JSON Library

JSON is a data interchange format that can be used to transmit data objects as human readable text. The innovaphone apps use JSON for all websocket protocols. This library can be used to encode and decode JSON in C++.

File information

Filecommon/ilib/json.h

Classes json_io
Defines JSON_ID_ROOT
JSON_ID_NONE

Examples Encoding
Decoding

Overview

JSON uses the following primitive data types.

JSON uses the following constructed data types.

Here are some hints how JSON messages are build up.

Classes

json_io

class json_io {
public:
    json_io(char * buffer);
    
    void reset();
    bool decode();
    word encode();
    word encode(word handle, char * buffer);
    void write(word current, char * & p, word incomplete = 0xffff);
    
    word add_object(word base, const char * name);
    word add_array(word base, const char * name);
    void add_string(word base, const char * name, const char * value, word len=0xffff);
    void add_string(word base, const char * name, const word * value, word len=0xffff);
    void add_replace_string(word base, const char * name, const char * value, word len=0xffff);
    void add_int(word base, const char * name, int c, char * & tmp);
    void add_unsigned(word base, const char * name, dword c, char * & tmp);
    void add_long64(word base, const char * name, long64 c, char * & tmp);
    void add_ulong64(word base, const char * name, ulong64 c, char * & tmp);
    void add_bool(word base, const char * name, bool value);
    void add_null(word base, const char * name);

    void add_printf(word base, const char * name, char * & tmp, const char * format, ...);
    void add_hexstring(word base, const char * name, const byte * hex, word hex_len, char * & tmp);
    void add_json(word base, const char * name, const char * value, word len=0xffff);
 
    word get_object(word base, const char * name);
    word get_object(word base, word & last);
    word get_array(word base, const char * name);
    word get_array(word base, word & last);
    const char * get_string(word base, const char * name);
    const char * get_string(word base, word & last);
    int get_int(word base, const char * name, bool * present=0);
    int get_int(word base, word & last, bool * present=0);
    dword get_unsigned(word base, const char * name, bool * present=0);
    dword get_unsigned(word base, word & last, bool * present=0);
    long64 get_long64(word base, const char * name, bool * present = 0);
    long64 get_long64(word base, word & last, bool * present = 0);
    ulong64 get_ulong64(word base, const char * name, bool * present = 0);
    ulong64 get_ulong64(word base, word & last, bool * present = 0);
    bool get_bool(word base, const char * name, bool * present=0);
    bool get_bool(word base, word & last, bool * present=0);
    bool get_bool_int(word base, const char * name, int & iret, byte * present=0);

    word to_url(word base, char * b, word l, const char * prefix = 0, bool cont = false);

    // advanced functions currently not documented
    word get_next(word base, word last, byte & type, byte & flags, const char * & name, const char * & info);
    word get_index();
    const char * get_name(word handle);
    const char * get_info(word handle);
    char * last;
    char * name_last;
    char * incomplete;
};

Overview

The json_io class can be used for encoding and decoding JSON messages to or from a char buffer.

Each element has a unique numeric ID word base. JSON_ID_ROOT represents the root element. It can be no or a single element. Often the root is an object that holds all the other data. Normally json_io can be used as a stack variable. A typical encoding flow looks like that:

char message[512];
json_io json(message);
word base = json.add_object(JSON_ID_ROOT, 0);
// add elements
json.encode();

Decoding is usually done like that:

json_io json(message);
json.decode();
word base = recv.get_object(JSON_ID_ROOT, 0);
// read elements

There are two different methods for traversing the JSON structure. Inside objects the elements are referenced by their name.

// adding elements
json.add_string(base, "name", "John Doe");
// reading elements
const char * name = json.get_string(base, "name");

Inside arrays the elements are enumerated.

// adding elements
json.add_string(base, 0, "programming");
json.add_string(base, 0, "tv series");
json.add_string(base, 0, "cooking");
// reading elements
word last = 0;
do {
    const char * interest = json.get_string(base, last);
}
while (last != JSON_ID_NONE);

Some add functions need an additional buffer for storing the values temporarily. Those buffers are called char * & tmp in the interface. Note that the buffer pointer reference is increased when adding an element.

char temp[128];
char * t = temp;
json.add_attrib_printf(base, "name", t, "%s %s", "John", "Doe");
json.add_unsigned(base, "age", 30, t);

Public functions

json_io (constructor)
Initializes the json_io structure.

