nlohmann/json 的主要用法

前言

nlohmann/json 是一个用于解析json的开源c++库，口碑一流，号称有业界最好的性能，并且使用非常方便直观，是很多c++程序员的首选。开源官网处有详细说明用法，但篇幅过长，不便于迅速阅读抓重点。而且，所举例的某些用法实践上其实比较少用到，而某些实践上常用到的一些用法，官网却缺例子。所以自己简要总结了一下它的主要用法，并加上自己写的示例，希望能给到新接触该库的朋友一些帮助。

准备工作：

下载 https://github.com/nlohmann/json/tree/develop/single_include/nlohmann/json.hpp，并加入本地工程

引用头文件：

#include "nlohmann/json.hpp"
using json = nlohmann::json;

主要用法：

#1 声明与构造

##1 纯粹声明

    json j1; 
    json j2 = json::object();
    json j3 = json::array();
    
    std::cout << j1.type_name() << std::endl; // output: null
    std::cout << j2.type_name() << std::endl; // output: object
    std::cout << j3.type_name() << std::endl; // output: 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}
    };

#2 解析与序列化

　　字符串：

解析

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

序列化

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

　　文件：// 比如有文件 c:\rankings.json，其内容如下

[
    {
        "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;

#3 引用和修改

举个栗子：

    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;

输出结果：

{"credits":1754500,"name":"Judd Trump","ranking":1}
{"credits":1754500,"name":"Ding Junhui","ranking":1}

#4 转换成C++对象

定义对象类型

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

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);
}

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;