使用方法

1. 读文件

#include <fstream>
#include <nlohmann/json.hpp>
using json = nlohmann::json;

// 读文件
std::ifstream f("example.json");
json data = json::parse(f);

2. parse 解析

// 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},
};

3. 定义

// 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}
  }}
};

4. object array

// a way to express the empty array []
json empty_array_explicit = json::array();

// ways to express the empty object {}
json empty_object_implicit = json({});
json empty_object_explicit = json::object();

// a way to express an _array_ of key/value pairs [["currency", "USD"], ["value", 42.99]]
json array_not_object = json::array({ {"currency", "USD"}, {"value", 42.99} });

5. dump

// explicit conversion to string
std::string s = j.dump();    // {"happy":true,"pi":3.141}

// serialization with pretty printing
// pass in the amount of spaces to indent
std::cout << j.dump(4) << std::endl;

6. 遍历查找

// special iterator member functions for objects
for (json::iterator it = o.begin(); it != o.end(); ++it) {
  std::cout << it.key() << " : " << it.value() << "\n";
}

// the same code as range for
for (auto& el : o.items()) {
  std::cout << el.key() << " : " << el.value() << "\n";
}

// even easier with structured bindings (C++17)
for (auto& [key, value] : o.items()) {
  std::cout << key << " : " << value << "\n";
}

// find an entry
if (o.contains("foo")) {
  // there is an entry with key "foo"
}

// or via find and an iterator
if (o.find("foo") != o.end()) {
  // there is an entry with key "foo"
}

7. 转换成string

std::string json_str = json_obj.dump();