C++ nlohmann/json库怎么用？从解析到写入的完整教程

include(fetchcontent)  
fetchcontent_declare(json  
        url https://github.com/nlohmann/json/releases/download/v3.11.3/json.tar.xz)  
fetchcontent_makeavailable(json)  
  
target_link_libraries(json_demo private nlohmann_json::nlohmann_json)

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

// method 1
std::ifstream f("example.json");
json data = json::parse(f);

// method 2
std::ifstream i("file.json");
json j;
i >> j;

// 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},
};

// 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";
}

if (j.contains("key")) {
}

if (j.find("foo") != o.end()) {
}

auto value = j["key"];
auto value = j.at("key");
std::string value = j["key"].template get<std::string>();

// c++17 
using namespace std::literals;
// 如果key不存在，则返回默认值0
int v_integer = j.value("integer"sv, 0); 

// delete an entry
o.erase("foo");

std::ofstream o("pretty.json");
o << std::setw(4) << j << std::endl;

// 按照四个空格缩进打印json
std::cout << j.dump(4) << std::endl;