json

https://github.com/nlohmann/json

官网仓库的test/src下边有很多例子可以参考，比如构造，解析等

说明

可以和 python，golang 的 json 库媲美了。
官网也支持单头文件的形式，坐标：官方仓库 single_include/nlohmann/json.hpp

用法

头文件

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

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

构造

json j{
    { "name", "Judd Trump"},
    { "credits", 1754500 },
    { "ranking", 1}
};

解析及序列号

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

读写文件

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

读取及修改

json j{
     { "name", "Judd Trump"},
     { "credits", 1754500 },
     { "ranking", 1}
 };

 std::cout << j << endl;   //  {"credits":1754500,"name":"Judd Trump","ranking":1}
 j["name"] = "Ding Junhui";
 std::cout << j << endl;   //  {"credits":1754500,"name":"Ding Junhui","ranking":1}

//字典中查找key是否存在
auto subjectIdIter = j.find("name");
if (subjectIdIter != j.end()){
    cout << "it is found" << endl;
}else{
    cout << "not found " << endl;
}
//如果确定存在，直接可以访问. 直接访问如果不存在会触发抛出异常
std::cout<< j["name"] <<std::endl;

结构体和json互转

struct Player{
    string name;
    int credits;
    int ranking;
};
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);
}
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;
    //name:    Ronnie O'Sullivan
	//credits: 954500
	//ranking: 3