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权双
发布于 2026-06-04 / 2 阅读
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nlohmann/json：让 C++ 像 Python 一样丝滑处理 JSON

#C++

nlohmann/json是目前 C++ 生态中最流行的 JSON 库之一。header-only（单头文件）、零依赖、接口像写原生 JS/Python 一样自然，支持 C++11+。

1 环境配置

1️⃣ 下载项目

项目地址：

👉 GitHub - nlohmann/json: JSON for Modern C++ · GitHub

git拉取：

git clone https://github.com/nlohmann/json.git

2️⃣ 文件需求说明

无需编译，只需要include下的纯文本文件，目录结构如下：

include
└─nlohmann
│ adl_serializer.hpp
│ byte_container_with_subtype.hpp
│ json.hpp
│ json_fwd.hpp
│ ordered_map.hpp
│
├─detail
│ │ abi_macros.hpp
│ │ exceptions.hpp
│ │ hash.hpp
│ │ json_custom_base_class.hpp
│ │ json_pointer.hpp
│ │ json_ref.hpp
│ │ macro_scope.hpp
│ │ macro_unscope.hpp
│ │ string_concat.hpp
│ │ string_escape.hpp
│ │ string_utils.hpp
│ │ value_t.hpp
│ │
│ ├─conversions
│ │ from_json.hpp
│ │ to_chars.hpp
│ │ to_json.hpp
│ │
│ ├─input
│ │ binary_reader.hpp
│ │ input_adapters.hpp
│ │ json_sax.hpp
│ │ lexer.hpp
│ │ parser.hpp
│ │ position_t.hpp
│ │
│ ├─iterators
│ │ internal_iterator.hpp
│ │ iteration_proxy.hpp
│ │ iterator_traits.hpp
│ │ iter_impl.hpp
│ │ json_reverse_iterator.hpp
│ │ primitive_iterator.hpp
│ │
│ ├─meta
│ │ │ cpp_future.hpp
│ │ │ detected.hpp
│ │ │ identity_tag.hpp
│ │ │ is_sax.hpp
│ │ │ logic.hpp
│ │ │ std_fs.hpp
│ │ │ type_traits.hpp
│ │ │ void_t.hpp
│ │ │
│ │ └─call_std
│ │ begin.hpp
│ │ end.hpp
│ │
│ └─output
│ binary_writer.hpp
│ output_adapters.hpp
│ serializer.hpp
│
└─thirdparty
└─hedley
hedley.hpp
hedley_undef.hpp

✅ 无需 .lib，无需编译

仅仅看着目录结构复杂，实际在使用时，仅需加载json.hpp这一个头文件即可。

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

2 示例

✅ 一、核心判断函数（记住这 4 个）

判断函数
是否是对象 {}j.is_object()
是否是数组 []j.is_array()
是否是字符串j.is_string()
是否是数字j.is_number()
是否是布尔j.is_boolean()
是否是nullj.is_null()
是否存在某个 keyj.contains("key")

✅ 七、nlohmann/json 类型对照表

JSONC++ 类型
{}json::object_t
[]json::array_t
"abc"std::string
123int / double
true/falsebool
nullnullptr_t

你也可以这样判断：

if (j.type() == json::value_t::object) {}
if (j.type() == json::value_t::array) {}

{
  "website": {
    "url": "https://pgmpocket.com",
    "title": "pgmpocket",
    "description": "Focusing on program development, embedded, and artificial intelligence technology sharing.",
    "author": "quan shuang",
    "language": "zh",
    "keywords": ["embedded", "CUDA", "AI"],
    "date_published": "2026-06-04T08:30:00Z"
  }
}

2.1 读取

普通刚发：

#include <iostream>
#include <fstream>
#include <string>
#include <nlohmann/json.hpp>

using json = nlohmann::json;

int main() {
    // 打开 JSON 文件
    std::ifstream file("test.json");
    if (!file.is_open()) {
        std::cerr << "无法打开 website.json 文件" << std::endl;
        return 1;
    }

    try {
        // 解析 JSON
        json website;
        file >> website;

