【三方库】jsoncpp

目录

Json基本结构

JsonCpp库

中间转储类型Json::Value

jsoncpp库部分源码

序列化示例

反序列化示例

---

Json是一种轻量级的数据交互格式，易于阅读和编写，也易于机器解析和生成。

Json基本结构

键：必须是字符串。

值：可以是字符串、数字、对象、数组、布尔值、null。

键值对："key" : value 。

对象：由键值对组成，使用{ }括住，多个键值对之间用逗号,分隔开。

数组：由值的有序集合组成，使用[ ]括住，多个值之间用逗号,隔开。

json数据格式示例：

{
        "address" : 
        {
                "city" : "xi'an",
                "country" : "china"
        },
        "age" : 22,
        "is_student" : true,
        "name" : "skk",
        "sex" : "male",
        "skills" : [ "c++", "linux", "mysql" ]
}

JsonCpp库

JsonCpp是一个高性能的Json库，用于处理数据，它提供完整的Json标准支持，包括解析Json字符串、生成Json字符串，读写Json对象和数组等功能。

特点

易于使用：JsonCpp提供了简单直观的API，使得在C++程序中处理Json数据变得容易。

高性能：快速生成和解析大量解析Json数据。

跨平台：支持多种操作系统，包括：Windows、Linux和macOS。

无依赖：JsonCpp不依赖其他外库，可以轻松集成到C++项目中。

完全支持C++11：支持智能指针。

安装指令(ubuntu下)

sudo apt install libjsoncpp-dev

头文件包含：

#include <jsoncpp/json/json.h>

注意：使用g++编译时要加上-ljsoncpp选项（l使小写L）。

中间转储类型Json::Value

Json::Value类型对象，不管是将数据进行序列化和反序化都要经过Json::Value类型对象。

Json序列化：将对象数据存储到Value对象中，然后数据就被转换为Json格式的数据。

Json反序列化：将Json格式数据存储到Value对象中，然后从Value对象中得到对象数据。

jsoncpp库部分源码

//Json::Value部分源码

namespace Json {
class JSON_API Value {
  Int asInt() const;
  UInt asUInt() const;
  Int64 asInt64() const;
  UInt64 asUInt64() const;
  float asFloat() const;
  double asDouble() const;
  bool asBool() const;

  bool isNull() const;
  bool isBool() const;
  bool isInt() const;
  bool isInt64() const;
  bool isUInt() const;
  bool isUInt64() const;
  bool isIntegral() const;
  bool isDouble() const;
  bool isNumeric() const;
  bool isString() const;
  bool isArray() const;
  bool isObject() const;

  //Number of values in array or object
  
  ArrayIndex size() const;

  bool empty() const;

  bool operator!() const;

  void clear();

  void resize(ArrayIndex size);

  Value& operator[](ArrayIndex index);
  
  Value& operator[](int index);
 
  const Value& operator[](ArrayIndex index) const;

  const Value& operator[](int index) const;

  /// If the array contains at least index+1 elements, returns the element
  /// value,
  /// otherwise returns defaultValue.
  Value get(ArrayIndex index, const Value& defaultValue) const;
  /// Return true if index < size().
  bool isValidIndex(ArrayIndex index) const;
  /// Append value to array at the end.
  /// Equivalent to jsonvalue[jsonvalue.size()] = value;
  Value& append(const Value& value);
};
}

//wirter.h
namespace Json {

class Value;

/**

Usage:
\code
  using namespace Json;
  void writeToStdout(StreamWriter::Factory const& factory, Value const& value) {
    std::unique_ptr<StreamWriter> const writer(
      factory.newStreamWriter());
    writer->write(value, &std::cout);
    std::cout << std::endl;  // add lf and flush
  }
\endcode
*/
class JSON_API StreamWriter {
  virtual int write(Value const& root, JSONCPP_OSTREAM* sout) = 0;

  class JSON_API Factory {
  public:
    virtual StreamWriter* newStreamWriter() const = 0;
  };  // Factory
};  // StreamWriter

class JSON_API StreamWriterBuilder : public StreamWriter::Factory {
public:
  Json::Value settings_;

  StreamWriterBuilder();
  ~StreamWriterBuilder() JSONCPP_OVERRIDE;

  /**
   * \throw std::exception if something goes wrong (e.g. invalid settings)
   */
  StreamWriter* newStreamWriter() const JSONCPP_OVERRIDE;

