c++中nlohmann?json的基本使用教程

更新時(shí)間：2022年09月05日 09:14:00 作者：tutu-hu

nlohmann/json 是一個(gè)C++實(shí)現(xiàn)的JSON解析器,使用非常方便直觀,下面這篇文章主要給大家介紹了關(guān)于c++中nlohmann?json基本使用的相關(guān)資料,文中通過實(shí)例代碼介紹的非常詳細(xì),需要的朋友可以參考下

一.json.hpp庫下載及安裝

1.1 開源地址及引入方法

nlohmann json的開源項(xiàng)目地址，其中有對(duì)json使用方法的詳細(xì)說明：

https://github.com/nlohmann/json#serialization–deserialization

對(duì)于我們項(xiàng)目中要使用nlohmann json工具，只需要引入json.hpp這一個(gè)文件，其中包含所有接口函數(shù)，正如其文檔中所述json.hpp文件在single_include/nlohmann目錄下，我們只需要下載該文件即可：

git clone https://github.com/nlohmann/json/blob/develop/single_include/nlohmann/json.hpp

如上圖片所示，使用json.hpp文件需要關(guān)注兩點(diǎn)：

一是：#include <nlohmann/json.hpp>頭文件路徑的引入，這里將json.hpp文件放到linux系統(tǒng)中的/usr/local/include路徑下，這是系統(tǒng)默認(rèn)頭文件路徑，在編譯時(shí)系統(tǒng)會(huì)自動(dòng)查找該路徑。我們?cè)?usr/local/include路徑下創(chuàng)建/nlohmann/json.hpp，如下圖所示：

二是：在編譯時(shí)需要指定c++11標(biāo)準(zhǔn)，-std=c++11。

1.2 demo程序測(cè)試

jsontest.cpp：

#include <iostream>
#include <nlohmann/json.hpp>   //引入json.hpp，該文件已經(jīng)放在系統(tǒng)默認(rèn)路徑：/usr/local/include/nlohmann/json.hpp
using namespace std;
// for convenience
using json = nlohmann::json;

int main() 
{
  auto config_json = json::parse(R"({"happy": true, "pi": 3.141})");  //構(gòu)建json對(duì)象
  cout << config_json << endl;   //輸出json對(duì)象值
  return 0;
}

編譯：

g++ jsontest.cpp -std=c++11

輸出結(jié)果：

{“happy”:true,“pi”:3.141}

二.nlohmann json基本操作

2.1 由basic value創(chuàng)建json

兩種方式創(chuàng)建json對(duì)象：賦值構(gòu)造+直接構(gòu)造

jsontest.cpp：

#include <iostream>
#include <nlohmann/json.hpp>   //引入json.hpp，該文件已經(jīng)放在系統(tǒng)默認(rèn)路徑：/usr/local/include/nlohmann/json.hpp

using namespace std;
using json = nlohmann::json;    // for convenience 

int main() 
{
  //方式一：賦值構(gòu)造
  json j1;
  j1["name"]="LeBorn Jame";//字符串  
  j1["number"]=23;         //整數(shù)
  j1["man"]=true;          //布爾值
  j1["children"]={"LeBorn Jr","Bryce Maximus","Zhuri"};//數(shù)組
  j1["behavior"]["funny"]="gigigigigigi";              //對(duì)象中元素值
  j1["wife"]={{"name","Savannah Brinson"},{"man",false}};//對(duì)象
  
  //方式二：直接構(gòu)造
  json j2={
    {"name","LeBorn Jame"},
    {"number",23},
    {"man",true},
    {"children",{"LeBorn Jr","Bryce Maximus","Zhuri"}},
    {"behavior",{{"funny","gigigigigigi"}}},
    {"wife",{{"name","Savannah Brinson"},{"man",false}}}
  };

  cout << "j1: "<<j1 << endl;   //輸出json對(duì)象值
  cout <<  "j2: "<<j2 << endl;   //輸出json對(duì)象值
  return 0;
}

編譯：

g++ jsontest.cpp -std=c++11

輸出結(jié)果：

j1: {“behavior”:{“funny”:“gigigigigigi”},“children”:[“LeBorn Jr”,“Bryce Maximus”,“Zhuri”],“man”:true,“name”:“LeBorn Jame”,“number”:23,“wife”:{“man”:false,“name”:“Savannah Brinson”}}
j2: {“behavior”:{“funny”:“gigigigigigi”},“children”:[“LeBorn Jr”,“Bryce Maximus”,“Zhuri”],“man”:true,“name”:“LeBorn Jame”,“number”:23,“wife”:{“man”:false,“name”:“Savannah Brinson”}}

