1. 编译

从官网https://github.com/nlohmann/json的Release页面下载单个json.hpp即可直接使用，无需单独编译。

2. 使用示例

下面以示例的方式罗列nlohmann/json库的基本使用方法。

2.1 生成JSON

方式1

int main()
{using json = nlohmann::json;json j;j["pi"] = 3.141;j["happy"] = true;j["name"] = "Niels";j["nothing"] = nullptr;j["answer"]["everything"] = 42;j["list"] = { 1, 0, 2 };j["object"] = { {"currency", "USD"}, {"value", 42.99} };// 转成字符串std::string strJSON = j.dump(2); // 2个空格的缩进std::cout << strJSON;return 0;
}

输出如下：

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

方式2

int main()
{using json = nlohmann::json;json j = {{"pi", 3.141},{"happy", true},{"name", "Niels"},{"nothing", nullptr},{"answer", {{"everything", 42}}},{"list", {1, 0, 2}},{"object", {{"currency", "USD"}, {"value", 42.99}}}};// 转成字符串std::string strJSON = j.dump(2);std::cout << strJSON;return 0;
}

输出内容与方式1一样。

方式3

int main()
{using json = nlohmann::json;json j;j["pi"] = 3.141;j["happy"] = true;j["name"] = "Niels";j["nothing"] = nullptr;json j_answer;j_answer["everything"] = 42;j["answer"] = j_answer;json j_list = json::array();j_list.push_back(1);j_list.push_back(0);j_list.push_back(2);j["list"] = j_list;json j_object;j_object["currency"] = "USD";j_object["value"] = 42.99;j["object"] = j_object;// 转成字符串std::string strJSON = j.dump(2);std::cout << strJSON;return 0;
}

输出内容与方式1一样。

2.2 解析JSON

int main()
{using json = nlohmann::json;std::string strJSON = u8R"({"answer": {"everything": 42},"happy": true,"list": [1,0,2],"name": "Niels","nothing": null,"object": {"currency": "USD","value": 42.99},"pi": 3.141})";auto jsonObj = json::parse(strJSON);std::cout << jsonObj["pi"].get<float>() << std::endl; // 3.141std::cout << jsonObj["pi"].get<double>() << std::endl; // 3.141std::cout << std::boolalpha << jsonObj["happy"].get<bool>() << std::endl; // truestd::cout << jsonObj["name"].get<std::string>() << std::endl; // Nielsassert(jsonObj["nothing"] == nullptr);std::cout << jsonObj["answer"]["everything"].get<int>() << std::endl; // 42std::cout << jsonObj["list"].size() << std::endl; // 3std::cout << jsonObj["list"][0].get<int>() << std::endl; // 1std::cout << jsonObj["list"][1].get<int>() << std::endl; // 0std::cout << jsonObj["list"][2].get<int>() << std::endl; // 2std::cout << jsonObj["object"]["currency"].get<std::string>() << std::endl; // USDstd::cout << jsonObj["object"]["value"].get<float>() << std::endl; // 42.99// 依次输出:// 1// 0// 2for (json::iterator it = jsonObj["list"].begin(); it != jsonObj["list"].end(); ++it) {std::cout << *it << std::endl;}return 0;
}

3. 异常处理

当解析和生成JSON出错时，nlohmann/json会抛出异常，因此在解析和生成JSON时，需要进行异常捕获。

int main()
{using json = nlohmann::json;std::string strJSON = u8R"({"pi": 3.141})";try {auto jsonObj = json::parse(strJSON);std::cout << jsonObj["ppp"].get<float>() << std::endl;}catch (std::exception& e) {std::cout << e.what() << std::endl;}return 0;
}

4. 判断成员是否存在

int main()
{using json = nlohmann::json;std::string strJSON = u8R"({"pi": 3.141})";auto jsonObj = json::parse(strJSON);std::cout << std::boolalpha << jsonObj.contains("pi") << std::endl; // truestd::cout << std::boolalpha << jsonObj.contains("ppp") << std::endl; // falsereturn 0;
}