c++ - 我如何用 C++ 解析 json 数组？

Question

std::stringstream ss;
ss << "{ \"values\": \"A\": 1, \"B\": 10 }";

我想把这个流变成下面这种格式。

{
  "values": [
    { "A": 1, "B": 10 }
    ...
  ]
}

有人知道如何使用 C++ 和 boost ptree 解析数组的值吗？

Answer 1

假设输入 input.json

使用 Boost Spirit V2.x

这里有一个简单的 boost 灵气的方法:

Live On Coliru

#include <boost/spirit/include/qi.hpp>
#include <boost/spirit/include/qi_match.hpp>
#include <boost/fusion/adapted/std_pair.hpp>
#include <fstream>
#include <map>

namespace {
    using Element = std::map<std::string, int>;
    struct Array : std::vector<Element> { };

    std::istream& operator>>(std::istream& is, Array& into) {
        using namespace boost::spirit::qi;
        using it = boost::spirit::istream_iterator;

        rule<it, std::string()> s;
        rule<it, Element(), space_type> r, e;

        s = '"' >> ~char_('"') >> '"';
        r = (s >> ':' >> int_) % ',';
        e = '{' >> r >> '}';

        return is >> phrase_match('{'
                    >> lit("\"values\"") >> ':' >> '[' >> (e % ',') >> ']'
                >> '}', space, into);
    }
}

int main() {
    std::ifstream ifs("input.json");
    ifs.unsetf(std::ios::skipws);

    Array array;
    if (ifs >> array) {
        std::cout << "Parsed " << array.size() << " elements:\n";

        for (auto& e : array) {
            std::cout << "\n--------------------\n{ ";
            for (auto& kv : e)
                std::cout << "\"" << kv.first << "\": " << kv.second << ", ";
            std::cout << " }\n";
        }
    } else {
        std::cout << "Parsing failed\n";
    }
}

打印

std::istream& {anonymous}::operator>>(std::istream&, {anonymous}::Array&)
Parsed 13 elements:
--------------------
{ "A": 1, "B": 10,  }
--------------------
{ "C": 3, "D": 12,  }
--------------------
{ "E": 5, "F": 14,  }
--------------------
{ "G": 7, "H": 16,  }
--------------------
{ "I": 9, "J": 18,  }
--------------------
{ "K": 11, "L": 20,  }
--------------------
{ "M": 13, "N": 22,  }
--------------------
{ "O": 15, "P": 24,  }
--------------------
{ "Q": 17, "R": 26,  }
--------------------
{ "S": 19, "T": 28,  }
--------------------
{ "U": 21, "V": 30,  }
--------------------
{ "W": 23, "X": 32,  }
--------------------
{ "Y": 25, "Z": 34,  }

使用Spirit X3

同样的交易:

Live On Coliru

namespace {
    using Element = std::map<std::string, int>;
    struct Array : std::vector<Element> { };

    namespace parser {
        using namespace boost::spirit::x3;
        rule<struct rule_key_t, std::string> s;
        rule<struct rule_element_t, Element> r;
        rule<struct rule_braced_t, Element>  e;

        auto s_def = '"' >> ~char_('"') >> '"';
        auto r_def = (s >> ':' >> int_) % ',';
        auto e_def = '{' >> r >> '}';

        BOOST_SPIRIT_DEFINE(s,r,e)
    }

    std::istream& operator>>(std::istream& is, Array& into) {
        using namespace parser;

        boost::spirit::istream_iterator f(is), l;

        if (!phrase_parse(f, l, '{'
                    >> lit("\"values\"") >> ':' >> '[' >> (e % ',') >> ']'
                >> '}', space, into))
        {
            is.setstate(is.rdstate() | std::ios::failbit);
        }

        return is;
    }
}

相同的 main() 输出相同

使用属性树

这有点不同，我选择不实现 operator>> 因为 Boost Property 并不能真正负担得起。

Live On Coliru

#include <boost/property_tree/json_parser.hpp>
#include <fstream>
#include <iostream>
#include <map>

namespace {
    using Element = std::map<std::string, int>;
    struct Array : std::vector<Element> { };

    Array read(std::string fname) {
        std::ifstream ifs(fname);
        Array into;

        using namespace boost::property_tree;
        ptree pt;
        read_json(ifs, pt);

        for (auto& entry : pt.get_child("values")) {
            Element e;
            for (auto& property : entry.second)
                e[property.first] = property.second.get_value(0);
            into.push_back(std::move(e));
        }

        return into;
    }
}

int main() {
    try {
        auto array = read("input.json");
        std::cout << "Parsed " << array.size() << " elements:\n";

        for (auto& e : array) {
            std::cout << "--------------------\n{ ";
            for (auto& kv : e)
                std::cout << "\"" << kv.first << "\": " << kv.second << ", ";
            std::cout << " }\n";
        }
    } catch (...) {
        std::cout << "Parsing failed\n";
    }
}

当然，输出还是和之前一样。