2.2 由json對(duì)象得到basic value

#include <iostream>
#include <string>

#include <nlohmann/json.hpp>   //引入json.hpp，該文件已經(jīng)放在系統(tǒng)默認(rèn)路徑：/usr/local/include/nlohmann/json.hpp

using namespace std;

using json = nlohmann::json;    // for convenience 

int main() 
{
  //構(gòu)建一個(gè)json對(duì)象hututu
  json hututu = {
      {"name","hututu"},
      {"age",18},
      {"gender",'m'},
      {"score",88.99},
      {"location",{"aaa","bbb","ccc"}},
  };
  //方式一
  auto name = hututu["name"].get<std::string>();  //獲取“name”對(duì)應(yīng)的value值，并轉(zhuǎn)為string類型
  cout<<"name = "<<name<<endl;
  cout<<"type name = "<<typeid(name).name()<<endl;
  cout<<"----------------------"<<endl;
  //方式二
  auto location0 = hututu["location"][0].get<std::string>();
  auto location1 = hututu["location"][1].get<std::string>();
  auto location2 = hututu["location"].at(2).get<std::string>();
  cout<<"location0 = "<<location0<<endl;
  cout<<"location1 = "<<location1<<endl;
  cout<<"location2 = "<<location2<<endl;
  return 0;
}

輸出結(jié)果：

name = hututu
type name = Ss
location0 = aaa
location1 = bbb
location2 = ccc

2.3 像操作stl container一樣操作json value

#include <iostream>
#include <string>

#include <nlohmann/json.hpp>   //引入json.hpp，該文件已經(jīng)放在系統(tǒng)默認(rèn)路徑：/usr/local/include/nlohmann/json.hpp

using namespace std;

using json = nlohmann::json;    // for convenience 

int main() 
{
  //構(gòu)建一個(gè)json對(duì)象animalArray
  json animalArray={"cat","dog"};//定義一個(gè)數(shù)組類型的json對(duì)象
  animalArray.push_back("pig");//添加元素
  animalArray.emplace_back("duck");//C++11新方式添加元素，減少申請(qǐng)內(nèi)存
  cout<<"animalArray: "<<animalArray<<endl;
  //使用is_array()函數(shù)判斷對(duì)象類型，使用empty函數(shù)判斷數(shù)量是否為空
  if(animalArray.is_array() && !animalArray.empty())
  {
      auto size=animalArray.size();          //使用size函數(shù)獲取元素?cái)?shù)量
      cout<<"animalArray size: "<<size<<endl;
      auto animalLast=animalArray.at(size-1).get<std::string>();
      cout<<"animalArray[size-1]: "<<animalLast<<endl;
      cout<<"/--------------------/"<<endl;
  }

  json animalObject={{"kind","dog"},{"height",50}};//定義一個(gè)對(duì)象類型的json對(duì)象
  animalObject.push_back({"color","red"});//插入元素
  animalObject.erase("kind");//刪除鍵值
  cout<<"animalObject: "<<animalObject<<endl;
  animalObject["height"] = 99;   //通過key修改value值
  //判斷是否含有某個(gè)鍵值方式一
  if(animalObject.contains("height"))//通過contains函數(shù)判斷是否包含某個(gè)key
  {
      auto height=animalObject["height"].get<double>();
      cout<<"方式一:height: "<<height<<endl;
  }
  //判斷是否含有某個(gè)鍵值方式二
  auto size=animalObject.count("height");//通過count函數(shù)計(jì)算某一個(gè)鍵的數(shù)量
  if(size>0)
  {
      cout<<"方式二:存在height鍵值"<<endl;
  }
  //判斷是否含有某個(gè)鍵值方式三
  auto iter=animalObject.find("height");//通過find函數(shù)查找某個(gè)鍵的迭代器
  if(iter!=animalObject.end())
  {
      cout<<"方式三:存在height鍵值"<<endl;
  }
  //遍歷輸出鍵值方式1
  cout<<"遍歷輸出鍵值方式1:"<<endl;
  for(auto item:animalObject.items())
  {
      std::cout<<item.key()<<" "<<item.value()<<std::endl;
  }
  //遍歷輸出鍵值方式2
  cout<<"遍歷輸出鍵值方式2:"<<endl;
  for(auto iter=animalObject.begin();iter!=animalObject.end();++iter)
  {
      cout<<iter.key()<<" "<<iter.value()<<std::endl;
  }
  return 0;
}

