Detailed explanation of cjson usage

Hello everyone , I meet you again , I'm your friend, Quan Jun .

because c In language , There is no direct dictionary , String arrays and other data structures , So with the help of structure definition , Handle json. It is more convenient if there is a corresponding data structure , Such as python of use json.loads(json) Just put json It is convenient to convert strings into built-in data structures .

An important concept :

stay cjson in ,json The object can be json, Can be a string , It could be a number ...

cjson Data structure definition ：

#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 The type of the structure is defined by the macro above 7 One of them */

    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 The type of the structure is defined by the macro above 7 One of them */

    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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One 、 analysis json The function used , stay cJSON.h You can find it all in the library ：

/* 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);// from   A given json Get from string cjson object 
/* Render a cJSON entity to text for transfer/storage. Free the char* when finished. */
extern char  *cJSON_Print(cJSON *item);// from cjson Object to get a formatted json object 
/* Render a cJSON entity to text for transfer/storage without any formatting. Free the char* when finished. */
extern char  *cJSON_PrintUnformatted(cJSON *item);// from cjson Object json object 

/* Delete a cJSON entity and all subentities. */
extern void   cJSON_Delete(cJSON *c);// Delete cjson object , Release the memory space occupied by the linked list 

/* Returns the number of items in an array (or object). */
extern int    cJSON_GetArraySize(cJSON *array);// obtain cjson Number of object array members 
/* Retrieve item number "item" from array "array". Returns NULL if unsuccessful. */
extern cJSON *cJSON_GetArrayItem(cJSON *array,int item);// Get... By subscript cjosn Objects in the object array 
/* Get item "string" from object. Case insensitive. */
extern cJSON *cJSON_GetObjectItem(cJSON *object,const char *string);// Get the corresponding value according to the key （cjson object ）

/* 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);// Get error string 

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To parse json

{
    "semantic": {
        "slots":    {
            "name": " Zhang San "
        }
    },
    "rc":   0,
    "operation":    "CALL",
    "service":  "telephone",
    "text": " Call Zhang San "
}

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

void printJson(cJSON * root)// Print recursively json The innermost key value pair of 
{
    for(int i=0; i<cJSON_GetArraySize(root); i++)   // Traverse the outermost layer json Key value pair 
    {
        cJSON * item = cJSON_GetArrayItem(root, i);        
        if(cJSON_Object == item->type)      // If the value of the corresponding key is still cJSON_Object Call recursively printJson
            printJson(item);
        else                                // Values are not for json Objects print out keys and values directly 
        {
            printf("%s->", item->string);
            printf("%s\n", cJSON_Print(item));
        }
    }
}

int main()
{
    char * jsonStr = "{\"semantic\":{\"slots\":{\"name\":\" Zhang San \"}}, \"rc\":0, \"operation\":\"CALL\", \"service\":\"telephone\", \"text\":\" Call Zhang San \"}";
    cJSON * root = NULL;
    cJSON * item = NULL;//cjson object 

    root = cJSON_Parse(jsonStr);     
    if (!root) 
    {
        printf("Error before: [%s]\n",cJSON_GetErrorPtr());
    }
    else
    {
        printf("%s\n", " Print in a format Json:");           
        printf("%s\n\n", cJSON_Print(root));
        printf("%s\n", " Print without format json：");
        printf("%s\n\n", cJSON_PrintUnformatted(root));

        printf("%s\n", " Step by step name  Key value pair :");
        printf("%s\n", " obtain semantic Under the cjson object :");
        item = cJSON_GetObjectItem(root, "semantic");//
        printf("%s\n", cJSON_Print(item));
        printf("%s\n", " obtain slots Under the cjson object ");
        item = cJSON_GetObjectItem(item, "slots");
        printf("%s\n", cJSON_Print(item));
        printf("%s\n", " obtain name Under the cjson object ");
        item = cJSON_GetObjectItem(item, "name");
        printf("%s\n", cJSON_Print(item));

        printf("%s:", item->string);   // to glance at cjson The meaning of these two members in the structure of the object 
        printf("%s\n", item->valuestring);
                        

        printf("\n%s\n", " Print json All innermost key value pairs :");
        printJson(root);
    }
    return 0;    
}

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Two 、 structure json：

structure json Relatively simple , add to json Object can . I can see from the examples .

Mainly for ,cJSON_AddItemToObject Function add json node .

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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Example ： To build json：

"semantic": {
        "slots":    {
            "name": " Zhang San "
        }
    },
    "rc":   0,
    "operation":    "CALL",
    "service":  "telephone",
    "text": " Call Zhang San "
}

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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));// Add... Under the root node 
    cJSON_AddItemToObject(root, "operation", cJSON_CreateString("CALL"));
    cJSON_AddItemToObject(root, "service", cJSON_CreateString("telephone"));
    cJSON_AddItemToObject(root, "text", cJSON_CreateString(" Call Zhang San "));
    cJSON_AddItemToObject(root, "semantic", item);//root Add under node semantic node 
    cJSON_AddItemToObject(item, "slots", next);//semantic Add under node item node 
    cJSON_AddItemToObject(next, "name", cJSON_CreateString(" Zhang San "));// add to name node 

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

    return 0;
}

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