Parameters

char * bufferThe buffer for the message.

Remarks

The buffer is mandatory for decoding. For encoding it is optional but if it's null a buffer must be passed to the encode function. Make sure it is big enough to contain the whole message. Note that the buffer will be modified for both encoding and decoding.
reset
Resets the internal state of the json_io structure. All added or parsed elements will be cleared from the internal state.
encode (overloaded)
Encodes the data structure into a null-terminated JSON string and writes it to the buffer specified with the constructor.

Return value

wordThe size of the encoded message.
encode (overloaded)
Encodes a subtree of the data structure into a null-terminated JSON string and writes it to the specified buffer.

Parameters

word handleThe ID of the root element of the subtree.
char * bufferThe buffer for the message.

Return value

wordThe size of the encoded message.
write
This function is used internally but it can also be used to encode the data structure to a JSON string in several chunks.

Parameters

word currentThe ID of the start element. Use 0 for complete data.
char *& pThe output buffer.
word incompleteThe incomplete argument can be used to write only the incomplete data so that new received data can be appended to the buffer an decoding started again. The incomplete argument should be the handle of the descriptor of the array, with potential incomplete elements.
add_object
Adds an object to the structure.

Parameters

word baseThe ID of the base element.
const char * nameThe name of the object. Only used when the object is inside another object. Set to 0 otherwise.

Return value

word The ID of the added object. Can be used to add elements to the object.

Remarks

If the object shall be added to root level, use JSON_ID_ROOT as the base.
add_array
Adds an array to the structure.

Parameters

word baseThe ID of the base element.
const char * nameThe name of the array. Only used when the array is inside an object. Set to 0 otherwise.

Return value

word The ID of the added array. Can be used to add elements to the array.

Remarks

If the array shall be added to root level, use JSON_ID_ROOT as the base.
add_string (overloaded)
Adds an UTF-8 string to the structure.

Parameters

word baseThe ID of the base element.
const char * nameThe name of the element. Only used when it's is inside an object. Set to 0 otherwise.
const char * valueThe value of the element. If value is 0 the element is not added.
word lenThe number of bytes in the value buffer. Only needed if value is not null-terminated.
add_string (overloaded)
Adds a string with 16-bit character representation to the structure.

Parameters

word baseThe ID of the base element.
const char * nameThe name of the element. Only used when it's is inside an object. Set to 0 otherwise.
const word * valueThe value of the element. If value is 0 the element is not added.
word lenThe number of bytes in the value buffer. Only needed if value is not null-terminated.
add_replace_string
Adds or replaces an existing UTF-8 string inside an object.

Parameters

word baseThe ID of the base element.
const char * nameThe name of the element (mandatory).
const char * valueThe value of the element.
word lenThe number of bytes in the value buffer. Only needed if value is not null-terminated.

Remarks

The function is only implemented for strings inside objects.
add_int
Adds a 32-bit integer to the structure.

Parameters

word baseThe ID of the base element.
const char * nameThe name of the element. Only used when it's is inside an object. Set to 0 otherwise.
int cThe value of the element.
char * & tmpThe temporary buffer to store the value until it's encoded.
add_unsigned
Adds a 32-bit unsigned integer to the structure.

Parameters

word baseThe ID of the base element.
const char * nameThe name of the element. Only used when it's is inside an object. Set to 0 otherwise.
unsigned cThe value of the element.
char * & tmpThe temporary buffer to store the value until it's encoded.
add_long64
Adds a 64-bit integer to the structure.

Parameters

word baseThe ID of the base element.
const char * nameThe name of the element. Only used when it's is inside an object. Set to 0 otherwise.
long64 cThe value of the element.
char * & tmpThe temporary buffer to store the value until it's encoded.
add_ulong64
Adds an 64-bit unsigned integer to the structure.

Parameters

word baseThe ID of the base element.
const char * nameThe name of the element. Only used when it's is inside an object. Set to 0 otherwise.
ulong64 cThe value of the element.
char * & tmpThe temporary buffer to store the value until it's encoded.
add_printf
Does an sprintf to a temporary buffer and adds the resulting string to the structure.