輸出結(jié)果：

animalArray: [“cat”,“dog”,“pig”,“duck”]
animalArray size: 4
animalArray[size-1]: duck
/--------------------/
animalObject: {“color”:“red”,“height”:50}
方式一:height: 99
方式二:存在height鍵值
方式三:存在height鍵值
遍歷輸出鍵值方式1:
color “red”
height 99
遍歷輸出鍵值方式2:
color “red”
height 99

三.json序列化與反序列化

3.1 json value和string

#include <iostream>
#include <string>

#include <nlohmann/json.hpp>   //引入json.hpp，該文件已經(jīng)放在系統(tǒng)默認(rèn)路徑：/usr/local/include/nlohmann/json.hpp

using namespace std;

using json = nlohmann::json;    // for convenience 

int main() 
{
  //反序列化構(gòu)建json對(duì)象，兩種方式
  json hututu1 = "{\"name\":\"hututu\",\"age\":18,\"score\":88.99}"_json;//方式1，通過"_json"實(shí)現(xiàn)反序列化
  auto temp = R"({"name":"hututu","age":18,"score":88.99})";//使用原生字符串關(guān)鍵字R來避免轉(zhuǎn)移字符，但這一句并沒有序列化，hututu2只保存字符串而已，需要結(jié)合方式3實(shí)現(xiàn)反序列化
  json hututu2 = json::parse(temp);//方式2，通過靜態(tài)函數(shù)"parse"實(shí)現(xiàn)反序列化
  cout<<"/----------反序列化-----------/"<<endl;
  cout<<"hututu1 = "<<hututu1<<endl;
  cout<<"hututu2 = "<<hututu2<<endl;
  cout<<"/----------序列化-----------/"<<endl;
  //序列化(Serialization)：dump(number)，number為打印出的空格數(shù)
  std::string hututu1_string=hututu1.dump();//animal1值為{"kind":"dog","height":50}
  std::string hututu2_string=hututu2.dump(4);
  cout<<"hututu1_string = "<<hututu1_string<<endl;
  cout<<"hututu2_string = "<<hututu2_string<<endl;
  
  return 0;
}

輸出結(jié)果：

/----------反序列化-----------/
hututu1 = {“age”:18,“name”:“hututu”,“score”:88.99}
hututu2 = {“age”:18,“name”:“hututu”,“score”:88.99}
/----------序列化-----------/
hututu1_string = {“age”:18,“name”:“hututu”,“score”:88.99}
hututu2_string = {
“age”: 18,
“name”: “hututu”,
“score”: 88.99
}

3.2 json對(duì)象和文件輸入輸出轉(zhuǎn)換

#include <iostream>        //文件操作頭文件
#include <string>
#include <fstream>
#include <nlohmann/json.hpp>   //引入json.hpp，該文件已經(jīng)放在系統(tǒng)默認(rèn)路徑：/usr/local/include/nlohmann/json.hpp

using namespace std;

using json = nlohmann::json;    // for convenience 

int main() 
{
  //上述操作適用于istream和ostream的子類，比如我們經(jīng)常會(huì)用到的ifstream和ofstream
  //從.json文件中讀取內(nèi)容到j(luò)son對(duì)象中
  std::ifstream in("./person.json");//打開文件，關(guān)聯(lián)到流in
  json hututu={"111","222"};      //定義一個(gè)json對(duì)象為hututu,有初始內(nèi)容，但是會(huì)被覆蓋
  in>>hututu;                     //從流in中(也就是./person.json文件)讀取內(nèi)容到j(luò)son對(duì)象中，會(huì)覆蓋之前內(nèi)容
  in.close();                     //關(guān)閉文件流in
  hututu["aaa"]="bbb";             //添加json對(duì)象內(nèi)容
  cout << hututu << endl;   //輸出json對(duì)象值

