JsonCpp使用方法详解

JSON全称为JavaScript ObjectNotation，它是一种轻量级的数据交换格式，易于阅读、编写、解析。jsoncpp是c++解析JSON串常用的解析库之一。

Json::Value：可以表示所有支持的类型，如：int,double,string,object,array等。其包含节点的类型判断(isNull,isBool,isInt,isArray,isMember,isValidIndex等),类型获取(type),类型转换(asInt,asString等),节点获取(get,[]),节点比较(重载<,<=,>,>=,==,!=),节点操作(compare,swap,removeMember,removeindex,append等)等函数。

Json::Writer：与JsonReader相反，将Json::Value转换成字符串流等，Writer类是一个纯虚类，并不能直接使用。在此我们使用 Json::Writer 的子类：Json::FastWriter(将数据写入一行,没有格式),Json::StyledWriter(按json格式化输出,易于阅读)。

Json::Reader可以通过对Json源目标进行解析，得到一个解析好了的Json::Value，通常字符串或者文件输入流可以作为源目标。


如下Json文件example.json：

{
    "encoding" : "UTF-8","plug-ins" : [
        "python","c++","ruby"
        ],"indent" : { "length" : 3,"use_space": true }
	"tab":null
}

Json::Value root;
Json::Reader reader;
std::ifstream ifs("example.json");//open file example.json

if(!reader.parse(ifs,root)){
   // fail to parse
}
else{
   // success
   std::cout<<root["encoding"].asString()<<endl;
   std::cout<<root["indent"]["length"].asInt()<<endl;
}

Json::Value root;
  Json::Reader reader;
  const char* s = "{"uploadid": "UP000000","code": 100,"msg": "","files": ""}"; 
  if(!reader.parse(s,root)){
    // "parse fail";
  }
  else{
      std::cout << root["uploadid"].asString();//print "UP000000"
  }

Json::Value root;
Json::Reader reader;
Json::FastWriter fwriter;
Json::StyledWriter swriter;

if(! reader.parse("example.json",root)){
// parse fail
    return 0;
}
std::string str = fwriter(root);
std::ofstream ofs("example_fast_writer.json");
ofs << str;
ofs.close();

str = swriter(root);
ofs.open("example_styled_writer.json");
ofs << str;
ofs.close();

结果1:example_styled_writer.json：
{
    "encoding" : "UTF-8","use_space": true }
	"tab":null
}

结果2:example_fast_writer.json：
{"encoding" : "UTF-8","plug-ins" : ["python","ruby"],"use_space": true}}

if(root.isMember("encoding")){
    std::cout<<"encoding is a member"<<std::endl;
}
else{
    std::cout<<"encoding is not a member"<<std::endl;
}

if(root["tab"].isNull()){
std::cout << "isNull" <<std::endl;//print isNull
}

完整例子使用举例来自于CSDN下载网友的程序：

源码下载地址：http://download.csdn.net/download/woniu222222/9966907

/********************************************************
Copyright (C),2016-2017,FileName: 	main
Author: 	woniu201
Email: 		wangpengfei.201@163.com
Created: 	2017/09/06
Description:use jsoncpp src,not use dll,but i also provide dll and lib.
********************************************************/

#include "stdio.h"
#include <string>
#include "jsoncpp/json.h"

using namespace std;

/************************************
@ Brief:		read file
@ Author:		woniu201 
@ Created: 		2017/09/06
@ Return:		file data  
************************************/
char *getfileAll(char *fname)
{
	FILE *fp;
	char *str;
	char txt[1000];
	int filesize;
	if ((fp=fopen(fname,"r"))==NULL){
		printf("open file %s fail n",fname);
		return NULL;
	}

	/*
	获取文件的大小
	ftell函数功能:得到流式文件的当前读写位置,其返回值是当前读写位置偏离文件头部的字节数.
	*/
	fseek(fp,SEEK_END); 
	filesize = ftell(fp);

	str=(char *)malloc(filesize);
	str[0]=0;

	rewind(fp);
	while((fgets(txt,1000,fp))!=NULL){
		strcat(str,txt);
	}
	fclose(fp);
	return str;
}

/************************************
@ Brief:		write file
@ Author:		woniu201 
@ Created: 		2017/09/06
@ Return:		    
************************************/
int writefileAll(char* fname,const char* data)
{
	FILE *fp;
	if ((fp=fopen(fname,"w")) == NULL)
	{
		printf("open file %s fail n",fname);
		return 1;
	}
	
	fprintf(fp,"%s",data);
	fclose(fp);
	
	return 0;
}

/************************************
@ Brief:		parse json data
@ Author:		woniu201 
@ Created: 		2017/09/06
@ Return:		    
************************************/
int parseJSON(const char* jsonstr)
{
	Json::Reader reader;
	Json::Value  resp;

	if (!reader.parse(jsonstr,resp,false))
	{
		printf("bad json format!n");
		return 1;
	}
	int result = resp["Result"].asInt();
	string resultMessage = resp["ResultMessage"].asString();
	printf("Result=%d; ResultMessage=%sn",result,resultMessage.c_str());

	Json::Value & resultValue = resp["ResultValue"];
	for (int i=0; i<resultValue.size(); i++)
	{
		Json::Value subJson = resultValue[i];
		string cpuRatio = subJson["cpuRatio"].asString();
		string serverIp = subJson["serverIp"].asString();
		string conNum = subJson["conNum"].asString();
		string websocketPort = subJson["websocketPort"].asString();
		string mqttPort = subJson["mqttPort"].asString();
		string ts = subJson["TS"].asString();

		printf("cpuRatio=%s; serverIp=%s; conNum=%s; websocketPort=%s; mqttPort=%s; ts=%sn",cpuRatio.c_str(),serverIp.c_str(),conNum.c_str(),websocketPort.c_str(),mqttPort.c_str(),ts.c_str());
	}
	return 0;
}

/************************************
@ Brief:		create json data
@ Author:		woniu201 
@ Created: 		2017/09/06
@ Return:		    
************************************/
int createJSON()
{
	Json::Value req;
	req["Result"] = 1;
	req["ResultMessage"] = "200";

	Json::Value	object1;
	object1["cpuRatio"] = "4.04";
	object1["serverIp"] = "42.159.116.104";
	object1["conNum"] = "1";
	object1["websocketPort"] = "0";
	object1["mqttPort"] = "8883";
	object1["TS"] = "1504665880572";
	Json::Value	object2;
	object2["cpuRatio"] = "2.04";
	object2["serverIp"] = "42.159.122.251";
	object2["conNum"] = "2";
	object2["websocketPort"] = "0";
	object2["mqttPort"] = "8883";
	object2["TS"] = "1504665896981";

	Json::Value jarray;
	jarray.append(object1);
	jarray.append(object2);

	req["ResultValue"] = jarray;

	Json::FastWriter writer;
	string jsonstr = writer.write(req);

	printf("%sn",jsonstr.c_str());

	writefileAll("createJson.json",jsonstr.c_str());
	return 0;
}

int main()
{
	/*读取Json串，解析Json串*/
	char* json = getfileAll("parseJson.json");
	parseJSON(json);
	printf("===============================n");

	/*组装Json串*/
	createJSON();

	getchar();
	return 1;
}

http://open-source-parsers.github.io/jsoncpp-docs/doxygen/index.html

http://blog.csdn.net/yc461515457/article/details/52749575

（编辑：李大同）

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