c++ nlohmann/json

news/2025/2/10 23:58:45/

json为JavaScript object notation 是一种数据格式，逐渐替换掉了传统的xml 。json数据格式的属性名称和字符串值需要用双引号引起来，用单引号或者不用引号会导致读取数据错误。json的另外一个数据格式是数组，和javascript中的数组字面量相同。

使用Json的格式与解析方便的可以表示一个对象信息，json有两种格式：

json中不能有注释，undefined，只要涉及到字符串的就必须双引号

①对象格式：{"key1":obj,"key2":obj,"key3":obj...}、

②数组/集合格式：[obj,obj,obj...]。

json.parse()解析将字符串解析成对应的值，

例子：

将nlohmann/json: JSON for Modern C++ (github.com)

中代码下载下来，然后将include文件夹添加到测试工程的包含路径下即可：

我是用的绝对路径：D:\excer\mfc\json\json\include\;

测试代码：

#include <iostream>
#include <fstream>
#include <ostream>
#include "json.hpp"
using json = nlohmann::json;int main()
{std::cout << "Hello World!\n";//读取json文件--------------------------------std::ifstream f("example.json");json data = json::parse(f);double pi = data["pi"];bool td = false; //td = data["happy"];if (!data["happy"].is_null()){td = data["happy"];}std::string sname = data["name"].get<std::string>(); ;std::cout << data;int c = 0;//输出json文件--------------------------------// create an empty structure (null)json j;//方式一---------------------------------------// add a number that is stored as double (note the implicit conversion of j to an object)j["pi"] = 3.141;// add a Boolean that is stored as boolj["happy"] = true;// add a string that is stored as std::stringj["name"] = "Niels";// add another null object by passing nullptrj["nothing"] = nullptr;// add an object inside the objectj["answer"]["everything"] = 42;// add an array that is stored as std::vector (using an initializer list)j["list"] = { 1, 0, 2 };// add another object (using an initializer list of pairs)j["object"] = { {"currency", "USD"}, {"value", 42.99} };//方式二---------------------------------------// instead, you could also write (which looks very similar to the JSON above)json j2 = {{"pi", 3.141},{"happy", true},{"name", "Niels"},{"nothing", nullptr},{"answer", {{"everything", 42}}},{"list", {1, 0, 2}},{"object", {{"currency", "USD"},{"value", 42.99}}}};//输出到文件中std::ofstream fout("out.json");fout << std::setw(4)<< j;fout << std::setw(4) << j2;fout.close();}

对应的：json格式

{"pi": 3.141,"happy": true,"name": "Niels","nothing": null,"answer": {"everything": 42},"list": [1, 0, 2],"object": {"currency": "USD","value": 42.99}
}

异常处理：

标签(关键字)happy2不存在，只有标签(关键字)happy，如果执行该语句bool td = data["happy2"]; 在直接读取标签happy2则解析器会崩溃。

解决办法：下面先对标签判断一下是否为空，为空的话，就不要直接读取了。标签不为空才能直接读取

//td = data["happy"];if (!data["happy"].is_null())//关键字不为空才能直接读取{td = data["happy"];}

主要看下面的文章：

写json格式

c++中nlohmann json的基本使用教程_C 语言_脚本之家 (jb51.net)

nlohmann/json: JSON for Modern C++ (github.com)https://github.com/nlohmann/json/tree/develop

JSON的三种格式https://blog.csdn.net/daxiong0816/article/details/125132404

json格式 (keoaeic.org)https://mip.keoaeic.org/unscramble_major/4394.htmlC++ json格式的书写_雪星途的博客-CSDN博客https://blog.csdn.net/weixin_45387966/article/details/122469835