nlohmann json for modern C++
Posted kaizen
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【1】作者简介
【2】库
https://github.com/nlohmann/json
【3】应用示例
（1）工程配置
（2）示例代码
1 #include <string> 2 #include <vector> 3 #include <fstream> 4 #include <iostream> 5 #include <iomanip> 6 7 // 为了调试中文显示 8 #pragma execution_character_set("utf-8") 9 10 #include "nlohmann/json.hpp" 11 // for convenience 12 using json = nlohmann::json; 13 14 void json_object() 15 { 16 // create a JSON object 17 json j = 18 { 19 {"pi", 3.141}, 20 {"happy", true}, 21 {"name", "Niels"}, 22 {"nothing", nullptr}, 23 { 24 "answer", { 25 {"everything", 42} 26 } 27 }, 28 {"list", {1, 0, 2}}, 29 { 30 "object", { 31 {"currency", "USD"}, 32 {"value", 42.99} 33 } 34 } 35 }; 36 37 // add new values 38 j["new"]["key"]["value"] = { "another", "list" }; 39 40 // count elements 41 auto s = j.size(); 42 j["size"] = s; 43 44 // pretty print with indent of 4 spaces 45 std::cout << std::setw(4) << j << \'\\n\'; 46 47 // create JSON values 48 json j_boolean = true; 49 json j_number_integer = 17; 50 json j_number_float = 23.42; 51 json j_object = { {"one", 1}, {"two", 2} }; 52 json j_object_empty(json::value_t::object); 53 json j_array = { 1, 2, 4, 8, 16 }; 54 json j_array_empty(json::value_t::array); 55 json j_string = "Hello, world"; 56 57 // call back() 58 std::cout << j_boolean.back() << \'\\n\'; 59 std::cout << j_number_integer.back() << \'\\n\'; 60 std::cout << j_number_float.back() << \'\\n\'; 61 std::cout << j_object.back() << \'\\n\'; 62 //std::cout << j_object_empty.back() << \'\\n\'; // undefined behavior 63 std::cout << j_array.back() << \'\\n\'; 64 //std::cout << j_array_empty.back() << \'\\n\'; // undefined behavior 65 std::cout << j_string.back() << \'\\n\'; 66 67 // back() called on a null value 68 try 69 { 70 json j_null; 71 j_null.back(); 72 } 73 catch (json::invalid_iterator & e) 74 { 75 std::cout << e.what() << \'\\n\'; 76 } 77 } 78 79 void json_arry() 80 { 81 // create JSON arrays 82 json j_no_init_list = json::array(); 83 json j_empty_init_list = json::array({}); 84 json j_nonempty_init_list = json::array({ 1, 2, 3, 4 }); 85 json j_list_of_pairs = json::array({ {"one", 1}, {"two", 2} }); 86 87 // serialize the JSON arrays 88 std::cout << j_no_init_list << \'\\n\'; 89 std::cout << j_empty_init_list << \'\\n\'; 90 std::cout << j_nonempty_init_list << \'\\n\'; 91 std::cout << j_list_of_pairs << \'\\n\'; 92 } 93 94 void json_object_at_key() 95 { 96 // create JSON object 97 json object = 98 { 99 {"the good", "il buono"}, 100 {"the bad", "il cattivo"}, 101 {"the ugly", "il brutto"} 102 }; 103 104 // output element with key "the ugly" 105 std::cout << object.at("the ugly") << \'\\n\'; 106 107 // change element with key "the bad" 108 object.at("the bad") = "liuy kaizen"; 109 object.at("the good") = 100; 110 111 // output changed array 112 std::cout << object << \'\\n\'; 113 114 // exception type_error.304 115 try 116 { 117 // use at() on a non-object type 118 json j_str = "I am a string"; 119 std::cout << j_str << \'\\n\'; 120 j_str.at("the good") = "Another string"; 121 122 const json jc_str = "I am a const json object"; 123 std::cout << jc_str << "\\n"; 124 // jc_str.at("the good") = "Const string"; const对象，禁止赋值 125 } 126 catch (json::type_error & e) 127 { 128 std::cout << e.what() << \'\\n\'; 129 } 130 131 // exception out_of_range.401 132 try 133 { 134 // try to write at a nonexisting key 135 object.at("the fast") = "il rapido"; 136 } 137 catch (json::out_of_range & e) 138 { 139 std::cout << e.what() << \'\\n\'; 140 } 141 } 142 143 void json_array_at_index() 144 { 145 // create JSON array 146 json array = { "first", "2nd", "third", "fourth" }; 147 148 // output element at index 2 (third element) 149 std::cout << array.at(2) << \'\\n\'; 150 151 // change element at index 1 (second element) to "second" 152 array.at(1) = "second"; 153 154 // output changed array 155 std::cout << array << \'\\n\'; 156 157 // exception type_error.304 158 try 159 { 160 // use at() on a non-array type 161 json str = "I am a string"; 162 str.at(0) = "Another string"; 163 } 164 catch (json::type_error & e) 165 { 166 std::cout << e.what() << \'\\n\'; 167 } 168 169 // exception out_of_range.401 170 try 171 { 172 // try