JsonCpp简单使用

作者：ilife

JsonCpp简单使用

1、相关概念总结

(1)解析json的方法

Json::Value json; //表示一个json格式的对象

Json::Reader reader; //json解析

reader.parse(json_buf/*json格式的字符串*/,json,false); //解析出json放到json中

jsoncpp库中的Reader类用来将字串或者流载入解析器。后期可以用Reader里面的解析方法把Json字串解码为C++认识的数据。可以用 Json::Reader来声明一个Reader实例。Reader中最常用的就是一个parse方法，该方法用来将载入的json字串解析为C++格式的数据。

(2) 数组访问

Json::Value //格式如下

[["key1":value1],["key2":value2]]

Json::Value::const_iterator iter; //迭代器

for(iter = input.begin(); iter != input.end(); iter++)

Json::Value::Members member=(*iter).getMemberNames();

*(member.begin()); // 输出 key1,key2

(*iter)[*(member.begin())]; //输出 value1,value2

Value类是库中的核心类，用于存储各样格式的数据，可以包括int,double,short,char *,string,bool,object,array等几乎所有格式的数据。该库的编码和解码的核心功能都是用Value类实现的。就用以上的 Reader的parse方法来说，需要传入一个Value类别的引用值，就是用来存储Json数据的根值，并且可以用这个根值来存取其他的所有值。

(3) 对象访问

直接用 value["key"]即可

(4) 输出json格式串

调用 Json::FastWriter的writer

writer是该库的一个虚类，没有真正的实现encode的功能。需要重载里头的方法来实现真正的encode功能。FastWriter是该库中真正实现encode功能的类，用来实现将Value编码称为Json串。Json::StyledWriter 是格式化后的json。

不支持utf-8格式的输出，需要自己调用writer之后，用iconv转化成utf-8字符串

2、示例代码

1）示例代码1

View Code

#include <stdio.h> #include <stdlib.h> #include <string.h> #include <string> #include <vector> #include <iostream> #include "json/json.h" using namespace std; typedef struct piece { string letter; string wild; }piece; string encode_msg(string token,int game_id,vector<piece> piece_array) { //Json::Value root; Json::Value var; //apply “token” and “game_id” value to json struct var["token"] = token; var["game_id"] = game_id; Json::Value pieces;//store all pieces for (int i=0;i < piece_array.size();i++) { Json::Value piece_ex;//here it store just one piece //next 4 lines to apply piece value to json struct piece_ex["letter"] = piece_array[i].letter; piece_ex["wild"] = piece_array[i].wild; //ok,yes we just have apply One piece ,then push back to the array pieces.append(piece_ex); } var["piece_array"] = pieces;//yes,store pieces in var [Value] //root.append(var); Json::FastWriter writer; return writer.write(var);//generate json string:),here all is done } int main() { piece one, two; one.letter = "1"; one.wild = "ont"; two.letter = "2"; two.wild = "two"; vector<piece> myp; myp.push_back(one); myp.push_back(two); string ret = encode_msg("mytoken", 123, myp); cout << ret << endl; return 1; }

View Code

{"game_id":123,"piece_array":[{"letter":"1","wild":"ont"},{"letter":"2","wild":"two"}],"token":"mytoken"}

结果显示

可以看到，直接用wirter输出的json为非格式化的数据，而通过root.toStyledString()后，代码就是格式化的。

2）示例3，来源于官网

View Code

// Configuration options { // Default encoding for text "encoding" : "UTF-8", // Plug-ins loaded at start-up "plug-ins" : [ "python", "c++", "ruby" ], // Tab indent size "indent" : { "length" : 3, "use_space": true } }

View Code

Json::Value root; // will contains the root value after parsing. Json::Reader reader; bool parsingSuccessful = reader.parse( config_doc, root ); if ( !parsingSuccessful ) { // report to the user the failure and their locations in the document. std::cout << "Failed to parse configuration " << reader.getFormattedErrorMessages(); return; } // Get the value of the member of root named 'encoding', return 'UTF-8' if there is no // such member. std::string encoding = root.get("encoding", "UTF-8" ).asString(); // Get the value of the member of root named 'encoding', return a 'null' value if // there is no such member. const Json::Value plugins = root["plug-ins"]; for ( int index = 0; index < plugins.size(); ++index ) // Iterates over the sequence elements. loadPlugIn( plugins[index].asString() ); setIndentLength( root["indent"].get("length", 3).asInt() ); setIndentUseSpace( root["indent"].get("use_space", true).asBool() ); // ... // At application shutdown to make the new configuration document: // Since Json::Value has implicit constructor for all value types, it is not // necessary to explicitly construct the Json::Value object: root["encoding"] = getCurrentEncoding(); root["indent"]["length"] = getCurrentIndentLength(); root["indent"]["use_space"] = getCurrentIndentUseSpace(); Json::StyledWriter writer; // Make a new JSON document for the configuration. Preserve original comments. std::string outputConfig = writer.write( root ); // You can also use streams. This will put the contents of any JSON // stream at a particular sub-value, if you'd like. std::cin >> root["subtree"]; // And you can write to a stream, using the StyledWriter automatically. std::cout << root;

参考

【1】讲解说明

http://hi.baidu.com/%B4%AB%CB%B5%D6%D0%B5%C4%C8%CC%D5%DF%C3%A8/blog/item/a6eb970c98a644d67acbe15a.html

【2】示例，该作者博文不错，涵盖了各个方面

http://hi.baidu.com/s_jqzhang/blog/item/a3c5df1f9408246ff624e4f5.html/cmtid/02e72e4fcb488039aec3ab28

【3】 boost库支持json比较好

http://freedomhui.com/?p=6

【3】对json的类型作了简单的小结，为json-c进行了介绍

http://developer.51cto.com/art/201001/176060.htm

【4】官网

http://jsoncpp.sourceforge.net/index.html