Parameters

word baseThe ID of the base element.
const char * nameThe name of the element. Only used when it's is inside an object. Set to 0 otherwise.
char * & tmpThe temporary buffer to store the value until it's encoded.
const char * formatA standard sprintf format string.
...Additional parameters to be used by sprintf as defined in the format string.
add_hexstring
Converts a binary buffer to a hex string and adds it to the structure, encoded as a JSON string.

Parameters

word baseThe ID of the base element.
const char * nameThe name of the element. Only used when it's is inside an object. Set to 0 otherwise.
const byte * hexArbitrary binary data.
word hex_lenThe number of bytes in the hex buffer.
char * & tmpThe temporary buffer to store the value until it's encoded.
add_json
Adds a raw JSON string to the structure. The string must have valid encoding. It will not be escaped by the library.

Parameters

word baseThe ID of the base element.
const char * nameThe name of the element. Only used when it's is inside an object. Set to 0 otherwise.
const char * valueThe JSON string to be added.
word lenThe number of bytes in value. Only needed if value is not null-terminated.
get_object (overloaded)
Gets an object by name.

Parameters

word baseThe ID of the base element.
const char * nameThe name of the element. Only used when it's is inside an object. Set to 0 otherwise.

Return value

wordThe ID of the object or JSON_ID_NONE if it's not found.
get_object (overloaded)
Gets the next object from an array.

Parameters

word baseThe ID of the base element.
word & lastA word reference that must have the value 0 for the first call. The value will be updated by the function call for each subsequent call. If the value is JSON_ID_NONE the end of the array is reached.

Return value

wordThe ID of the object or JSON_ID_NONE if there are no more objects in the array.
get_array (overloaded)
Gets an array by name.

Parameters

word baseThe ID of the base element.
const char * nameThe name of the element. Only used when it's is inside an object. Set to 0 otherwise.

Return value

wordThe ID of the array or JSON_ID_NONE if it's not found.
get_array (overloaded)
Gets the next array from an array.

Parameters

word baseThe ID of the base element.
word & lastA word reference that must have the value 0 for the first call. The value will be updated by the function call for each subsequent call. If the value is JSON_ID_NONE the end of the array is reached.

Return value

wordThe ID of the array or JSON_ID_NONE if there are no more objects in the array.
get_string (overloaded)
Gets a string by name.

Parameters

word baseThe ID of the base element.
const char * nameThe name of the element. Only used when it's is inside an object. Set to 0 otherwise.

Return value

const char *The value of the string or 0 if it's not found.
get_string (overloaded)
Gets the next array from an array.

Parameters

word baseThe ID of the base element.
word & lastA word reference that must have the value 0 for the first call. The value will be updated by the function call for each subsequent call. If the value is JSON_ID_NONE the end of the array is reached.

Return value

const char *The value of the string or 0 if there are no more objects in the array.
get_int (overloaded)
Gets a 32-bit signed integer by name.

Parameters

word baseThe ID of the base element.
const char * nameThe name of the element. Only used when it's is inside an object. Set to 0 otherwise.
bool * presentIf specified, the value will be set to true if the element existed. false otherwise.

Return value

intThe value of the element or 0 if it's not found.
get_int (overloaded)
Gets the next 32-bit signed integer from an array.

Parameters

word baseThe ID of the base element.
word & lastA word reference that must have the value 0 for the first call. The value will be updated by the function call for each subsequent call. If the value is JSON_ID_NONE the end of the array is reached.
bool * presentIf specified, the value will be set to true if the element existed. false otherwise.

Return value

intThe value of the element or 0 if there are no more objects in the array.
get_unsigned (overloaded)
Gets a 32-bit unsigned integer by name.

Parameters

word baseThe ID of the base element.
const char * nameThe name of the element. Only used when it's is inside an object. Set to 0 otherwise.
bool * presentIf specified, the value will be set to true if the element existed. false otherwise.

Return value

dwordThe value of the element or 0 if it's not found.
get_unsigned (overloaded)
Gets the next 32-bit unsigned integer from an array.

Parameters

word baseThe ID of the base element.
word & lastA word reference that must have the value 0 for the first call. The value will be updated by the function call for each subsequent call. If the value is JSON_ID_NONE the end of the array is reached.
bool * presentIf specified, the value will be set to true if the element existed. false otherwise.

Return value

dwordThe value of the element or 0 if there are no more objects in the array.
get_long64 (overloaded)
Gets a 64-bit signed integer by name.