}

//reaser.h

class JSON_API CharReader {
public:
  virtual ~CharReader() {}
  /** \brief Read a Value from a <a HREF="http://www.json.org">JSON</a>
   document.
   * The document must be a UTF-8 encoded string containing the document to read.
   *
   * \param beginDoc Pointer on the beginning of the UTF-8 encoded string of the
   document to read.
   * \param endDoc Pointer on the end of the UTF-8 encoded string of the
   document to read.
   *        Must be >= beginDoc.
   * \param root [out] Contains the root value of the document if it was
   *             successfully parsed.
   * \param errs [out] Formatted error messages (if not NULL)
   *        a user friendly string that lists errors in the parsed
   * document.
   * \return \c true if the document was successfully parsed, \c false if an
   error occurred.
   */
  virtual bool parse(
      char const* beginDoc, char const* endDoc,
      Value* root, JSONCPP_STRING* errs) = 0;

  class JSON_API Factory {
  public:
    virtual ~Factory() {}
    /** \brief Allocate a CharReader via operator new().
     * \throw std::exception if something goes wrong (e.g. invalid settings)
     */
    virtual CharReader* newCharReader() const = 0;
  };  // Factory
};  // CharReader

/** \brief Build a CharReader implementation.

Usage:
\code
  using namespace Json;
  CharReaderBuilder builder;
  builder["collectComments"] = false;
  Value value;
  JSONCPP_STRING errs;
  bool ok = parseFromStream(builder, std::cin, &value, &errs);
\endcode
*/

class JSON_API CharReaderBuilder : public CharReader::Factory {
public:

  Json::Value settings_;

  CharReaderBuilder();
  ~CharReaderBuilder() JSONCPP_OVERRIDE;

  CharReader* newCharReader() const JSONCPP_OVERRIDE;
};

序列化示例

#include <iostream>
#include <sstream>
#include <memory>
#include <jsoncpp/json/json.h>

int main()
{
    Json::Value val;
    val["name"] = "skk";
    val["age"] = 22;
    val["sex"] = "male";
    val["is_student"] = true;
    val["skills"].append("c++");
    val["skills"].append("linux");
    val["skills"].append("mysql");
    Json::Value obj;
    obj["country"] = "china";
    obj["city"] = "xi'an";
    val["address"] = obj;

    Json::StreamWriterBuilder builder;
    builder["indentation"] = ""; // 缩进空格数，默认是2个空格，设置为""表示不缩进
    //builder["commentStyle"] = "None";// 注释风格，默认是"All"，设置为"None"表示不注释
    std::unique_ptr<Json::StreamWriter> writer(builder.newStreamWriter());
    std::stringstream ss;
    writer->write(val, &ss);
    //std::cout << std::endl;
    //std::cout << ss.str() << std::endl;
}

反序列化示例

int main()
{
    std::string str = R"({
        "name": "skk",
        "age": 22,
        "sex": "male",
        "is_student": true,
        "skills": [
            "c++",
            "linux",
            "mysql"
        ],
        "address": {
            "country": "china",
            "city": "xi'an"
        }
    })";
   Json::Value val;
   Json::CharReaderBuilder builder;
   std::string err = "";
   std::unique_ptr<Json::CharReader> reader(builder.newCharReader());
   reader->parse(&str[0], &str[str.size()], &val, &err);
   if (!err.empty()) {
       std::cout << "parse error: " << err << std::endl;
   }
   if(!val["name"].isNull()){
        std::cout << "name: " << val["name"].asString() << std::endl;
   }
   if(!val["age"].isNull()){
        std::cout << "age: " << val["age"].asInt() << std::endl;
   }
   if(!val["sex"].isNull()){
        std::cout << "sex: " << val["sex"].asString() << std::endl;
   }
   if(!val["is_student"].isNull()){
        std::cout << "is_student: " << val["is_student"].asBool() << std::endl;
   }
   if(!val["skills"].isNull()){
        for(int i = 0; i < val["skills"].size(); i++){
            std::cout << "skill: " << val["skills"][i].asString() << std::endl;
        }
   }
   if(!val["address"].isNull()){
        std::cout << "country: " << val["address"]["country"].asString() << std::endl;
        std::cout << "city: " << val["address"]["city"].asString() << std::endl;
   }
   return 0;
  }