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【3rd Party】nlohmann json 基础用法

发布时间 2023-08-31 16:53:07作者: Koshkaaa

什么是nlohman json ?

nlohman json GitHub - nlohmann/json: JSON for Modern C++ 是一个为现代C++（C++11）设计的JSON解析库，主要特点是

  1. 易于集成，仅需一个头文件，无需安装依赖
  2. 易于使用，可以和STL无缝对接，使用体验近似python中的json

Minimal Example

CMakeLists.txt 编写教程：Here

# CMakeLists.txt
cmake_minimum_required(VERSION 3.24)
project(nlohmannJson)

set(CMAKE_CXX_STANDARD 17)

include_directories("include")
add_executable(${PROJECT_NAME} src/main.cpp)

// main.cpp
#include <iostream>
#include "nlohmann/json.hpp"

using namespace std;
using namespace nlohmann;

int main() {
    auto config_json = json::parse(R"({"Happy": true, "pi": 3.1415})");
    cout << config_json << endl;

    return 0;
}

示范用法

nlohman::json 库操作的基本对象是 json Object，全部操作围绕此展开

更多示例在 GitHub - nlohmann/json: JSON for Modern C++

从文件读取json信息

// read json from file
std::ifstream input_json_file("../configs/config.json");
nlohmann::json config_json;
input_json_file >> config_json;

从字符串中读取json信息

// from string, just use _json
auto config_json = R"({"A" : "a", "B" : "b", "Pi" : 1234 })"_json;

// parse explicitly
std::string info = R"({"A" : "a", "B" : "b", "Pi" : 1234 })";
auto config_json = nlohmann::json::parse(info);

直接创建json对象

json j1 = {
    {"pi", 3.141},
    {"happy", true},
    {"name", "Niels"},
    {"nothing", nullptr},
    {"answer", {
      {"everything", 42}
    }},
    {"list", {1, 0, 2}},
    {"object", {
      {"currency", "USD"},
      {"value", 42.99}
    }}
};

json j2;  // create null object
j2["pi"] = 3.1415;
j2["list"] = {1,2,3,4,5};
j2["object"] = { {"currency", "USD"}, {"value", 42.99} };

从json对象获取键值对

auto config_json = R"({"A" : "a", "B" : "b", "Pi" : 1234 })"_json;

// 遍历键值对
for(auto elem : config_json.items()){
    std::cout << elem.key() << " , " << elem.value() << std::endl;
}

// C++ 17
for (auto& [key, value] : j1.items()) {
  cout << key << " : " << value << "\n";
}

// 直接访问
std::string s1 = config_json["a"];

和std::vector交互

json j1 = {
    {"list", {1, 0, 2}},
    {"object", {
      {"currency", "USD"},
      {"value", 42.99}
    }}
};  
std::vector<int> a = j1["list"];
for (auto& elem : a){
    cout << elem << endl;
}
// output
//1
//0
//2

json j2;
j2["vector"] = a;
std::vector<string> b{"s1","s2","s3"};
j2["string vector"] = b;
cout << j2;
// output
// {"string vector":["s1","s2","s3"],"vector":[1,0,2]}

序列化

auto j = json::parse(R"({"happy": true, "pi": 3.141})")

// 写入字符串
std::string s = j.dump();

// 写入文件
std::ofstream o("pretty.json");
o << j << "\n;