https://github.com/nlohmann/json
<1>
展开json
#include <json.hpp> using json = nlohmann::json; int main() { // create JSON value json j_flattened = { {"/answer/everything", 42}, {"/happy", true}, {"/list/0", 1}, {"/list/1", 0}, {"/list/2", 2}, {"/name", "Niels"}, {"/nothing", nullptr}, {"/object/test0", "maya"}, {"/object/test1", "houdini"}, {"/pi", 3.141} }; // call unflatten() std::cout << std::setw(4) << j_flattened.unflatten() << ‘\n‘; }
./out >> test.txt 展开如下。
{ "answer": { "everything": 42 }, "happy": true, "list": [ 1, 0, 2 ], "name": "Niels", "nothing": null, "object": { "test0": "maya", "test1": "houdini" }, "pi": 3.141 }
<2>JSON Array 操作:
#include <json.hpp> using json = nlohmann::json; int main() { // create JSON arrays json j_no_init_list = json::array(); json j_empty_init_list = json::array({}); json j_nonempty_init_list = json::array({1, 2, 3, 4}); json j_list_of_pairs = json::array({ {"one", 1}, {"two", 2} }); // serialize the JSON arrays std::cout << j_no_init_list << ‘\n‘; std::cout << j_empty_init_list << ‘\n‘; std::cout << j_nonempty_init_list << ‘\n‘; std::cout << j_list_of_pairs << ‘\n‘; }
./out >> test.txt
[] [] [1,2,3,4] [["one",1],["two",2]]
<3> change value use _json_pointer or change array value use slice
_json_pointer前面必须是以linux / 符号才能改变值
当然at()也能返回值,如果作为返回就为const类型输出。
#include <json.hpp> using json = nlohmann::json; int main() { // create a JSON value json j = { {"number", 1}, {"string", "foo"}, {"array", {1, 2}} }; // read-only access std::cout << j.dump(4) <<std::endl; // print all as 4 spaces // output element with JSON pointer "/number" std::cout << j.at("/number"_json_pointer) << ‘\n‘; // output element with JSON pointer "/string" std::cout << j.at("/string"_json_pointer) << ‘\n‘; // output element with JSON pointer "/array" std::cout << j.at("/array"_json_pointer) << ‘\n‘; // output element with JSON pointer "/array/1" std::cout << j.at("/array/1"_json_pointer) << ‘\n‘; // writing access // change the string j.at("/string"_json_pointer) = "bar"; // output the changed string std::cout << j["string"] << ‘\n‘; // change an array element j.at("/array/0"_json_pointer) = 21; j.at("/array/1"_json_pointer) = 31; // output the changed array std::cout << j["array"] << ‘\n‘; // print out all std::cout << j["array"][0] << ‘\n‘; //print first element std::cout << j["array"][1] << ‘\n‘; //print secend element // change value direct slice get j["array"][0] = 11; std::cout << j["array"] << ‘\n‘; // print out all }
./out >> text.txt
{ "array": [ 1, 2 ], "number": 1, "string": "foo" } 1 "foo" [1,2] 2 "bar" [21,31] 21 31 [11,31]
<4>不用_json_pointer来改变,仅仅使用at(),如果用修改补存在的key-value,则会抛出异常
#include <json.hpp> using json = nlohmann::json; int main() { // create JSON object json object = { {"the good", "il buono"}, {"the bad", "il cattivo"}, {"the ugly", "il brutto"}, {"array",{1,2,3,4,5}} }; // output element with key "the ugly" std::cout << object.at("the ugly") << ‘\n‘; // change element with key "the bad" object.at("the bad") = "il cattivo"; object.at("array")[0] = 1000; object.at("/array"_json_pointer)[1] = 2000; //如果使用/ ,才能用_json_pointer // output changed array std::cout << object << ‘\n‘; // try to write at a nonexisting key try { object.at("the fast") = "il rapido"; } catch (std::out_of_range& e) //修改不存在的值抛出异常 { std::cout << "out of range: " << e.what() << ‘\n‘; } }
./out >> test.txt
"il brutto"{"array":[1000,2000,3,4,5],"the bad":"il cattivo","the good":"il buono","the ugly":"il brutto"} out of range: key ‘the fast‘ not found
<5>at()按照取标号来改变值,或者取值
#include <json.hpp> using json = nlohmann::json; int main() { // create JSON array json array = {"first", "2nd", "third", "fourth"}; // output element at index 2 (third element) std::cout << array.at(2) << ‘\n‘; // change element at index 1 (second element) to "second" array.at(1) = "second"; // output changed array std::cout << array << ‘\n‘; // try to write beyond the array limit try { array.at(5) = "sixth"; } catch (std::out_of_range& e) { std::cout << "out of range: " << e.what() << ‘\n‘; } }
./out >> test.txt
"third"
["first","second","third","fourth"]
out of range: array index 5 is out of range
<6>
时间: 2024-10-13 11:57:28