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。

    cjson库文件下载：

 

    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;

 

 

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" }

  代码：

#include  #include  #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//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" }

代码：

#include  #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;
}