cJSON 使用详解

由于c语言中，没有直接的字典，字符串数组等数据结构，所以要借助结构体定义，处理json。如果有对应的数据结构就方便一些， 如python中用json.loads(json)就把json字符串转变为内建的数据结构处理起来比较方便。

一个重要概念:

在cjson中，json对象可以是json，可以是字符串，可以是数字。。。

cjson数据结构定义：

 #define cJSON_False 0

 #define cJSON_True 1

 #define cJSON_NULL 2

 #define cJSON_Number 3

#define cJSON_String 4

 #define cJSON_Array 5

#define cJSON_Object 6

 typedef struct cJSON {

    struct cJSON *next,*prev;    /* next/prev allow you to walk array/object chains. Alternatively, use GetArraySize/GetArrayItem/GetObjectItem */

    struct cJSON *child;        /* An array or object item will have a child pointer pointing to a chain of the items in the array/object. */

    int type;                    /* The type of the item, as above. cjson结构的类型上面宏定义的7中之一*/

    char *valuestring;            /* The item's string, if type==cJSON_String */

    int valueint;                /* The item's number, if type==cJSON_Number */

    double valuedouble;            /* The item's number, if type==cJSON_Number */

    char *string;                /* The item's name string, if this item is the child of, or is in the list of subitems of an object. */

} cJSON;

#define cJSON_False 0

 #define cJSON_True 1

 #define cJSON_NULL 2

 #define cJSON_Number 3

 #define cJSON_String 4

#define cJSON_Array 5

 #define cJSON_Object 6

 typedef struct cJSON {

    struct cJSON *next,*prev;    /* next/prev allow you to walk array/object chains. Alternatively, use GetArraySize/GetArrayItem/GetObjectItem */

    struct cJSON *child;        /* An array or object item will have a child pointer pointing to a chain of the items in the array/object. */

    int type;                    /* The type of the item, as above. cjson结构的类型上面宏定义的7中之一*/

    char *valuestring;            /* The item's string, if type==cJSON_String */

    int valueint;                /* The item's number, if type==cJSON_Number */

    double valuedouble;            /* The item's number, if type==cJSON_Number */

    char *string;                /* The item's name string, if this item is the child of, or is in the list of subitems of an object. */

 } cJSON;
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一、解析json 用到的函数，在cJSON.h中都能找到：

 /* Supply a block of JSON, and this returns a cJSON object you can interrogate. Call cJSON_Delete when finished. */

extern cJSON *cJSON_Parse(const char *value);//从 给定的json字符串中得到cjson对象

/* Render a cJSON entity to text for transfer/storage. Free the char* when finished. */

extern char  *cJSON_Print(cJSON *item);//从cjson对象中获取有格式的json对象

 /* Render a cJSON entity to text for transfer/storage without any formatting. Free the char* when finished. */

extern char  *cJSON_PrintUnformatted(cJSON *item);//从cjson对象中获取无格式的json对象

/* Delete a cJSON entity and all subentities. */

extern void   cJSON_Delete(cJSON *c);//删除cjson对象，释放链表占用的内存空间

/* Returns the number of items in an array (or object). */

extern int    cJSON_GetArraySize(cJSON *array);//获取cjson对象数组成员的个数

/* Retrieve item number "item" from array "array". Returns NULL if unsuccessful. */

extern cJSON *cJSON_GetArrayItem(cJSON *array,int item);//根据下标获取cjosn对象数组中的对象

 /* Get item "string" from object. Case insensitive. */

 extern cJSON *cJSON_GetObjectItem(cJSON *object,const char *string);//根据键获取对应的值（cjson对象）

/* For analysing failed parses. This returns a pointer to the parse error. You'll probably need to look a few chars back to make sense of it. Defined when cJSON_Parse() returns 0. 0 when cJSON_Parse() succeeds. */

 extern const char *cJSON_GetErrorPtr(void);//获取错误字符串

要解析的json

 {

    "semantic": {

        "slots":    {

            "name": "张三"

        }

    },

    "rc":   0,

    "operation":    "CALL",

    "service":  "telephone",

    "text": "打电话给张三"

