看json串1： {"system":{"version":"v2.6.1","name":"value"}}

废话少说， 直接撸代码：

#include <iostream>
#include <stdio.h>
#include<unistd.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include<sstream>

// 请自己下载开源的rapidjson
#include "rapidjson/prettywriter.h"
#include "rapidjson/rapidjson.h"
#include "rapidjson/document.h"
#include "rapidjson/stringbuffer.h"
#include "rapidjson/writer.h"
#include "rapidjson/memorystream.h"

using namespace std;
using rapidjson::Document;
using rapidjson::StringBuffer;
using rapidjson::Writer;
using namespace rapidjson;

string getVersion(const string &jvStr)
{
	Document document;
	if (document.Parse(jvStr.c_str()).HasParseError() || !document.HasMember("system")) 
	{
		return "";
	}

	const rapidjson::Value &jvObject = document["system"];
	if(!jvObject.IsObject())
	{
		return "";
	}

	if(!jvObject.HasMember("version"))
	{
		return "";
	}
	
	const rapidjson::Value &jv = jvObject["version"];
	
	return jv.GetString();
}


int main(int argc,char *argv[])
{
	string s = "{\"system\":{\"version\":\"v2.6.1\",\"name\":\"value\"}}";
	cout << s << endl;
	cout << getVersion(s) << endl;
	return 0;
}

{"system":{"version":"v2.6.1","name":"value"}}
v2.6.1

再看字符串：{"system": "{\"version\":\"v2.6.1\",\"name\":\"value\"}"}

直接上马：

#include <iostream>
#include <stdio.h>
#include<unistd.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include<sstream>

// 请自己下载开源的rapidjson
#include "rapidjson/prettywriter.h"
#include "rapidjson/rapidjson.h"
#include "rapidjson/document.h"
#include "rapidjson/stringbuffer.h"
#include "rapidjson/writer.h"
#include "rapidjson/memorystream.h"

using namespace std;
using rapidjson::Document;
using rapidjson::StringBuffer;
using rapidjson::Writer;
using namespace rapidjson;

string getStringFromJson(const string &jsStr,const string &strKey)  
{  
    Document document;  
    if (document.Parse(jsStr.c_str()).HasParseError() || !document.HasMember(strKey.c_str()))   
    {  
        return "";  
    }  

    const rapidjson::Value &jv = document[strKey.c_str()];  
    return jv.GetString();  
}  

int main(int argc,char *argv[])
{
	string s = "{\"system\": \"{\\\"version\\\":\\\"v2.6.1\\\",\\\"name\\\":\\\"value\\\"}\"}";
	cout << s << endl;
	string str = getStringFromJson(s,"system");
	cout << str << endl;
	cout << getStringFromJson(str,"version") << endl;
	
	return 0;
}

{"system": "{\"version\":\"v2.6.1\",\"name\":\"value\"}"}
{"version":"v2.6.1","name":"value"}
v2.6.1

第二种方式的json串， 看起来太恶心了。

另外， 再次强调一下， json串解析的时候， 容易core dump,所以要做好异常判断， 也要注意类型。