        // 访问 website 对象
        const auto& site = website["website"];

        std::cout << "URL: " << site["url"].get<std::string>() << std::endl;
        std::cout << "标题: " << site["title"].get<std::string>() << std::endl;
        std::cout << "描述: " << site["description"].get<std::string>() << std::endl;
        std::cout << "作者: " << site["author"].get<std::string>() << std::endl;
        std::cout << "语言: " << site["language"].get<std::string>() << std::endl;

        std::cout << "关键词: ";
        for (const auto& kw : site["keywords"]) {
            std::cout << kw.get<std::string>() << " ";
        }
        std::cout << std::endl;

        std::cout << "发布时间: " << site["date_published"].get<std::string>() << std::endl;
        std::cout << "Canonical URL: " << site["canonical_url"].get<std::string>() << std::endl;

    }
    catch (const json::exception& e) {
        std::cerr << "JSON 解析错误: " << e.what() << std::endl;
        return 1;
    }

    return 0;
}

from_json方法：

#include <iostream>
#include <fstream>
#include <string>
#include <vector>
#include <nlohmann/json.hpp>

using json = nlohmann::json;

// 网站信息结构体
struct Website {
    std::string url;
    std::string title;
    std::string description;
    std::string author;
    std::string language;
    std::vector<std::string> keywords;
    std::string date_published;
    std::string canonical_url;
};

// from_json 反序列化
void from_json(const json& j, Website& w) {
    j.at("url").get_to(w.url);
    j.at("title").get_to(w.title);
    j.at("description").get_to(w.description);
    j.at("author").get_to(w.author);
    j.at("language").get_to(w.language);
    j.at("keywords").get_to(w.keywords);
    j.at("date_published").get_to(w.date_published);
    j.at("canonical_url").get_to(w.canonical_url);
}

// 根对象
struct Root {
    Website website;
};

void from_json(const json& j, Root& r) {
    j.at("website").get_to(r.website);
}

int main() {
    std::ifstream file("test.json");
    if (!file) {
        std::cerr << "无法打开 test.json" << std::endl;
        return 1;
    }

    json j;
    file >> j;

    Root root = j.get<Root>();
    const auto& w = root.website;

    std::cout << "URL: " << w.url << std::endl;
    std::cout << "标题: " << w.title << std::endl;
    std::cout << "描述: " << w.description << std::endl;
    std::cout << "作者: " << w.author << std::endl;
    std::cout << "语言: " << w.language << std::endl;

    std::cout << "关键词: ";
    for (const auto& k : w.keywords) {
        std::cout << k << " ";
    }
    std::cout << std::endl;

    std::cout << "发布时间: " << w.date_published << std::endl;
    std::cout << "Canonical URL: " << w.canonical_url << std::endl;

    return 0;
}

2.2 写入

#include <iostream>
#include <fstream>
#include <nlohmann/json.hpp>

using json = nlohmann::json;

int main() {
    json j;
    j["website"]["url"] = "https://example.com";
    j["website"]["title"] = "JSON";
    j["website"]["language"] = "zh";
    j["website"]["keywords"] = { "JSON", "SEO", "C++" };

    std::ofstream file("output.json");
    if (!file) {
        std::cerr << "无法写入文件" << std::endl;
        return 1;
    }

    file << j.dump(4); // 4 空格缩进
    return 0;
}

#include <string>
#include <vector>
#include <fstream>
#include <nlohmann/json.hpp>

using json = nlohmann::json;

struct Website {
    std::string url;
    std::string title;
    std::string description;
    std::string author;
    std::string language;
    std::vector<std::string> keywords;
    std::string date_published;
    std::string canonical_url;
};

NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE(
    Website,
    url,
    title,
    description,
    author,
    language,
    keywords,
    date_published,
    canonical_url
)

int main() {
    Website w;
    w.url = "https://example.com/blog/json-metadata";
    w.title = "JSON";
    w.description = "JSON JSON JSON";
    w.author = "qs";
    w.language = "zh";
    w.keywords = { "JSON", "SEO", "config" };
    w.date_published = "2026-06-04T08:30:00Z";
    w.canonical_url = "https://example.com/blog/json-metadata";

    json j;
    j["website"] = w;

    std::ofstream file("website.json");
    file << j.dump(4);

    return 0;
}
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