How do I select a value from json - c++

Question

Here json
{"ts":1827908701,"updates":[[4,30623409,17,81282347579,1425823449632," ... ","tfs"],[80,1,0],[7,81282347579,30652308]]}
How can I get the value 4 of updates using the library boost?
I know how to take such a value of "ts". But I do not understand how to take the value of the two brackets

Answer 1

Technically, this answer shows how to use Boost to extract that value:
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#include <boost/fusion/adapted/std_pair.hpp>
#include <boost/spirit/include/qi.hpp>
#include <boost/spirit/include/phoenix.hpp>
#include <map>
namespace qi = boost::spirit::qi;
std::string const sample = R"({"ts":1827908701,"updates":[[4,30623409,17,81282347579,1425823449632," ... ","tfs"],[80,1,0],[7,81282347579,30652308]]})";
namespace qd_json { // quick and dirty JSON handling
struct null {};
using text = std::string;
using value = boost::make_recursive_variant<
null,
text, // "string" (roughly!)
double, // number
std::map<text, boost::recursive_variant_>, // object
std::vector<boost::recursive_variant_>, // array
bool
>::type;
using member = std::pair<text, value>;
using object = std::map<text, value>;
using array = std::vector<value>;
template <typename It, typename Skipper = qi::space_type>
struct grammar : qi::grammar<It, value(), Skipper>
{
grammar() : grammar::base_type(value_) {
using namespace qi;
text_ = '"' >> raw [*('\\' >> char_ | ~char_('"'))] >> '"';
null_ = "null" >> attr(null{});
bool_ = "true" >> attr(true) | "false" >> attr(false);
value_ = null_ | bool_ | text_ | double_ | object_ | array_;
member_ = text_ >> ':' >> value_;
object_ = '{' >> -(member_ % ',') >> '}';
array_ = '[' >> -(value_ % ',') >> ']';
////////////////////////////////////////
// Bonus: properly decoding the string:
text_ = lexeme [ '"' >> *ch_ >> '"' ];
ch_ = +(
~char_("\"\\")) [ _val += _1 ] |
qi::lit("\x5C") >> ( // \ (reverse solidus)
qi::lit("\x22") [ _val += '"' ] | // " quotation mark U+0022
qi::lit("\x5C") [ _val += '\\' ] | // \ reverse solidus U+005C
qi::lit("\x2F") [ _val += '/' ] | // / solidus U+002F
qi::lit("\x62") [ _val += '\b' ] | // b backspace U+0008
qi::lit("\x66") [ _val += '\f' ] | // f form feed U+000C
qi::lit("\x6E") [ _val += '\n' ] | // n line feed U+000A
qi::lit("\x72") [ _val += '\r' ] | // r carriage return U+000D
qi::lit("\x74") [ _val += '\t' ] | // t tab U+0009
qi::lit("\x75") // uXXXX U+XXXX
>> _4HEXDIG [ append_utf8(qi::_val, qi::_1) ]
);
BOOST_SPIRIT_DEBUG_NODES((text_)(value_)(member_)(object_)(array_)(null_)(bool_))
}
private:
qi::rule<It, text()> text_, ch_;
qi::rule<It, null()> null_;
qi::rule<It, bool()> bool_;
qi::rule<It, value(), Skipper> value_;
qi::rule<It, member(), Skipper> member_;
qi::rule<It, object(), Skipper> object_;
qi::rule<It, array(), Skipper> array_;
struct append_utf8_f {
template <typename...> struct result { typedef void type; };
template <typename String, typename Codepoint>
void operator()(String& to, Codepoint codepoint) const {
auto out = std::back_inserter(to);
boost::utf8_output_iterator<decltype(out)> convert(out);
*convert++ = codepoint;
}
};
boost::phoenix::function<append_utf8_f> append_utf8;
qi::uint_parser<uint32_t, 16, 4, 4> _4HEXDIG;
};
template <typename Range, typename It = typename boost::range_iterator<Range const>::type>
value parse(Range const& input) {
grammar<It> g;
It first(boost::begin(input)), last(boost::end(input));
value parsed;
bool ok = qi::phrase_parse(first, last, g, qi::space, parsed);
if (ok && (first == last))
return parsed;
throw std::runtime_error("Remaining unparsed: '" + std::string(first, last) + "'");
}
namespace accessors {
static double dbl_(qd_json::value const&v) { return boost::get<double>(v); }
static int int_(qd_json::value const&v) { return boost::get<double>(v); }
static std::string txt_(qd_json::value const&v) { return boost::get<qd_json::text>(v); }
static qd_json::array arr_(qd_json::value const&v) { return boost::get<qd_json::array>(v); }
static qd_json::object obj_(qd_json::value const&v) { return boost::get<qd_json::object>(v); }
}
}
using It = std::string::const_iterator;
int main()
{
using namespace qd_json::accessors;
auto root = obj_(qd_json::parse(sample));
for(auto& updates : arr_(root["updates"]))
for(auto& first : arr_(updates))
{
std::cout << int_(first) << "\n";
return 0;
}
}
Prints:
4
The parser was of course courtesy older answers (Getting values from a json file using boost/property_tree, with multiple elements/arrays/sub-arrays C++) and if you need a more specific answer, I suggest you show the code that you had already got.