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：

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;
    string intcredits;
    string intranking;
};

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 players = j.get<:vector>>();
std::cout<< "name:" << players[2].name <<:endl>
std::cout<< "credits:" << players[2].credits <<:endl>
std::cout<< "ranking:" << players[2].ranking << std::endl;

输出结果：

name: Ronnie O'Sullivan

credits: 954500

ranking: 3