本节主要介绍 json是什么以及jsoncpp库的使用。
（1）JSON是什么
json 是一种轻量级的文本数据交换格式；
json 独立于语言、平台，使用java script语法来描述对象；
json 解析器和json库对多种不同语言均提供了支持；
json (JavaScript Object Notation) 指的是javascript对象表示方法.
（2）c++JSON书写范例
1.书写c++代码：
// main.cpp
#include <iostream> #include "json/reader.h" #include "json/value.h"
using namespace std; int main(int argc,char *argv[]) { Json::Value person; person["name"] = "MenAngel"; person["sex"] = "男"; person["age"] = 23; person["height"] = 178; cout<<person.toStyledString()<<endl; return 0; }
2.头文件及库文件所在路径如下：
头文件：/data01/bm80/ob_rel/include/3rd
库文件：/data01/bm80/ob_rel/lib
库名：libjsoncppD.so
3.使用g++编译链接：
g++ main.cpp -ljsoncppD -I /data01/bm80/ob_rel/include/3rd -L /data01/bm80/ob_rel/lib -o test
4.执行结果如下：
{ "name" : "MenAngel", "sex" : "男", "age" : 23, "height" : 178 }
（3）html中使用javascript脚本创建java对象
1.书写html:
<html>
<body>
<h2>在 JavaScript 中创建 JSON 对象</h2>
<p> Name: <span id="jname"></span><br /> sex: <span id="jage"></span><br /> age: <span id="jstreet"></span><br />
</p>
<script type="text/javascript">
var JSONObject= { "name":"MenAngel", "sex":"男", "age":23}; document.getElementById("jname").innerHTML=JSONObject.name document.getElementById("jage").innerHTML=JSONObject.sex document.getElementById("jstreet").innerHTML=JSONObject.age </script>
</body>
</html>
用浏览器打开结果如下：
（4）几个重要的jsoncpp的类
Json::Value 可以表示所有的类型，int、uint、string、object、array,boolean等； Json::Reader 将json文件流或字符串解析到Json::Value, 主要函数有Parse； Json::Writer 将Json::Value转化成字符串流， Json::FastWriter 输出不带格式的json Json::StyleWriter 输出带格式的json
（5）jsoncpp使用详细范例：
1.从字符串中解析json：
// main.cpp
#include "json/reader.h" #include "json/value.h"
#include "stdlib.h"
using namespace std; int main(int argc,char *argv[]) { //创建json value 并转化为字符串
Json::Value person; person["name"] = "MenAngel"; person["isMarriged"] = false; person["age"] = 23; person["height"] = "178"; string strJson = person.toStyledString(); //解析字符串
Json::Reader reader; Json::Value root; string name; bool isMarriged; int age; int height,weight; if(reader.parse(strJson,root)) { if(!root["name"].isNull()) name = root["name"].asString(); if(!root["isMarriged"].isNull()) isMarriged = root["isMarriged"].asBool(); if(!root["age"].isNull()) age = root["age"].asInt(); if(!root["height"].isNull()) height = atoi(root["height"].asString().c_str()); weight = root["weight"].asInt(); } cout<<"name = " << name <<" "<< root["name"].isString() <<endl <<"isMarriged = " << isMarriged <<" "<< root["isMarriged"].isBool() <<endl <<"age = " << age <<" "<< root["age"].isInt() <<endl <<"height = " << height <<" "<< root["height"].isObject() <<endl //当key不存在时,返回nullValue ,isObject() is 1
<<"height = " << height <<" "<< root["height"].isInt() <<endl <<"height = " << height <<" "<< root["height"].isArray() <<endl <<"height = " << height <<" "<< root["height"].isNumeric() <<endl <<"weight = " << weight <<" "<< root["weight"].isObject() <<endl; return 0; }
2.从文件中解析json
#include <iostream> #include <fstream> #include "stdlib.h" #include "json/reader.h" #include "json/value.h"
using namespace std; int main(int argc,char *argv[]) { //创建json value 并转化为字符串
Json::Value person; person["name"] = "MenAngel"; person["isMarriged"] = false; person["age"] = 23; person["height"] = "178"; string strJson = person.toStyledString(); const char * filename = "./json.txt"; //将json字符串写入文件
ofstream ofile; ofile.open(filename); ofile<<strJson<<endl; ofile.flush(); ofile.close(); //从文件中解析json字符串
