cJSON 使用详解

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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;

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#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

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要解析的json

{
    "semantic": {
        "slots":    {
            "name": "张三"
        }
    },
    "rc":   0,
    "operation":    "CALL",
    "service":  "telephone",
    "text": "打电话给张三"
}

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#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;    
}

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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);

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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"
}

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#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;
}

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