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是一个用于解析json的开源c++库，是github上start最多的c++ 的json库，并且使用非常方便直观。

安装过程就不叙述了，不同的开发工具安装的过程也不太一样。

一、引入头文件

#include "nlohmann/json.hpp"

二、主要用法

1、声明与构造

json j1;
json j2 = json::object();
json j3 = json::array();

2、内容构造

json j = R"(
{
    "name": "Judd Trump",
    "credits": 1754500,
    "ranking": 1
}
)"_json;
	
或
	
json j{
    { "name", "Judd Trump"},
    { "credits", 1754500 },
    { "ranking", 1}
};

3、解析与序列化

（1）解析

std::string s = R"(
{
    "name": "Judd Trump",
    "credits": 1754500,
    "ranking": 1
}
)";
auto j = json::parse(s);

（2）序列化

 std::string s = j.dump();

（3）文件式

[
    {
        "name": "Judd Trump",
        "credits": 1754500,
        "ranking": 1
    },
    {
        "name": "Neil Robertson",
        "credits": 1040500,
        "ranking": 2
    },
    {
        "name": "Ronnie O'Sullivan",
        "credits": 954500,
        "ranking": 3
    }	
]

// 解析 
json j;
std::ifstream("c:\\rankings.json") >> j;
// 序列化
std::ofstream("d:\\rankings.json") << j;

4、引用和修改

json j{
        { "name", "Judd Trump"},
        { "credits", 1754500 },
        { "ranking", 1}
    };
   
    std::cout << j << endl;

    j["name"] = "Ding Junhui"; //j.at("name") = "Ding Junhui";　　 
    std::cout << j << endl;

5、转换成c++对象

（1）定义对象类型

struct Player{
    string name;
    int credits;
    int ranking;
};

（2）重载转换函数

void to_json(nlohmann::json& j, const Player& p) {
    j = json{  {"name", p.name}, {"credits", p.credits}, {"ranking", p.ranking} };
}

void from_json(const nlohmann::json& j, Player& p) {
    j.at("name").get_to(p.name);
    j.at("credits").get_to(p.credits);
    j.at("ranking").get_to(p.ranking);
}

（3）转换示例

auto j = R"([
        {
            "name": "Judd Trump",
            "credits": 1754500,
            "ranking": 1
        },
        {
            "name": "Neil Robertson",
            "credits": 1040500,
            "ranking": 2
        },
            {
            "name": "Ronnie O'Sullivan",
            "credits": 954500,
            "ranking": 3
        }    
        ])"_json; 
    
    std::vector<Player> players = j.get<std::vector<Player>>();
    std::cout << "name:    " << players[2].name << std::endl;
    std::cout << "credits: " << players[2].credits << std::endl;
    std::cout << "ranking: " << players[2].ranking << std::endl;

6、char数组转json对象

std::vector<char> jBuf(jLen - 4 + 1); // +1 为了空终止符
    if (inputFile.read(jBuf.data(), jLen - 4)) {
        jBuf[jLen - 4] = '\0'; // 确保以空终止符结尾
        try {
            json jsonObject = json::parse(jBuf.data());
            std::cout << "JSON 对象: " << jsonObject.dump(4) << std::endl;
        } catch (json::parse_error& e) {
            std::cerr << "JSON 解析错误: " << e.what() << std::endl;
            return 1;
        }
    } else {
        std::cerr << "读取 JSON 数据失败" << std::endl;
        return 1;
    }