ifstream ifile; ifile.open(filename,ios::binary); Json::Reader reader; Json::Value root; string name; bool isMarriged; int age; int height,weight; if(reader.parse(ifile,root)) { if(!root["name"].isNull()) name = root["name"].asString(); if(!root["isMarriged"].isNull()) isMarriged = root["isMarriged"].asBool(); if(!root["age"].isNull()) age = root["age"].asInt(); if(!root["height"].isNull()) height = atoi(root["height"].asString().c_str()); weight = root["weight"].asInt(); } cout<<"name = " << name <<" "<< root["name"].isString() <<endl <<"isMarriged = " << isMarriged <<" "<< root["isMarriged"].isBool() <<endl <<"age = " << age <<" "<< root["age"].isInt() <<endl <<"height = " << height <<" "<< root["height"].isObject() <<endl //当key不存在时,返回nullValue ,isObject() is 1
<<"height = " << height <<" "<< root["height"].isInt() <<endl <<"height = " << height <<" "<< root["height"].isArray() <<endl <<"height = " << height <<" "<< root["height"].isNumeric() <<endl <<"weight = " << weight <<" "<< root["weight"].isObject() <<endl; //remove(filename); return 0; }
//json.txt
{ "name" : "MenAngel", "isMarriged" : false, "age" : 23, "height" : "178" }
3.FastWriter将一个Value对象格式化为JSON格式的字符串 (FastWriter、StyledWriter、StyledStreamWriter)
// main.cpp
#include <iostream> #include <json/json.h>
using namespace std; int main(int argc,char *argv[]) { Json::Value person; person["name"] = "MenAngel"; person["sex"] = "男"; person["age"] = 23; person["height"] = 178; Json::Writer *writer1 = new Json::FastWriter(); Json::Writer *writer2 = new Json::StyledWriter(); string str1 = writer1->write(person); string str2 = writer2->write(person); string str3 = person.toStyledString();//Json::StyledStreamWriter();
cout<<"str1 : "<<endl <<str1<<endl; cout<<"str2 : "<<endl <<str2<<endl; cout<<"str3 : "<<endl <<str3<<endl; cout<<"str4 : "<<endl <<person<<endl; return 0; }
str1 :
{"name":"MenAngel","sex":"男","age":23,"height":178}
str2 :
{
"name" : "MenAngel",
"sex" : "男",
"age" : 23,
"height" : 178
}
str3 :
{
"name" : "MenAngel",
"sex" : "男",
"age" : 23,
"height" : 178
}
str4 :
{
"name" : "MenAngel",
"sex" : "男",
"age" : 23,
"height" : 178
}
4.在JsonCpp中对Json:value对象中array、object、member、number、int的操作
// main.cpp
#include <iostream> #include <json/json.h>
using namespace std; int main(int argc,char *argv[]) { //构建json对象
Json::Value family; Json::Value members; family["family_id"] = 1314; family["single_parent"] = false; family["age"] = 20; family["money"] = 13.14; for(int i = 0 ;i < 4; ++i) { Json::Value member; member["id"] = i + 1; member["name"] = "name"; members.append(member); } family["members"] = members; //打印json value
string strJson = family.toStyledString(); cout<<family<<endl; //解析json value
Json::Reader reader; Json::Value root; if(reader.parse(strJson,root)) { std::vector<std::string> list_strMembers; if(!root.isNull() && root.isObject()) list_strMembers = root.getMemberNames(); for(auto str:list_strMembers) { if(!root.isMember(str)) continue; cout<<str<<" : "; if(!root[str].isNull()) { if(root[str].isInt()) { int tempInt = root[str].asInt(); cout<<" is Int,value = "<<tempInt<<endl; } if(root[str].isBool()) { bool tempBool = root[str].asBool(); cout<<"is Bool,value = "<<tempBool<<endl; } if(root[str].isString()) { string tempString = root[str].asString(); cout<<"is String,value = "<<tempString<<endl; } if(root[str].isObject()) { Json::Value tempValue = root[str]; cout<<"is Object,value = "<<tempValue<<endl; } if(root[str].isArray()) { Json::Value tempMember = root[str]; cout<<"is Array,size = "<<tempMember.size()<<endl; for(int j = 0;j < tempMember.size();j++) { Json::Value tempValue = tempMember[j]; cout<<" person "<<j+1<<":"<<endl; cout<<" id = "<<tempValue["id"]; cout<<" name = "<<tempValue["name"]; } } if(root[str].isNumeric() && !root[str].isBool()) { //布尔值在使用[]获取时返回的即是整型又是数值类型，其中整型是数值类型的一种
double tempDouble = root[str].asDouble(); cout<<"is Double,value = "<<tempDouble<<endl; } } } } //解析
return 0; }
{ "family_id" : 1314, "single_parent" : false, "age" : 20, "money" : 13.140, "members" : [ { "id" : 1, "name" : "name" }, { "id" : 2, "name" : "name" }, { "id" : 3, "name" : "name" }, { "id" : 4, "name" : "name" } ] } family_id : is Int,value = 1314 single_parent : is Bool,value = 0 age : is Int,value = 20 money : is Double,value = 13.14 members : is Array,size = 4 person 1: id = 1 name = "name" person 2: id = 2 name = "name" person 3: id = 3 name = "name" person 4: id = 4 name = "name"
在main.cpp中使用了c++11的特性因此编译时要进行指定 -std=c++11
g++ main.cpp -std=c++11 -ljsoncppD -I /data01/bm80/ob_rel/include/3rd -L /data01/bm80/ob_rel/lib -o test
5.对json value的修改，删除
// main.cpp
#include <iostream> #include <json/json.h>
using namespace std; int main(int argc,char *argv[]) { //构建json对象
Json::Value family; Json::Value members; family["family_id"] = 1314; family["single_parent"] = false; family["age"] = 20; family["money"] = 13.14; for(int i = 0 ;i < 4; ++i) { Json::Value member; member["id"] = i + 1; member["name"] = "name"; members.append(member); } family["members"] = members; //打印json value
string strJsonBefore = family.toStyledString(); cout<<strJsonBefore<<endl; family.removeMember("age"); family["money"] = 521; //Json::Value tempDelete; //新版本中删除json数组中元素的方法 //family["members"].removeIndex(3,tempDelete);
string strJsonAfter = family.toStyledString(); cout<<strJsonAfter<<endl; return 0; }
{ "family_id" : 1314, "single_parent" : false, "age" : 20, "money" : 13.140, "members" : [ { "id" : 1, "name" : "name" }, { "id" : 2, "name" : "name" }, { "id" : 3, "name" : "name" }, { "id" : 4, "name" : "name" } ] } { "family_id" : 1314, "single_parent" : false, "money" : 521, "members" : [ { "id" : 1, "name" : "name" }, { "id" : 2, "name" : "name" }, { "id" : 3, "name" : "name" }, { "id" : 4, "name" : "name" } ] }
6.处理不合法的json字符串时
// main.cpp
#include <iostream> #include <json/json.h>
using namespace std; int main(int argc,char *argv[]) { string errorStrJson1 = "{\"key1\":\"value1\",\"}"; string errorStrJson2 = "1111 {}"; Json::Reader reader; Json::Value root; if(!reader.parse(errorStrJson1,root)) { cout<<"errorStrJson1 parse error!"<<endl; } if(!reader.parse(errorStrJson2,root)) { cout<<"errorStrJson2 parse error!"<<endl; }else { cout<<"errorStrJson2 parse success!"<<endl; //root.getMemberNames();会core掉
}
//启用严格模式，让非法的json解析时直接返回false，不自动容错。这样，在调用parse的时候就会返回false。 Json::Reader *pJsonParser = new Json::Reader(Json::Features::strictMode()); if(!pJsonParser->parse(errorStrJson1,root)) { cout<<"errorStrJson1 parse error!"<<endl; } if(!pJsonParser->parse(errorStrJson2,root)) { cout<<"errorStrJson2 parse error!"<<endl; }else { cout<<"errorStrJson2 parse success!"<<endl; //root.getMemberNames();会core掉
} return 0; }
