7、JsonCpp简单使用(3)

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\n"
               << 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;