 }

#include <stdio.h>

#include <stdlib.h>

#include "cJSON.h"

 void printJson(cJSON * root)//以递归的方式打印json的最内层键值对

 {

    for(int i=0; i<cJSON_GetArraySize(root); i++)   //遍历最外层json键值对

    {

        cJSON * item = cJSON_GetArrayItem(root, i);        

        if(cJSON_Object == item->type)      //如果对应键的值仍为cJSON_Object就递归调用printJson

            printJson(item);

        else                                //值不为json对象就直接打印出键和值

        {

            printf("%s->", item->string);

            printf("%s\n", cJSON_Print(item));

        }

    }

 }

int main()

{

    char * jsonStr = "{\"semantic\":{\"slots\":{\"name\":\"张三\"}}, \"rc\":0, \"operation\":\"CALL\", \"service\":\"telephone\", \"text\":\"打电话给张三\"}";

    cJSON * root = NULL;

    cJSON * item = NULL;//cjson对象

    root = cJSON_Parse(jsonStr);     

    if (!root) 

    {

        printf("Error before: [%s]\n",cJSON_GetErrorPtr());

    }

    else

    {

        printf("%s\n", "有格式的方式打印Json:");           

        printf("%s\n\n", cJSON_Print(root));

        printf("%s\n", "无格式方式打印json：");

        printf("%s\n\n", cJSON_PrintUnformatted(root));

        printf("%s\n", "一步一步的获取name 键值对:");

        printf("%s\n", "获取semantic下的cjson对象:");

        item = cJSON_GetObjectItem(root, "semantic");//

        printf("%s\n", cJSON_Print(item));

        printf("%s\n", "获取slots下的cjson对象");

        item = cJSON_GetObjectItem(item, "slots");

        printf("%s\n", cJSON_Print(item));

        printf("%s\n", "获取name下的cjson对象");

        item = cJSON_GetObjectItem(item, "name");

        printf("%s\n", cJSON_Print(item));

        printf("%s:", item->string);   //看一下cjson对象的结构体中这两个成员的意思

        printf("%s\n", item->valuestring);

        printf("\n%s\n", "打印json所有最内层键值对:");

        printJson(root);

    }

    return 0;    

}

  二、构造json：

    构造 json比较简单，添加json对象即可。参照例子一看大概就明白了。

    主要就是用，cJSON_AddItemToObject函数添加json节点。

      xtern cJSON *cJSON_CreateObject(void);

        extern void cJSON_AddItemToObject(cJSON *object,const char *string,cJSON *item);

        extern cJSON *cJSON_CreateNull(void);

        extern cJSON *cJSON_CreateTrue(void);

        extern cJSON *cJSON_CreateFalse(void);

        extern cJSON *cJSON_CreateBool(int b);

        extern cJSON *cJSON_CreateNumber(double num);

        extern cJSON *cJSON_CreateString(const char *string);

        extern cJSON *cJSON_CreateArray(void);

        extern cJSON *cJSON_CreateObject(void);

 例子：     要构建的json：

        "semantic": {

        "slots":    {

            "name": "张三"

        }

            },

            "rc":   0,

            "operation":    "CALL",

            "service":  "telephone",

            "text": "打电话给张三"

        }

        #include <stdio.h>

        #include "cJSON.h"

        int main()

        {

    cJSON * root =  cJSON_CreateObject();

     cJSON * item =  cJSON_CreateObject();

    cJSON * next =  cJSON_CreateObject();

     cJSON_AddItemToObject(root, "rc", cJSON_CreateNumber(0));//根节点下添加

     cJSON_AddItemToObject(root, "operation", cJSON_CreateString("CALL"));

    cJSON_AddItemToObject(root, "service", cJSON_CreateString("telephone"));

     cJSON_AddItemToObject(root, "text", cJSON_CreateString("打电话给张三"));

     cJSON_AddItemToObject(root, "semantic", item);//root节点下添加semantic节点

    cJSON_AddItemToObject(item, "slots", next);//semantic节点下添加item节点

    cJSON_AddItemToObject(next, "name", cJSON_CreateString("张三"));//添加name节点

    printf("%s\n", cJSON_Print(root));

    return 0;

}

原文连接：https://blog.csdn.net/qq_32172673/article/details/88305781
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