Parameters

word baseThe ID of the base element.
const char * nameThe name of the element. Only used when it's is inside an object. Set to 0 otherwise.
bool * presentIf specified, the value will be set to true if the element existed. false otherwise.

Return value

long64The value of the element or 0 if it's not found.
get_long64 (overloaded)
Gets the next 64-bit signed integer from an array.

Parameters

word baseThe ID of the base element.
word & lastA word reference that must have the value 0 for the first call. The value will be updated by the function call for each subsequent call. If the value is JSON_ID_NONE the end of the array is reached.
bool * presentIf specified, the value will be set to true if the element existed. false otherwise.

Return value

long64The value of the element or 0 if there are no more objects in the array.
get_ulong64 (overloaded)
Gets a 64-bit signed uninteger by name.

Parameters

word baseThe ID of the base element.
const char * nameThe name of the element. Only used when it's is inside an object. Set to 0 otherwise.
bool * presentIf specified, the value will be set to true if the element existed. false otherwise.

Return value

ulong64The value of the element or 0 if it's not found.
get_ulong64 (overloaded)
Gets the next 64-bit unsigned integer from an array.

Parameters

word baseThe ID of the base element.
word & lastA word reference that must have the value 0 for the first call. The value will be updated by the function call for each subsequent call. If the value is JSON_ID_NONE the end of the array is reached.
bool * presentIf specified, the value will be set to true if the element existed. false otherwise.

Return value

ulong64The value of the element or 0 if there are no more objects in the array.
get_bool (overloaded)
Gets a boolean by name.

Parameters

word baseThe ID of the base element.
const char * nameThe name of the element. Only used when it's is inside an object. Set to 0 otherwise.
bool * presentIf specified, the value will be set to true if the element existed. false otherwise.

Return value

boolThe value of the element or false if it's not found.
get_bool (overloaded)
Gets the next boolean from an array.

Parameters

word baseThe ID of the base element.
word & lastA word reference that must have the value 0 for the first call. The value will be updated by the function call for each subsequent call. If the value is JSON_ID_NONE the end of the array is reached.
bool * presentIf specified, the value will be set to true if the element existed. false otherwise.

Return value

boolThe value of the element or false if there are no more objects in the array.
get_bool_int (overloaded)
Gets a boolean or interger by name.

Parameters

word baseThe ID of the base element.
const char * nameThe name of the element. Only used when it's is inside an object. Set to 0 otherwise.
int & iretReturns the integer value
bool * presentIf specified, the value will be set to 1 if the element existed and is bool, 2 if the value existed and is integer. Set to 0 otherwise

Return value

boolThe value of the element or false if it's not found.
to_url
Helper function to encodes the structure not as JSON but as URL arguments.

Parameters

word baseThe ID of the base element.
char * bThe output buffer.
word lThe size of the output buffer.
const char * prefixPrefix to be used for the single elements. For elements nested within arrays or objects another prefix of the name of the array/object is added. The prefixes are sepearted by '.'
bool conttrue indicates that the arguments are to be added to other arguments. This means a '&' is put at the beginning

Return value

wordThe number of bytes that have been written to the output buffer.

Data types

Defines

JSON_ID_ROOTRepresents the top-level element.
JSON_ID_NONEUsed if an element is not found.

Code Examples

The following examples demonstrate the library on the following JSON structure.

{ 
    "name": "John Doe", 
    "age": 30, 
    "interests": [ 
        "programming", 
        "tv series", 
        "cooking"
    ] 
}

Encoding

char message[256];
json_io json(message);

word base = json.add_object(JSON_ID_ROOT, 0);
json.add_string(base, "name", "John Doe");
json.add_unsigned(base, "age", 30);

word interests = json.add_array(base, "interests");
json.add_string(interests, "programming");
json.add_string(interests, "tv series");
json.add_string(interests, "cooking");

json.encode();

Decoding

json_io json(message);
json.decode();

word base = json.get_object(JSON_ID_ROOT, 0);
const char * name = json.get_string(base, "name");
dword age = json.get_unsigned(base, "age");

const char * interest_values[4];
word interest_count = 0;
word interests = json.get_array(base, "interests");
word last = 0;

while (last != JSON_ID_NONE && interest_count < 4) {
    const char * value = = json.get_string(interests, last);
    if (value) {
        interest_values[interest_count++] = value;
    }
}