nlohmann-json使用

2023-12-17 08:20

文章标签 使用 json nlohmann

本文主要是介绍nlohmann-json使用，希望对大家解决编程问题提供一定的参考价值，需要的开发者们随着小编来一起学习吧！

nlohmann/json

nlohmann/json是一个使用现代c++编写的一个json库，该库是head-only的。

json介绍

关于json的介绍可以参考：CJSON简单介绍

使用

直接包含

single_include/
└── nlohmann├── json_fwd.hpp└── json.hpp

一般只要包含json.hpp，如果需要forward-declarations那么可以包含json_fwd.hpp。

注意，因为库使用了c++11特性，所以在编译的时候需要用C++11以上的标准编译。

#include <iostream>
#include "include/single_include/nlohmann/json.hpp"using json = nlohmann::json;int main()
{// create the different JSON values with default valuesjson j_null(json::value_t::null);json j_boolean(json::value_t::boolean);json j_number_integer(json::value_t::number_integer);json j_number_float(json::value_t::number_float);json j_object(json::value_t::object);json j_array(json::value_t::array);json j_string(json::value_t::string);// serialize the JSON valuesstd::cout << j_null << '\n';std::cout << j_boolean << '\n';std::cout << j_number_integer << '\n';std::cout << j_number_float << '\n';std::cout << j_object << '\n';std::cout << j_array << '\n';std::cout << j_string << '\n';
}

运行结果：

$./a.out 
null
false
0
0.0
{}
[]
""

例子

从文件中导入json对象

#include <fstream>
#include <nlohmann/json.hpp>
using json = nlohmann::json;// ...std::ifstream f("example.json");
json data = json::parse(f);

使用json字面量创建json对象

// Using (raw) string literals and json::parse
json ex1 = json::parse(R"({"pi": 3.141,"happy": true}
)");// Using user-defined (raw) string literals
using namespace nlohmann::literals;
json ex2 = R"({"pi": 3.141,"happy": true}
)"_json;// Using initializer lists
json ex3 = {{"happy", true},{"pi", 3.141},
};

直接创建并操作json对象

// create an empty structure (null)
json j;// 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 bool
j["happy"] = true;// add a string that is stored as std::string
j["name"] = "Niels";// add another null object by passing nullptr
j["nothing"] = nullptr;// add an object inside the object
j["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} };// 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}}}
};