to write beyond the array limit 173 array.at(5) = "sixth"; 174 } 175 catch (json::out_of_range & e) 176 { 177 std::cout << e.what() << \'\\n\'; 178 } 179 } 180 181 void json_at_pointer() 182 { 183 // create a JSON value 184 json j = 185 { 186 {"number", 1}, {"string", "foo"}, {"array", {1, 2}} 187 }; 188 189 // read-only access 190 191 // output element with JSON pointer "/number" 192 std::cout << j.at("/number"_json_pointer) << \'\\n\'; 193 // output element with JSON pointer "/string" 194 std::cout << j.at("/string"_json_pointer) << \'\\n\'; 195 // output element with JSON pointer "/array" 196 std::cout << j.at("/array"_json_pointer) << \'\\n\'; 197 // output element with JSON pointer "/array/0" 198 std::cout << j.at("/array/0"_json_pointer) << \'\\n\'; 199 // output element with JSON pointer "/array/1" 200 std::cout << j.at("/array/1"_json_pointer) << \'\\n\'; 201 202 // writing access 203 204 // change the string 205 j.at("/string"_json_pointer) = "bar"; 206 // output the changed string 207 std::cout << j["string"] << \'\\n\'; 208 209 // change an array element 210 j.at("/array/1"_json_pointer) = 21; 211 // output the changed array 212 std::cout << j["array"] << \'\\n\'; 213 214 // out_of_range.106 215 try 216 { 217 // try to use an array index with leading \'0\' 218 json::reference ref = j.at("/array/01"_json_pointer); 219 } 220 catch (json::parse_error & e) 221 { 222 std::cout << e.what() << \'\\n\'; 223 } 224 225 // out_of_range.109 226 try 227 { 228 // try to use an array index that is not a number 229 json::reference ref = j.at("/array/one"_json_pointer); 230 } 231 catch (json::parse_error & e) 232 { 233 std::cout << e.what() << \'\\n\'; 234 } 235 236 // out_of_range.401 237 try 238 { 239 // try to use a an invalid array index 240 json::reference ref = j.at("/array/4"_json_pointer); 241 } 242 catch (json::out_of_range & e) 243 { 244 std::cout << e.what() << \'\\n\'; 245 } 246 247 // out_of_range.402 248 try 249 { 250 // try to use the array index \'-\' 251 json::reference ref = j.at("/array/-"_json_pointer); 252 } 253 catch (json::out_of_range & e) 254 { 255 std::cout << e.what() << \'\\n\'; 256 } 257 258 // out_of_range.403 259 try 260 { 261 // try to use a JSON pointer to an nonexistent object key 262 json::const_reference ref = j.at("/foo"_json_pointer); 263 } 264 catch (json::out_of_range & e) 265 { 266 std::cout << e.what() << \'\\n\'; 267 } 268 269 // out_of_range.404 270 try 271 { 272 // try to use a JSON pointer that cannot be resolved 273 json::reference ref = j.at("/number/foo"_json_pointer); 274 } 275 catch (json::out_of_range & e) 276 { 277 std::cout << e.what() << \'\\n\'; 278 } 279 } 280 281 void json_compare() 282 { 283 // create a JSON array 284 json j1 = { "one", "two", 3, 4.5, false }; 285 286 // create a copy 287 json j2(j1); 288 289 // serialize the JSON array 290 std::cout << j1 << " = " << j2 << \'\\n\'; 291 std::cout << std::boolalpha << (j1 == j2) << \'\\n\'; 292 } 293 294 void json_list_init() 295 { 296 // create JSON values 297 json j_empty_init_list = json({}); 298 json j_object = { {"one", 1}, {"two", 2} }; 299 json j_array = { 1, 2, 3, 4 }; 300 json j_nested_object = { {"one", {1}}, {"two", {1, 2}} }; 301 json j_nested_array = { {{1}, "one"}, {{1, 2}, "two"} }; 302 303 // serialize the JSON value 304 std::cout << j_empty_init_list << \'\\n\'; // {} 305 std::cout << j_object << \'\\n\'; // {"one":1,"two":2} 306 std::cout << j_array << \'\\n\'; // [1,2,3,4] 307 std::cout << j_nested_object << \'\\n\'; // {"one":[1],"two":[1,2]} 308 std::cout << j_nested_array << \'\\n\'; // [[[1],"one"],[1,2],"two"] 309 } 310 311 void json_move_constructor() 312 { 313 // create a JSON value 314 json a = 23; 315 316 // move contents of a to b 317 json b(std::move(a)); 318 319 // serialize the JSON arrays 320 std::cout << a << \'\\n\'; // null 321 std::cout << b << \'\\n\'; // 23 322 } 323 324 void json_copy_assignment() 325 { 326 // create JSON values 327 json a = 23; 328 json b = 42; 329 330 // copy-assign a to b 331 b = a; 332 333 // serialize the JSON arrays 334 std::cout << a << \'\\n\'; // 23 335 std::cout << b << \'C++ 下使用的 JSON 库 JSON for Modern C++ | 软件推介简单好用的C++ json库——JSON for Modern C++
c++11:nlohmann::json进阶使用ordered_json