免责声明：下面的示例不是完整的 JSON 解析器。考虑使用支持您需求的库。你可以在这里看到一个更进化的 JSON 解析器https://github.com/sehe/spirit-v2-json
一种简单粗暴的 Spirit 语法（假设您不需要太多的一致性）是：
text_ = '"' >> raw [*('\\' >> char_ | ~char_('"'))] >> '"'; // ¹
value_ = null | bool | text_ | double_ | object_ | array_; // ²
member_ = text_ >> ':' >> value_;
object_ = '{' >> -(member_ % ',') >> '}';
array_ = '[' >> -(value_ % ',') >> ']';
// ¹ as a bonus I added utf8 escape decoding in the full sample
// ² as another bonus I threw in the missing `null` and `bool` types
使用 AST 无需进一步努力即可转换为 C++ 类型，例如：
using text = std::string;
using value = boost::make_recursive_variant<
null,
bool,
text, // "string" (roughly!)
double, // number
std::map<text, boost::recursive_variant_>, // object
std::vector<boost::recursive_variant_> // array
>::type;
using member = std::pair<text, value>;
using object = std::map<text, value>;
using array = std::vector<value>;
如果您有两个 qd_json::value 对象，您可以比较它们：
qd_json::value local_tree, remote_tree;
if (local_tree == remote_tree)
{
std::cout << "the tree is unchanged\n";
}
这是一个演示程序：
更新演示
更新了演示，向您展示了如何获得您在问题编辑中建议的“用户友好”数据结构：
int main() {
auto json = qd_json::parse(sample);
// extract into user friendly datastructure from the question:
auto extracted = Data::extract_from(json);
for (auto& e : extracted.folders) std::cout << "folder:\t" << e.id << "\t" << e.path << "\n";
for (auto& e : extracted.files) std::cout << "file:\t" << e.id << "\t" << e.path << "\t" << e.name << "\n";
}
Live On Coliru
#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;
static std::string const sample = R"(
{
"folders" :
[{
"id" : 109,
"parent_id" : 110,
"path" : "\/1\/105\/110\/"
},
{
"id" : 110,
"parent_id" : 105,
"path" : "\/1\/105\/"
}
],
"files" :
[{
"id" : 26,
"parent_id" : 105,
"name" : "picture.png",
"hash" : "md5_hash",
"path" : "\/1\/105\/"
},
{
"id" : 25,
"parent_id" : 110,
"name" : "another_picture.jpg",
"hash" : "md5_hash",
"path" : "\/1\/105\/110\/"
}
]
})";
namespace qd_json { // quick and dirty JSON handling
struct null {
bool operator==(null) const { return true; }
};
inline static std::ostream& operator<<(std::ostream& os, null) { return os << "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) + "'");
}
}
#include <boost/range/algorithm.hpp>
#include <boost/range/adaptors.hpp>
#include <boost/range/algorithm_ext/push_back.hpp>
struct Data {
struct Folder { int id, parent_id; std::string path; };
struct File { int id, parent_id; std::string path, name, md5_hash; };
using Folders = std::vector<Folder>;
using Files = std::vector<File>;
Folders folders;
Files files;
static Data extract_from(qd_json::value const& json) {
using namespace boost::adaptors;
return {
boost::copy_range<Folders>(arr(obj(json).at("folders")) | transformed(obj) | transformed(&Data::extract_folder)),
boost::copy_range<Files> (arr(obj(json).at("files")) | transformed(obj) | transformed(&Data::extract_file)),
};
}
private:
static Folder extract_folder(qd_json::object const& obj) {
return {
id (obj.at("id")),
id (obj.at("parent_id")),
text (obj.at("path"))
};
}
static File extract_file(qd_json::object const& obj) {
return {
id (obj.at("id")),
id (obj.at("parent_id")),
text (obj.at("path")),
text (obj.at("name")),
text (obj.at("hash")),
};
}
static int id (qd_json::value const&v) { return boost::get<double>(v); };
static std::string text(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); };
};
int main()
{
auto json = qd_json::parse(sample);
// compare json documents
qd_json::value clone = json;
assert(json == clone);
// extract into user friendly datastructure from the question:
auto extracted = Data::extract_from(json);
for (auto& e : extracted.folders) std::cout << "folder:\t" << e.id << "\t" << e.path << "\n";
for (auto& e : extracted.files) std::cout << "file:\t" << e.id << "\t" << e.path << "\t" << e.name << "\n";
}
输出：
folder: 109 /1/105/110/
folder: 110 /1/105/
file: 26 /1/105/ picture.png
file: 25 /1/105/110/ another_picture.jpg