  //輸出json對(duì)象內(nèi)容到文件中，并生成新的文件
  std::ofstream out("./new.json");    //創(chuàng)建文件./new.json，并關(guān)聯(lián)到流out
  hututu["name"]="new name";          //更改hututu對(duì)象的內(nèi)容
  out<<std::setw(4)<<hututu;          //輸出json對(duì)象hututu信息到文件./new.json中，std::setw(4)用于設(shè)置增加打印空格
  out.close();         //關(guān)閉文件流out

  return 0;
}

./person.json文件內(nèi)容

{
“name”:“hututu”,
“age”:18,
“gender”:“m”,
“score”:88.99
}

執(zhí)行程序后，輸出的json對(duì)象內(nèi)容如下，也就是從./person.json文件中讀取的信息：

{“aaa”:“bbb”,“age”:18,“gender”:“m”,“name”:“hututu”,“score”:88.99}

同時(shí)在當(dāng)前目錄下生成新的文件./new.json，內(nèi)容如下所示：

{
“aaa”: “bbb”,
“age”: 18,
“gender”: “m”,
“name”: “new name”,
“score”: 88.99
}

3.3 json value和自定義對(duì)象

在自定義對(duì)象命名空間中定義兩個(gè)函數(shù)即可像basic value一樣進(jìn)行反序列化和序列化：from_json(const json& j,T& value)、to_json(json& j,const T& value)

#include <iostream>        //文件操作頭文件
#include <string>
#include <nlohmann/json.hpp>   //引入json.hpp，該文件已經(jīng)放在系統(tǒng)默認(rèn)路徑：/usr/local/include/nlohmann/json.hpp

using namespace std;

using json = nlohmann::json;

class person
{
    public:
        person(){}       //默認(rèn)構(gòu)造函數(shù)
        person(string m_name,int m_age,double m_score):name(m_name),age(m_age),score(m_score){};
    public:
      string name;
      int age;
      double score;
    void display()
    {
      cout<<"person name = "<<this->name<<endl;
      cout<<"person age = "<<this->age<<endl;
      cout<<"person score = "<<this->score<<endl;
    }
};
//定義from_json(const json& j,T& value)函數(shù)，用于序列化
//json對(duì)象----->class對(duì)象
void from_json(const json& j,person& hututu)
{
    hututu.name=j["name"].get<std::string>();
    hututu.age=j["age"].get<int>();
    hututu.score=j["score"].get<double>();
}

//定義to_json(json& j,const T& value)函數(shù)，用于反序列化
//class對(duì)象----->json對(duì)象
void to_json(json& j,const person& hututu)
{
    j["name"]=hututu.name;
    j["age"]=hututu.age;
    j["score"]=hututu.score;
}
// void to_json(json& j, const person& p) 
// {
// 	j = json{ {"name", p.name}, {"address", p.address}, {"age", p.age} };
// }
 
// void from_json(const json& j, person& p) {
// 	j.at("name").get_to(p.name);
// 	j.at("address").get_to(p.address);
// 	j.at("age").get_to(p.age);
// }

//main.cpp文件
int main()
{
    person hututu{"hututu",18,88.99};//定義一個(gè)person對(duì)象為hututu
    cout<<"/----------to json,方式1:json=class隱式轉(zhuǎn)換-----------/"<<endl;
    json j1=hututu;                //class to json,隱式調(diào)用to_json函數(shù) 
    cout<<"j1 = "<<j1<<endl;       //輸出json對(duì)象值
    cout<<"/----------to json,方式2:調(diào)用to_json函數(shù)-----------/"<<endl;
    json j2;
    to_json(j2,hututu);          //to json,調(diào)用to_json函數(shù)
    cout<<"j2 = "<<j2<<endl;     //輸出json對(duì)象值
    cout<<"/----------from json,方式1:調(diào)用from_json函數(shù)-----------/"<<endl; 
    j1["name"]="new name";        //修改json對(duì)象數(shù)據(jù)
    cout<<"new j1 = "<<j1<<endl;        //輸出json對(duì)象值
    person hututu_new;
    from_json(j1,hututu_new);   //json---->class
    hututu_new.display();     //輸出person對(duì)象內(nèi)容
    cout<<"/----------from json,方式2:調(diào)用.get函數(shù)-----------/"<<endl; 
    person hututuNew = j2.get<person>();//像basic value一樣通過get函數(shù)獲取值，將其值直接賦值給自定義對(duì)象
    hututuNew.display();
    return 0;
}

