::space, v);
for (auto& p : v) {
std::cout << "(" << p.first << ", " << p.second << ")\n";
}
}
Prints
(20, 10)
(5, 3)
Pro Tip 1: Validity
If you want to make sure there's no unwanted/unexpected input, check for remaining data:
check the iterators after parsing
auto f = line.cbegin(), l = line.cend();
qi::phrase_parse(f, l, *(qi::int_ >> qi::int_), qi::space, v);
if (f!=l)
std::cout << "Unparsed input '" << std::string(f,l) << "'\n";
or simple require qi::eoi as part of the parser expression and check the return value:
bool ok = qi::phrase_parse(line.cbegin(), line.cend(), *(qi::int_ >> qi::int_) >> qi::eoi, qi::space, v);
Pro Tip 2: "Look ma, no hands"
Since the grammar is trivially the simplest thing that could parse into this datastructure, you can let Spirit do all the guesswork:
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qi::phrase_parse(line.begin(), line.end(), qi::auto_, qi::space, v);
That's, a grammar consisting of nothing but a single qi::auto_. Output is still:
(20, 10)
(5, 3)