使用 cJSON.h 创建 json 对象

Question

我正在尝试创建如下所示的 JSON 对象，但我无法在其中添加第二个项目，例如:

"CarType": "mercedes",
"carID": "merc123"

还有其他项目。

我想像这样创建 JSON:

{
  cars: [
    {
      "CarType": "BMW",
      "carID": "bmw123"
    },
    {
      "CarType": "mercedes",
      "carID": "merc123"
    },
    {
      "CarType": "volvo",
      "carID": "vol123r"
    },
    {
      "CarType": "ford",
      "carID": "ford123"
    }
  ]
};

到目前为止我已经尝试过:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include "cJSON.h"

int main (void){
    char field_name[32], value[32], *out;
    cJSON *root,*car;

    root  = cJSON_CreateObject();
    car=  cJSON_CreateArray();

    cJSON_AddItemToObject(root, "CarType", cJSON_CreateString("BMW"));
    cJSON_AddItemToObject(root, "carID", cJSON_CreateString("bmw123"));
    cJSON_AddItemToArray(car, root);

    out = cJSON_Print(car);
    printf("%s\n",out);

    return 0;
}

我的输出是这样的(缩进与这里显示的完全一样):

[{
        "CarType":  "BMW",
        "carID":    "bmw123"
    }]

Answer 1

以下代码将向您展示如何使用 cJSON 函数，如 cJSON_CreateObject()、cJSON_CreateArray()、cJSON_AddItemToObject() 和 cJSON_AddItemToArray()。

您必须将 cars 数组添加到 root 对象。之后，您必须将每辆 car 创建为包含 CarType 和 carID 项的对象。每个 car 对象都必须添加到 cars 数组中。

它也有很好的示例记录 here at GitHub .

编辑#1:

正如@JonnySchubert 指出的那样，有必要释放分配的资源。但在这种情况下释放根节点就足够了，因为将项目添加到数组或对象会转移它的所有权。换句话说:释放根节点将导致根下的所有节点也被释放。来 self 上面链接的 GitHub 资源:

For every value type there is a cJSON_Create... function that can be used to create an item of that type. All of these will allocate a cJSON struct that can later be deleted with cJSON_Delete. Note that you have to delete them at some point, otherwise you will get a memory leak. Important: If you have added an item to an array or an object already, you mustn't delete it with cJSON_Delete. Adding it to an array or object transfers its ownership so that when that array or object is deleted, it gets deleted as well.

编辑#2:

@lsalamon 提到你必须释放 cJSON_Print 的返回值，参见 here on SO for example和 the documentation .

代码:

#include <stdio.h>
#include <stdlib.h>
#include "cJSON.h"

int main()
{
   char *out;
   cJSON *root, *cars, *car;

   /* create root node and array */
   root = cJSON_CreateObject();
   cars = cJSON_CreateArray();

   /* add cars array to root */
   cJSON_AddItemToObject(root, "cars", cars);

   /* add 1st car to cars array */
   cJSON_AddItemToArray(cars, car = cJSON_CreateObject());
   cJSON_AddItemToObject(car, "CarType", cJSON_CreateString("BMW"));
   cJSON_AddItemToObject(car, "carID", cJSON_CreateString("bmw123"));

   /* add 2nd car to cars array */
   cJSON_AddItemToArray(cars, car = cJSON_CreateObject());
   cJSON_AddItemToObject(car, "CarType", cJSON_CreateString("mercedes"));
   cJSON_AddItemToObject(car, "carID", cJSON_CreateString("mercedes123"));

   /* print everything */
   out = cJSON_Print(root);
   printf("%s\n", out);
   free(out);

   /* free all objects under root and root itself */
   cJSON_Delete(root)

   return 0;
}

输出:

{
    "cars": [{
            "CarType":  "BMW",
            "carID":    "bmw123"
        }, {
            "CarType":  "mercedes",
            "carID":    "mercedes123"
        }]
}

此代码仅添加 2 辆汽车以显示用法。在您的实际应用程序中，您应该使用 C 数组和 for 循环来实现这一点。