執(zhí)行結(jié)果：

/----------to json,方式1:json=class隱式轉(zhuǎn)換-----------/
j1 = {“age”:18,“name”:“hututu”,“score”:88.99}
/----------to json,方式2:調(diào)用to_json函數(shù)-----------/
j2 = {“age”:18,“name”:“hututu”,“score”:88.99}
/----------from json,方式1:調(diào)用from_json函數(shù)-----------/
new j1 = {“age”:18,“name”:“new name”,“score”:88.99}
person name = new name
person age = 18
person score = 88.99
/----------from json,方式2:調(diào)用.get函數(shù)-----------/
person name = hututu
person age = 18
person score = 88.99

四.NLOHMANN_DEFINE_TYPE_INTRUSIVE宏的使用

4.1 宏的定義

JSON for Modern C++ 中為方便序列化和反序列化定義了兩宏，如下

NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE(name, member1, member2, …) 將在要為其創(chuàng)建代碼的類/結(jié)構(gòu)的命名空間內(nèi)定義。
NLOHMANN_DEFINE_TYPE_INTRUSIVE(name, member1, member2, …) 將在要為其創(chuàng)建代碼的類/結(jié)構(gòu)中定義。該宏還可以訪問私有成員。

進(jìn)一步查看代碼:

/*!
@brief macro
@def NLOHMANN_DEFINE_TYPE_INTRUSIVE
@since version 3.9.0
*/
#define NLOHMANN_DEFINE_TYPE_INTRUSIVE(Type, ...)  \
    friend void to_json(nlohmann::json& nlohmann_json_j, const Type& nlohmann_json_t) { NLOHMANN_JSON_EXPAND(NLOHMANN_JSON_PASTE(NLOHMANN_JSON_TO, __VA_ARGS__)) } \
    friend void from_json(const nlohmann::json& nlohmann_json_j, Type& nlohmann_json_t) { NLOHMANN_JSON_EXPAND(NLOHMANN_JSON_PASTE(NLOHMANN_JSON_FROM, __VA_ARGS__)) }

/*!
@brief macro
@def NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE
@since version 3.9.0
*/
#define NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE(Type, ...)  \
    inline void to_json(nlohmann::json& nlohmann_json_j, const Type& nlohmann_json_t) { NLOHMANN_JSON_EXPAND(NLOHMANN_JSON_PASTE(NLOHMANN_JSON_TO, __VA_ARGS__)) } \
    inline void from_json(const nlohmann::json& nlohmann_json_j, Type& nlohmann_json_t) { NLOHMANN_JSON_EXPAND(NLOHMANN_JSON_PASTE(NLOHMANN_JSON_FROM, __VA_ARGS__)) }

4.2 宏的使用

可以看出上述的宏主要實(shí)現(xiàn)了from_json和to_json兩個(gè)函數(shù)的功能，使用時(shí)需要在一個(gè)類中調(diào)用該宏，并傳入(類名，參數(shù)1，參數(shù)2，參數(shù)3…)使用，這樣在json對(duì)象和class對(duì)象之間之間直接賦值可以完成相互轉(zhuǎn)換，具體用法如下：

#include <iostream>        //文件操作頭文件
#include <string>
#include <nlohmann/json.hpp>   //引入json.hpp，該文件已經(jīng)放在系統(tǒng)默認(rèn)路徑：/usr/local/include/nlohmann/json.hpp

using namespace std;

using json = nlohmann::json;

class person
{
   public:
      string name;
      int age;
      double score;
      void display()
      {
        cout<<"person name = "<<this->name<<endl;
        cout<<"person age = "<<this->age<<endl;
        cout<<"person score = "<<this->score<<endl;
      }
      // 類名，成員1，成員2，成員3
      NLOHMANN_DEFINE_TYPE_INTRUSIVE(person, name, age, score);
};

//main.cpp文件
int main()
{
    person hututu{"hututu",18,88.99};//定義一個(gè)person對(duì)象為hututu
    cout<<"/----------調(diào)用宏實(shí)現(xiàn):to json-----------/"<<endl;
    json j1 = hututu;
    cout << j1<< endl;
    cout << j1.dump() << endl;
    cout<<"/----------調(diào)用宏實(shí)現(xiàn):from json-----------/"<<endl;
    j1["name"]="new name";
    person hututu_new = j1;
    hututu_new.display();

    return 0;
}

輸出結(jié)果：

/----------調(diào)用宏實(shí)現(xiàn):to json-----------/
{“age”:18,“name”:“hututu”,“score”:88.99}
{“age”:18,“name”:“hututu”,“score”:88.99}
/----------調(diào)用宏實(shí)現(xiàn):from json-----------/
person name = new name
person age = 18
person score = 88.99